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Back to Home 		  Some Thoughts on Off-The-Grid (Solar, Micro‑Hydro or Wind Powered) Repeater Systems
By Mike Morris WA6ILQ 		  Print this Page

This page was written in 2006 and there is an update at the bottom.
I'm presenting some random thoughts, not a really polished article.

Comments, updates and contributions are welcome.

Over the last few decades the majority of the efficiency engineering on small power systems has been done by the "off-the-grid" homeowners... those that live far from the power lines and depend on solar electric panels, wind generators, micro‑hydro water systems, etc. The "bible" of their community is the 6-times-per-year Home Power Magazine (at http://www.homepower.com), published by Richard and Karen Perez (N7BCR and KA7ETV) in Oregon. If you are interested in solar power for a repeater site, or in cutting your home power bill, this is the magazine for you. Since the first issue in October 1987, they have published hundreds of articles on solar, wind, and microhydro electricity, energy efficiency, solar hot water systems, space heating and cooling, green building materials and home design, efficient transportation, and more. Their web site is worth a visit for subscriptions, CD and book orders. And the magazine staff walks the walk as well - the entire magazine production is done with off-the-grid computers. Their web site is a fount of information, as are their back issues which they sell in PDF form on CDs. Yes, every issue ends up a single searchable PDF file, and they sell multi-year collections on CDs. No, I've no interest in the magazine except as a long-time subscriber. The advertising pages in the magazine pay for the production and are useful as a source of vendors of panels, charge controllers, disconnects, batteries, energy efficient appliances, and more.

No, they didn't pay me to write and publish the above paragraph. I'm a long-time subscriber, feel it's a valuable resource, and wanted to share the information that they exist.

//
// Update:
// This article was written in 2006. Home Power Magazine has folded; the last issue was November 2018.
// The complete 31 years of publication - every issue - is available for download from their web site at https://www.homepower.com.
//

A second source of DC efficiency engineering has been the RV community and the RV manufacturers and the made-for-RV equipment manufacturers. One of those manufacturers is Victron. More on this later in this writeup.

Back to repeaters (amateur, GMRS, even commercial). If you try some experimentation you will be amazed at just how much transmitter power you DON'T need if you have a good site, and a good antenna system. An antenna with 6dB to 9dB of gain will give you a much better performance boost than a more powerful transmitter since it will benefit you on receive as well. As an example of reduced power, locally we have a UHF repeater sitting at the top of a 4,900 foot (1500m) mountain. The system has two antennas on the tower: a shared receive antenna at 120 feet (36.5m) with 1 5/8 inch Heliax feedline then an AngleLinear preamp, then a multicoupler / distribution amplifier that drives the receivers of several different repeaters that range from 400-512 MHz.
The transmitter feeds a circulator and a second 8 inch or 10 inch pass cavity then a run of 1 5/8 inch Heliax feedline to the transmit antenna at 80 feet (24.4m). Why transmitting from the 80 foot level? The 1 5/8 transmit feedline and transmit antenna was leftover on the tower from a previous tenant.
One day I noticed that the system was not as audible as I drove into the parking garage at work... it was "scratchy". Later that day I asked the owner about it and he commented that the 100w transmitter power amplifier had died the previous week and the system had been running on the exciter since. Hmmm.... and the output power was? About five watts into the feedline! Yes, it was weaker and had some flutter but it was usable.

As evident by the previous paragraph, the efficiency and gain of the antenna system is a major contributor. Antenna pattern and antenna gain is a frequently misunderstood term. Every dB you get in antenna "gain" is a dB that isn't going somewhere else. If that "somewhere else" is some place that you will never need coverage, then it's A Good Thing. The gain in an antenna comes from limiting the pattern in some way - by tilting it, flattening it, or by changing the directivity.
Look at this web page: http://www.marcspages.co.uk/tech/antgain.htm.   (local copy)

So before you go to all the effort of engineering a ultra low drain 50 watt repeater, do an FCC web search for geographically nearby systems and see what else is at the same site (even commercial or public safety), and then put an appropriate attenuator on a receiver (even a scanner) to give yourself the performance of an effective 50 watts (or whatever your situation calls for) ERP from that site and drive around as you do a lot of listening. You might find that you can get by with a 20-30w ERP system instead of the 100-150w ERP you think you need. That means a lot less transmitter, which means a lot less battery (i.e. fewer dollars spent) and fewer solar panels... (i.e. even fewer dollars spent). Or a lot more run time for the battery you do buy / have.

Another way of saying the above is that many times, cutting your transmitters power by 3 dB or 6 dB and using 3 dB, 6 dB or 9 dB of antenna gain can have a bigger impact on the power consumption. And as a bonus the 3 dB, 6 dB or 9 dB increase in antenna improves your system receive performance.

Transmitter efficiency is another place to look at. I've seen two different transmitters on the bench where one used 20% less DC power compared to the other for the same output power... That translates to that much more battery life.

You might get some ideas for your design if you peruse the manual for a Daniels Electronics MT-3 or MT-4 or an old Motorola or GE fire tower radio (also known as a "lookout repeater"). The Motorola firetower radio was originally developed in the 1960s for the Forest Service and was specifically designed to run for as long as possible on a car battery (or two or more in parallel). The unit was switchable between base station mode and repeater mode. The first iteration used HT200 handheld / PT200 packset receiver and transmitter boards and was redesigned as needed as the radios went through various generations. Depending on the model and options it ran from 2, 10 or 20 watts - but was very stingy with DC power. Squelched receive current draw was about 7 milliamps.

I've seen the manuals on a couple of generations of the firetower radios and seen a couple of the firetower radios on eBay (the latest manual I've been able to acquire is on the Motorola MX page at this web site - look for the MTR-300 FireTower repeater PDF file). Aerotron, Repco and others also made fire tower radios. GE made one based on their PortaMobile radio. Then there was the suitcase repeater Motorola made from some MX series handhelds originally for a 3-letter agency but it became a regular product... But not all Motorola radio suitcases are handheld based; they made some that were mobile radio based.

One of the most useful features in the more advanced repeater controllers is analog voltage inputs (i.e. remote metering). You can measure battery voltage, battery temperature, charging current, discharge current, and more. You can trigger announcements based on certain measurements - i.e. when the AC goes away the repeater can announce that fact and switch to a different courtesy beep, a shorter carrier delay (hang-in timer), a different ID (perhaps suffixed with "/B" to indicate on battery), or a combination of those signals. This change in repeater characteristics can clue the users to keep their transmissions short. When the battery discharges to a certain voltage threshold point the ID could change pitch and change the suffix to "/LB" indicating Low Battery). Additional trigger points can make more pointed announcements until the repeater transmitter is disabled completely by a LVD (low voltage disconnect) protecting an exhausted battery).

The most common technology for powering off-the-grid repeaters is solar panels. Don't think for a moment that you will have a solar powered repeater - you won't. It's not solar powered, it's battery powered, and the batteries are solar charged. If the sun doesn't shine your batteries die, and the repeater is off the air. If your power source is a micro-hydro system, then the charging is dependent on the water flow. If your power source is a wind generator then the charging is dependent on the wind. There is no reason that you can't have a mixed source - micro-hydro, wind and solar. The batteries are the critical part, and will probably cost you as much as the power source does, and require more maintenance. More on them later.

The power generating difference between a horizontal flat-on-the-ground solar panel and a properly angled (tilted) panel can easily be over four to one... as proven in my own personal experience: Back in the early 1980s I had an array of four 3-4 amp panels in parallel (i.e. 12 volts 12-16 amps) lying on the ground in my back yard charging a parallel bank of three 12v 20ah gell cells and saw a charge rate of 1.4 to 2 amps. Tilting the 4-panel array by hand while watching the ammeter I saw about 8 amps at the best angle - and to maintain the peak current I had to reset the panel angle about once per hour as the sun traversed across the sky.

There are tracking solar panel mounts that maximize the power generation but they are not practical at rural, occasionally visited repeater sites (moving parts need maintenance, and most of my repeater sites get visited only when necessary, and there's one site that I've not been to in over 4 years). Vandalism is another concern - I've seen photos of an multi-panel array that had about 30 bullet holes in it, plus a shotgun slug through the tracker drive. The entire solar array was a total loss.

Batteries, Charge Controllers, and more:

Abbreviation time.... State-of-charge (SOC) is a term used to describe how much stored energy is in a battery. The SOC of a fully charged battery is 100%. Normal operation of the battery occurs between 0 and 100% state of charge. A fully discharged battery, 0% SOC, is defined by the conditions of its capacity measurement and not by a complete lack of charge. The method of measuring it is different in nickel-cadmium, lead-acid, and lithium batteries. It's inverse is the depth-of-discharge (DOD).

Before you buy a battery charge controller do your research - there are at least two basic types of solar charge controllers: PWM and MPPT.

The Pulse Width Modulation (PWM) controller senses the SOC of a lead acid battery by measuring the voltage and then reduces the power from the PV panel to charge the battery by interrupting the current into a rapid series of power pulses. The pulses can vary in width and repetion rate. The rapid rise and fall time of the power pulses can be a source of EMI in poorly designed and suppressed units.

The MPPT (Maximum Power Point Tracking) charge controller adjusts the input impedance of the DC controller on the fly to match the output impedance of the solar panel (which varies drastically with the amount of sun falling on it). It looks at the voltage of the the PV panel(s) and the battery and continuously adjusts itself so as to maximize charging amps into the battery. Internally it's a high frequency dc to dc switching converter and hence can be a source of EMI if poorly designed, not suppressed or isolated. It takes the output of the PV panel, changes it to high frequency AC, runs it through a transformer, and then rectifies it back to DC that matches the battery. It varies the AC voltage and waveshape (duty cycle) to optimize the charging voltage and current.

One advantage of the MPPT design is that you can have the panels in series (i.e. 24, 36, 48 volts or even higher) and use smaller diameter wire for the same power delivered. I remember reading one article about a home system where the solar panel array was several hundred feet from the house and to minimize the IR voltage drop in the cabling the array output was in the 150 volt range. Another advantage so series panels is that an overcast day will still deliver enough voltage to the controller to put some change in the batteries... i.e. a 15% impact on a 14 volt array of panels is 11.9 volts, and that may not be enough. The same 15% drop on a 28 volt array is 23.8 volts and that will still charge a 12 volt battery bank.

Both controller designs can give you as much of a boost (or more) in power generation as a good tracker mount. But be careful - both designs can be RF hash generators. If you can, try before you buy, or have an exchange arrangement before you hand over your money.

//
// 2019 update: MPPT has won the battle and is the preferred type.
//

If you have a cold site then make sure that all of your equipment - the radio, the solar charge controller, etc. are rated for the coldest temperatures you will ever see.

Back to solar panels...
The most effective fixed mount is to point your panel(s) facing the equator and perpendicular to the sun at solar noon... which translates to true south (not magnetic south) plus a variable number of degrees depending on the lattitude.

Lattitude Angle
0-4° 10°
5-20° Lattitude + 5°
21-45° Lattitude + 10°
45-65° Lattitude + 15°
65-75° 80°
The USA ranges from 25 to about 48 degrees. At my location (which is near the 34th parallel) I'd use an angle of about 44±5 degrees... but the truly optimum angle changes with the seasons.

However.... if you think about it the tilt angle for best power is NOT the best winter snow / frost / dirt shedding angle. So in snowy areas you either end up making a site visit twice a year to change the angle, or you end up mounting the solar panels at the optimum winter shedding angle and adding enough panels to compensate for the non-optimum angle and keep the system working in the winter (short solar days), and taking the charge reduction in the summer knowing that the more intense sunlight (higher solar flux) and the longer solar day will more than make up for the non-optimum mounting angle. And even with the longer solar day you will have to use a solar charge controller to prevent overcharging. Some controller just pinch off the current flow and that can raise the operating temperature of the panels. Others have an output that connects to an auxiliary load... that load bleeds off the excess DC power (one popular method is to run a fan or two to blow some cooling air through the building, another is mounting several resistive loads in a well-ventilated rooftop housing... and maybe add a fan blowing across the resistors?). The off-the-grid community used that aux load output to run a water heater, then they took their showers before they went to bed.

When you build up the mounting frame for your solar panels for the repeater site you may want to plan on tower mounting them. Not only will it get you above the snow on the ground, but it will get you above the trees and some of the vandals and thieves. And when you mount the panels you may want to survey the tower for additional panel(s) (i.e. plan for system growth). And don't forget that the panel(s) will add wind loading and torque loading to the tower, and the weight of the snow will add even more load.

And as was mentioned above, if you use solar panels on your system, it's not solar powered, it's battery powered, and the batteries are solar recharged. Just as in your car, the battery acts as a sponge for electricity... the load squeezes it out, and the charging source pours it back in, but you lose a little in each direction. If the inbound does not keep up with the outbound plus the losses, it's going to run dry.

In the daytime your solar panels get to both run the repeater and charge the batteries. You may only have 4 to 5 solar hours a day, but you have 24×7×365 repeater drain. The batteries are all you have at night or on cloudy days (and what if you get 10 cloudy days in a row? Or 15? That's not unheard of in many parts of the country (especially in those where the seasons are measured in almost winter, real winter, late winter, and road repair). In those areas you may need 300 watts of solar to run a repeater that draws 50 watts of DC power...

One fact that bites the behinds of some folks that are just starting out with solar panels and batteries is that running some batteries down to under 50% charge will shorten their life (the exact percentage depends on the battery chemistry and the ambient temperature). Yes, your 400 AH battery bank may only have 200 AH of usable capacity, and even less when it gets cold. And cold temperatures can do a real number on battery SOC, as anyone who lives in Alaska, Canada or the northern states and parks their car outside, and then tries to start it in the morning can attest. You can easily lose 1/2 to 2/3 of your battery capacity in the temperature swing from 75 degrees F (about 25 degrees C) to 33 degrees F (about 1 degree C), and if your batteries freeze they can be destroyed.

So if your repeater site is in an area where temperatures get down to freezing and you use FLA or AGMs that can be discharged to low levels you will have to buy or build an insulated battery box. And some extreme areas may require a heater inside the battery box (which is just that much more energy that you will have to generate). And don't scrimp on the insulation - you will discover that styrofoam is MUCH cheaper than batteries and solar panels. Don't forget the insulation under and over as well as around the batteries. I saw one mountaintop site where the four 200 AH batteries were sitting on the concrete floor... a huge thermal mass (heat sink). The room might be 50 degrees F (about 10 degrees C) due to the heating by the equipment but the batteries were around 35 degrees F (about 1.5 degrees C) becasue the concrete floor was effectively at the outside temperature. Installing a plywood battery box with several inches of closed-cell (semi-hard) styrofoam under, around, and above the batteries made a big difference.

Battery choices: Lead / Acid, or Lithium?

a) Flooded Lead Acid (FLA) batteries are "wet" (i.e. each cell has a fill cap). This type includes many automotive batteries (12 Volt) and golf cart batteries (6 Volt). Many RVs use a pair (or multiple pairs in parallel) of 6 volt 225 AH FLAs in series (the common name is a "GC2" battery, and they are widely available). You may need to add water on a regular basis and FLAs are characterized by venting of corrosive (sulfuric acid) gases. And the gases can condense on the top of the batteries and on the terminals.
For solar systems, FLA batteries can give the most Amp-Hours (AH) for the money but you must consider the hazards involved. They can self-discharge as much as 5 percent per month, or higher if there is damp dirt across the top of the battery.

b) The second type are factory Sealed Lead Acid units. They are commonly called SLA or SLAB (Sealed Lead Acid Battery). In addition to not needing the distilled water the sealed top means that they stay cleaner longer... And no corrosion damage to the battery box or tray or brackets from sulphuric acid fumes.

The Sealed Lead Acid batteries come in two or three sub-types:

The first sub-type of SLA is a sealed lead-calcium battery; these are the "sealed maintenance free" batteries that are the original equipment for most automotive, commercial vehicle and motor home chassis (starting) battery positions. A lead-calcium battery is a still a lead acid battery, the manufacturers tweaked the chemistry by using calcium instead of antimony in the plates of the battery plates to give it some advantages. There are no fill caps on top but these are not AGM.

The second sub-type of SLA is called an "Absorbed Glass Mat" or AGM... the name comes from how they are constructed internally. These AGM batteries are true sealed batteries and are preferred over lead-calcium or gell-cell. Recharging is same as for FLA batteries, plus they have lower self discharge rates, low internal resistance when under heavy loads (helps prevent heat buildup). Interestingly, they are the only type that is allowed in power wheelchairs on passenger carrying airplanes.

c) There is an older sub-type that predated AGM; it was a sealed battery that was called a gell-cell and was popular is the 1970s through the 1990s... You will probably never see one... There were both lead-acid and nickel-cadmium gell-cells. The manufacturer used a jelly electrolyte and sealed the battery so as to prevent spillage and to allow "any position" mounting. However, they must be more slowly charged, never above 1/10 of the AH (i.e. a 20 AH gell cell was limited to 2 amps) and they use slightly lower charge voltages to prevent gas bubbles from forming within the cell. Both the lead-acid and nickel cadmium versions had similar charging current limitations. Gell-cells have lost most of their market share to AGM batteries and there are few left.

We mentioned State of Charge above...
The SOC of a lead-acid battery can be estimated by by measuring the battery voltage at rest (defined as diconnecting the battery for three hours and measuring the voltage). An RV magazine had this table:
Lead-Acid Battery State of Charge at Rest
State of
Charge
at Rest		 Battery Voltage
Flooded (FLA)
(has twist-off
fill caps)		 Sealed
(SLA)		 Absorbed
(AGM)
100% 12.6-12.7 12.85+ 12.8+
75% 12.4 12.7 12.6
50% 12.2 12.4 12.3
25% 12.0 12.0 12.0
0% 11.8 11.8 11.8
Note that the difference between 25% and 100% is about 3/4 of a volt.

IMPORTANT NOTES:

Then there are the lithium-based batteries... they are available in several different lithium-based chemistries.
Lithium batteries cost two to three times more than FLA or SLA or AGM but in most cases will last three to five times longer and have many additional advantages, including 1/3 to 1/4 the weight for the same capacity, greatly increased storage capacity for the same physical size, able to accept higher charge and discharge currents - given a charger with enough amperage a lithium battery can recharge in 1/4 to 1/3 of the time of the same capacity FLA or AGM. Depending on which manufacturers literature you read the lithium batteries can be discharged down to 15% or even 10% SOC (i.e. 85% or 90% discharged).
The charge profile of a lithium is very different than for an FLA, SLA or AGM. The battery charger must be designed for the lithium battery chemistry. The lithium batteries are better but not perfect, one problem is that many lithium batteries do not do well at low temperatures, in fact several RV manufacturers that offer lithium have moved the lithium batteries from an outside storage compartment (translation: outside air temperature) to inside the RV (60-90 degrees F / 16-32 degrees C).

As mentioned above the SOC of a lead-acid battery can be loosely approximated by measuring the battery voltage at rest, but the SOC of a lithium cannot... it will be almost full voltage until almost dead. The SOC of a lithium battery can best be determined by using a shunt based battery monitor. This technology adds a milliohm resistor between the battery negative terminal and system ground. The monitor continuously reads the voltage across the shunt resistor... positive millivolts as the battery is charging and negative millivolts during discharging. These millivolts are measured over time and custom software in a microprocessor develops a "gas gauge" display that shows real-time SOC. Victron is a very popular manufacturer of these units for RVs, and some models can feed a logging / graphing program on a laptop, others talk to a cellphone application display.

Lithium Summary:

Back to solar...
If you have a large enough array of solar panels to generate a decent charge in the winter you will have serious overkill in the summer despite the increased repeater usage caused by the longer days.... Depending on your latitude the lower angle of the sun in the winter sky means that the sunlight hours in December provide as little as 1/3 of the charge available for the same hours in July. And in Alaska, Canada, or the northern states like Washington, Montana, or Maine it's worse than the southern states... the higher latitude means fewer sun hours, plus there are a lot more overcast days, lots of rain, plus snow.... You may need to include a low voltage disconnect sensor that protects the battery bank by either shedding the load (shutting off portions of, or even the entire system) or fires up an automatic "helper" generator - be it micro-hydro, wind, gasoline, propane or diesel... The most common ones I've seen are large propane or diesel standby generators; however there are several little single cylinder auto-start and auto-shutdown generators made for the marine industry (especially sailboats). They resemble an oversize horizontal driveshaft lawn mower engine mated to an oversize automotive alternator. These come in 1800 rpm and 3600 rpm, the 1800 rpm ones are generally more durable. They come in gasoline, propane, and diesel. Note that diesel is a ignition by compression, and you can have a propane diesel or a petroleum diesel. Petroleum diesel and gasoline have a limited storage life, which can be extended with additives where the propane life is dependent only on leakage. In general, petroleum diesel is preferred over gasoline as the fuel lasts longer in storage, has a lower evaporation rate, has more energy per gallon, and the engines are simpler and last a lot longer. However propane has less energy per gallon than gasoline or petroleum diesel. Another point that was made to me over lunch one time is that you can backpack several gallons of diesel or gasoline into a site and pour it into the site tank a lot easier than you can backpack an equivalent amount of propane and transfer it over. And you can pack in several multiple-gallon cans in one day. But if you do choose propane, there are both "gas" engines (i.e. spark-ignited) and diesel engines (i.e. compression-ignited) that run on propane. When you go searching the catalogs, look for an 1800 rpm industrial or marine unit instead of a 3600 rpm "consumer" version as it is built much more rugged and will last a lot longer. For example, there are a number of cheap single cylinder industrial engines that are just piston and cylinder; they do not have piston rings and therefore are not rebuildable. I suggest that visit your local library - the various farm magazines are a good source of advertisements and information articles on small diesel generators, and the various Yachting / boating and RV magazines have advertisements on both gas and diesel remote start (i.e. electric start) and remote shutdown generators. Some of the magazines have annual buyers guide issues that you might find in your local library magazine collections (or as a downloadable PDF, or as a purchasable back issue).

If you are engineering a custom repeater system then you should create a system power budget spreadsheet. Don't just read the device specification sheets, measure how much current each item draws, both idle and active. I've seen spec sheets that claimed a device pulled 1 amp and when measured it drew 0.45 amp idle and 1.4 amps active. Once you have that then project the idle and active run time of each and every item and calculate the AH that each item requires on both a best-case and a worst-case basis. Then total up the amp-hours you will need to run the system for a full day, and then determine the solar hours for your location, and remember that peak power is only for a few hours around solar noon each day, especially in the winter (plan on 3 to 5 hours per day of charge). Then figure the AH you will need - and then decide what size and chamistry battery plant will deliver that number of amp-hours. Then take into account the minimum SOC you will tolerate, voltage sag curve of the battery and the cutoff voltage of the repeater and the controller.

And don't forget that there are losses going into the battery and more losses coming out, in other words while you may have 100 AH going in, you might only have 85 AH deliverable to the load. This is controlled by the battery type, chemistry and ambient temperature.

You must consider whether the end voltage upon which the battery capacity is based is the same or higher than the voltage at which the repeater shuts down. If you overlook that and the repeater were transmitting continuously the current draw times the hours would suggest a longer backup capability than you actually get. And because repeaters are normally not continuously transmitting the batteries will last longer than the calculations.

When you get into the actual repeater and controller electronics you will want to look at what can be powered off, even temporarily. Even a commercial repeater can have some massaging done on it. For one, a crystal based radio uses a lot less power than the same synthesized radio, and since a repeater never has to change frequency (and the duplexer would need a full retuning if it did) so a synthesized receiver and transmitter isn't absolutely needed. Crystal radios can be a lot cleaner (spectrally) than synthesized, and generally don't need as much duplexer (or get better performance from the same duplexer).

One "battery saver" trick that was popular in handhelds in the 1970s and 1980s was to shut off the entire receiver for anywhere from 50% to 90% of the time (depending on the manufacturer and the model), then to power it up and look for a carrier. If none was seen it went back to sleep. The "on" timing was based on how long it took for the receiver to recognize that there was a signal on the channel after it was powered up. Once the receiver saw a valid signal then the receiver stayed on until the signal went away. Yes, in a worst-case scenario where someone keyed up and started talking at the same instant the receiver powered off you might miss the first half-word but that was the price you paid for the greatly extended battery life.

Then there are radio-specific modifications that can be done to save power - for example, a Motorola Micor station receiver can save a lot of idle current (over 1/4 amp, even at low volume) by disabling the receiver audio amplifier. Since repeat audio in a Micor receiver is taken off before the volume control, the audio amplifier is only used to run the local monitor speaker. The speaker amplifier can be switched on only when you are there and need it. The disabling can be done by cutting the trace to pin 16 of the Audio & Squelch board, then jumpering the cut with a switch (one trick is to swap the volume control for one that has a switch on it, and use that switch to enable the audio amplifier). When you aren't there, just rotate the volume control to the off position. This will work with any Micor, be it a converted mobile or a real station, on any band as pin 16 of the board is always Audio A+ (speaker amplifier) power.

Or if you want to have a full-time speaker and low drain, just remove the audio speaker amplifier and replace it with an LM386 chip - and even that can be turned off with a switch. Or an electronic switch - the Wilson 1402 hand-held switched the LM386 audio amplifier chip with a series PNP transistor that was driven by the squelch circuit.

Likewise, some of the receive audio circuits (like the CTCSS and DTMF decoders) in the controller can be power-switched by the channel busy (COR) signal. If you are running a repeater that will require full time CTCSS then the DTMF decoder doesn't need to be powered up until the CTCSS decoder is active.
The controllers transmit audio circuits, in fact the entire transmitter exciter could be switched by PTT so that there is zero power drain when the repeater transmitter is off (the stock Micor runs the lowest level transmitter circuits full time 24x7).

After you have modified the radio gear you can turn your attention to the repeater controller hardware and software design. For example, the entire DVR system can be switched on or off by the DVR access signal. When you sit down and examine the schematics closely it's amazing how many power consuming circuits can be switched off by judicious use of the proper control signals. I once worked with a homebrew multiport controller that had the CPU spend 99% of the time stopped. Twenty times per second a hardware clock pulse started the CPU, it swept through the code and then halted the CPU (which used almost no power when halted). The code was table driven, and each pass through the code resulted in different paths being taken, different subroutines being called, different things happening, all depending on if a COR was active, if a touchtone digit had been received, if the IDer was active (and the CPU switched the tone on and off for each dit or dah), which audio crosspoints were off, in monitor, or in transcieve, etc. But the key was that the controller (one version was four radio ports, another was six ports, the remote base option added a one more) used less than 60 ma when idle with everything that wasn't needed powered off, and less than 440 ma at worst case - when every transmitter was IDing and each receiver had a touchtone digit in the buffer). Sixty milliamps of idle current seems high today, but this was the late 1970 to late 80s when the circuitry was mostly TTL as CMOS didn't have the wide chip variety or availability. The point I am trying to make is that even today a power reduction of as high as 5 to 1 or even 8 to 1 is possible by careful controller design (or custom design).

Power consumption engineering in handheld radios used to be a lot higher priority... dry cells or nickel cadmium batteries were common and had less than 1/3 the amp-hours of lithium cells of the same size. I always liked the innovative design of the Motorola HT-200 handheld receiver (late 1960s)... If I remember correctly the entire radio drew less than 7 ma when squelched, and it didn't have a pulsed power save circuit - it actually was designed to have the battery run the receiver for over five days when squelched. The stock radio was a two channels max (but I've worked on one that was six channels). The audio stages and the PL decoder were powered off when squelched, each IF stage was designed for 3 volt operation and the receiver had four separate stages in DC series across the 12v battery, and the same 3 ma ran all four stages. I've not seen anything like it since. It was a really nice low DC power design that didn't sacrifice performance. Too bad the receiver and transmitter boards were a real pain to work on (each one was built in three or four "layers" of parts) and they were positive ground... One of the early USFS fire lookout base / repeater designs was based on the HT200 handheld / PT200 packset boards, with at least 3 RF amplifier power output options - they got around the positive ground situation by transformer coupling the audio in and out of it, and transformer coupling the RF.

You can't really say that your modifications you have cut the power drain in half until you actually measure the before and after power consumption. You can measure DC power with a voltmeter and an ammeter, or by a West Mountain Radio "Whatt Meter" but this product has been discontinued. It has been replaced by the "PWRcheck". You can measure AC mains power consumption with a meter called a "Kill-a-Watt", made by a company called "P3 International" and available on Amazon and at Home Depot. The model number of the one I used is a P4400.

Several mountaintop radio sites I visited in central and northern Oregon and southern Washington used thermoelectric generators (zero moving parts) just because the snow would have built up on solar panels if they had used them. The propane tanks at each radio site were sized to last the entire winter (I saw 1,500 gallons at one site, and there probably were more tanks that I didn't see). Even so, the local propane supplier had an all-wheel-drive tank truck to refuel his more remote remote / rural customers including the forestry radio and firetower sites (dual rear axles, 4 tires per rear axle plus the front axle - a 10 wheel drive propane truck is not a common sight, but obviously was needed in that area).

If all you are looking for is a battery-backup system for a regular AC Mains powered repeater you can use a quality Solar Charge Controller with a Low Voltage Disconnect circuit as part of it (or a separate Charge Controller and a Low Voltage Disconnect add-on). Get one that will handle the repeater system load current (plus some headroom for later expansion), and has three ports: Solar panels, Battery, and Load. Connect it as per the instructions except connect the system power supply to the Solar Panel connections, and the entire repeater hardware package (including the repeater controller) is on the load connection. The solar charge controller will do its main job of managing the battery charging and increasing the life of your battery bank AND provide the low voltage disconnect function.

Once the total repeater site load is optimized then the budget determines the solar panel array size and the battery bank size. The larger the panel array the more it costs, and there is a tradeoff between array size, battery capacity, square (or cubic) footage occupied in the building, and more. If the battery voltage drops too low too often then it is obvious that the amount of stored power is insufficient. The weather can be "normal" for years, and then Mother Nature can throw you a curve ball and deliver a solid month of cloudy weather. Installing an array that is sized for a one-in-ten years event is overkill and a waste of money (not to mention the snow load and wind load if the array is mounted on the tower). It will be more cost effective to plan to take a 5-gallon can of fuel and a small portable rope-start generator (like a borrowed Honda 2000 or 2200) to the site and cable it in for a few hours to do a battery recharge, then take it home when you are done (or leave it there if you can). Such a generator can even be home-brew, QST had an article called "The 12 Volt Pup: A DC Generator You Can Build" that was essentially a lawn mower engine, a 50 amp car alternator and some control circuitry. It was in the June 1997 QST.

If an auxiliary generator becomes a regular necessity you can chose a permanently mounted gasoline or diesel. You can hook it to the repeater controller and remote start it and kill it via DTMF. I saw just such a system at one of the Oregon sites - an electric-start Yamaha single cylinder diesel engine mated to a 2 KW generator. I was told that it was purchased out of a magazine geared towards small sailboats. It was mounted in the bottom of a home-made welded angle iron frame with a 55-gallon drum of diesel in the upper part of the frame. This configuration provided gravity feed - therefore no need for a fuel pump (one less thing to break and / or require maintenance). It also provided an easy method for refueling the generator - just swap the almost-empty 55-gallon drum for a full one. There are additives that can be mixed with gasoline or diesel to increase the storage life... but the longest life stored liquid fuel is propane.

In closing...

I really suggest that you do some serious research before you spend your first dollar. Visit the Home Power Magazine web site and the web sites for some of their advertisers. Spend a day at a large public library, look in the farm, RV and sailing / cruising / boating magazines / web sites. If you can find a firetower or lookout repeater manual you can look at the overall design and see how the total battery drain is minimized (i.e. the power control circuitry).
Google for solar powered repeaters... the earliest one I found is the VA3PLA repeater here which has been in continuous operation since September of 1994.   (Off-site pointer, opens in a new browser tab)
There is a lot of information available for free... like this: Solar Electric Power for Instruments at Remote Sites   Note that this was written in the late 1990s and first posted in 2000. Some of the information is outdated.

There are three Solar Charge Controller articles here on repeater-builder:
1) The first is A Solar Charge Controller for Medium power Applications by G. Forrest Cook, WBØRIO and was published in February / March 1998 issue of Home Power magazine
2) The second is The Micro M+ Charge Controller from Michael Bryce WB8VGE and was published in the October 2001 issue of QST.
3) And lastly The FET Charge Controller also by Mr. Bryce in the January 1992 issue of QST.

Mr. Cook also wrote an LVD article for the August / September 1997 issue of Home Power: A Homebrew Low Voltage Battery Disconnect / Automatic Battery Shutoff For Medium Power DC Loads by G. Forrest Cook, WBØRIO

Update 2021:
Where I recommended libraries back in 2006 you will find that almost everything is available through a search engine. Many magazine articles that were pre-internet may require a library visit.

An acquaintance has a motor home with 400 watts of solar panels mounted flat on the roof. He made these comments in an email:
>
> My hands-on experience suggests that the practical reality of RV solar generation is 60%-70% of specs, maybe less.
> It can be worse in northern latitudes.
>
> Panel specs @ 56° F: Vpm 20 volts, Ipm 10 amp = 200 watts. (Vpm is voltage at the back if the panel)
> I'm using a Victron MPPT Controller and a matching Victron shunt-based battery monitor. It displays a 0-to-100% battery "gas gauge".
> My location: 35° latitude in central California.
> Outside temperature: at the time of these tests: 77° F.
> Peak measured power: 266.56 watts (19.6 amps @ 13.6 volts DC).
> Total amp-hours generated in one solar day: 151 AH.
>
> Observations:
> First, you should know solar panels generate less power as they get hotter.
> The spec sheet for my panels said the test temperature was 56° F.
> It was 77° F outside at ground level and the panel itself was hotter due to the heating from the sun.
> My panels are mounted flat and a couple of inches above my RV roof.
> The angle of the sun as little as 15° off of perpendicular to the panel causes significant degradation in power generated.
> (If I were to do the install over I'd seriously consider tiltable mounts - locked flat while in motion, park it, then manually tilt the panels to face the sun)
> My 400 watts of panels only generates 266-270 watts at best.
> That is 66.5% of the rated power for the panels.
> The summer solstice, the longest day of the year is June 20 which also means that solar noon is when the sun will be at its highest in the sky.
> Theoretically the panel will generate max power for 20 minutes around solar noon but might not if cloudy and if the panels are hot from solar heating.
> Expect less solar flux (less power) in locations further north latitude, and expect more solar flux (more power, up to the panel(s) maximum) further south.

Contact Information:

The author, Mike Morris WA6ILQ, can be contacted here.

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This page originally posted on 10-Dec-2006 and updated occasionally.

Text, artistic layout and hand-coded HTML © Copyright 2006 and date of last update by Mike Morris WA6ILQ.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.