Just my opinion / experience : Optima Batteries as back up power source.
I had purchased 5 new yellow top Optima batteries (deep cycle) for my home ham shack backup power system (thinking I was doing the smart thing). They were quite expensive. I've always believed 'you get what you pay for'. Honestly, I wish I would have never purchased them. They Suck! I previously had a single AC DELCO deep cycle family 27 sized battery prior, and it worked better under load then all 5 of these Optimas tied together. The Optimas I had purchased were all fresh (with a date code within the last month or two at the time). I ran these batteries on the same load as I had on the AC DELCO deep cycle battery, but the voltage on these yellow tops sinks faster than I can sneeze under any real load (yes, I'm exaggerating). Maybe the other ham is correct in stating that the Red Top starting battery is a better choice for this application.
Sometime later, while at a customers, I saw that they were using yellow topped Optimas inside of their black and white patrol fleet. Curious, I asked one of the mechanics from their maintenance garage how they liked them. He shook his head and stated that he didn't like them at all, but they are sealed, small, so that's just what we have to use when place batteries inside the vehicles. I then shared with him my experience, and he just kept nodding his head in agreement.
I use to operate my HF station at 100W on the AC DELCO, and now with these five Optima yellow tops, I had to turn my HF stations output power down to 40 watts in order to sustain a QSO.
I currently use and am very happy with four 6V 210AH AGM batteries. I just had to shoot this e-mail out before someone else made the same mistake I did, and a very expensive one at that. I would only use the yellow tops in an application where your running a base VHF/UHF radio at 5-10W.
I have also since installed two 6V 210AH AGM batteries on an open UHF repeater.
And if your using old hand me down batteries on your repeaters / home stations, you get what you pay for - those batteries were retired for a reason. A friend of mine had me attach two (100lb) each deep cycle commercial batteries on his repeater up at one hill top site. Well after attaching them (with nice heavy gauge wire, all connections soldered), I pulled the AC plug to see how they'd handle the repeater. The batteries dropped down to 10 volts and the repeater started convulsing just 45 seconds into a solid key down of the transmitter. Also to mention is that these batteries had just been fully charged.
In the past, I have also been handed 'hand me down batteries' (of different makes and models pulled from various applications). Some would only last two months before losing a cell and becoming useless. A friend of mine made a comment once about Optima batteries (which I have not verified) is that when an Optima is fully depleted, and re-charged, it will never fully recover. One must never let Optima batteries get below a specific voltage.
So all in all, if you really want your equipment to actually work when all else fails . . . .
Don't chance it, buy new, and choose wisely.
Sure it will be expensive, but you are buying peace of mind. How much is that worth to you?
Please do not tie a back-up battery directly across the main output terminals of any DC power supply (which provides no current limiting to the battery while re-charging). I have found this actually done at a hill top site. What had happened was that AC power to a repeater had been off for three days or so. The repeater was running exclusively off the battery for 3 days and finally depleted it. Well, when AC power was restored, the battery started pulling an enormous amount of amperes from the power supply (since it was now depleted it looked almost like a short). I was there at the time the AC was restored, and within a matter of a minute of the power being restored, I smelled something burning. After sniffing out it's point of origin in the room, I found that the power supply tied to the battery was too hot to touch, and that the wires connecting it to the battery were just about melting the insulation off. What a fire hazard! I shudder to think of what would have happened if I hadn't been there! I disconnected that battery right then and there and notified the owner of the repeater. So please don't burn down our repeater sites. It's hard enough trying to get into commercial sites as it is. Many commercial site owners will never again accept an Amateur Radio repeater station due to their (or others) previous bad experiences with hams. Please don't add to that.
A so called marine deep cycle/starting battery is nothing more than a little heavier starting battery.
A true deep cycle battery has much larger plates than a starting battery.
A starting battery has thinner plates (and more of them). This construction provides more surface area - and that allows the battery to release energy faster than batteries with thick plates. The thin plate batteries can release very high currents in a short time but there is the danger that the excessive current will warp the plates. Warped plates can touch, short and athat kills the cell.
A true deep cycle battery with heavy plates can not supply as much current as a starting battery but it can be discharged to a lower point without harm.
A regular starting battery should not be discharged below around 80% and should be immediately recharged after the draw down.
Deep cycle batteries have a finite number of charge / discharge cycles. Every time you discharge your battery you use some of that cycle life. As an example, if you discharge them to a 25% state of charge (SOC), every time, you can expect about 1500+ cycles (varies with manufacturer, chemistry, temperature). Drain it down to 50% before recharge and expect about 1000 cycles. Discharge them down to 20% before recharge and you can expect about 500 cycles.
For the sake of simplicity, let’s say we have a 100ah battery. We can extract 80ah 500 times by taking it to 20% SOC, for a lifetime total of 40,000ah. Or, we can extract 50ah 1000 times by taking it to 50% SOC, for a lifetime total of 50,000ah. That’s a 25% increase using the typical manufacturers 50% DOD recommendation.
Every deep discharge (beyond 50%) will shorten the life a battery. For best life a deep cycle battery should be immediately recharged after use. Leaving it in the discharged state for a long period of time is sure death of the battery.
Proper charging of batteries needs to be controlled. Too much charging current will warp the plates just like too much discharge current and ruin the battery.
Batteries need to be overcharged a little so that there is some gassing or they will never reach full charge. However gassing uses water, so you need to "top off" the battery every so often. Use distilled water. You have no idea what the local impurities are, or what the reaction with the batttery electrolyte will be.
Batteries also need to be floated at the proper voltage to maintain their life. 13.6 volts is about right for most deep cycle batteries and some regular automotive types (check the data sheet). It depends on the material used in the plates. In most cases using a 13.8 volt float voltage will kill these batteries over time. Plus or minua a tenth of a volt is critical with the float voltage. AGM batteries require a different float voltage. Look at the manufacturers literature to see what it should be, then set the charger to it.
A battery that is designed to be floated at 13.6 volts will never reach full charge at 13.6 volts after a discharge. It will take at least 14 volts to charge it properly (with appropriate current limiting). A number of data sheets say to charge with 14.4 to 14.5 volts and that 12.7 volts is a fully charged battery. Batteries have a negative temperature coefficient so as they get colder the fully charged voltage goes up, warmer the voltage goes down. In order for the idle voltage indication to have useful accuracy for state of charge the battery must be completely "off" (zero charge or discharge - disconnect everything from one post) for at least 3 hours.
There are a lot of old 13.8 volt chargers out there that have been around for years. These were used a lot on boats. They were big heavy beasts that could supply lots of current when needed. Most were the fero-resonant regulated type. 13.8 was a compromise between charge and float voltage. They are responsible for more battery sales than any other cause. Using 13.8 volts will not fully charge the battery but at the same time it will allow it to gas and boil off the water over time which exposes the plates and kills the battery. If the batteries were floated at 13.6 they would last a long time but will not fully charge at this voltage unless they were first charged at a higher voltage.
Just because you have a 140 amp-hour battery doesn't mean that you can draw that much power from it in an hour. That specification is usually a 10 or 20 hour rating (the manufacturers literature will tell you if it is 10 or 20). A 10 hour rating means that you could draw 14 amps from it for 10 hours. Then it would be 100% dead. Since you don't want to go below 50% that means that it would only last around 5 hours. Yes, your 140 AH battery only has a usable 70 AH.
Note that a 10 hour verses a 20 hour curve is different. You can't just half or double one to get the other. They are not linear. It all depends on the internal construction and the chemistry.
The 10.5 volt rate for a dead battery is with no load. A starting battery will drop down to 7 or 8 volts sometimes under starter load. It all depends upon the internal resistance of the battery. As they get old the internal resistance goes up as the plates become sulfated and they are not able to supply as much current without a large voltage drop.
Think of it this way: If you need large amounts of current for a short time then a regular starting battery is best. If you need moderate amounts of current for a long time then a deep cycle battery is what to use.
Find out what the recommended discharge rate is for the batteries that you are going to use for backup and match that against the current draw of your equipment. You can't just arbitrarily throw batteries in (especially mismatched batteries) and expect good results. And the mismatch usually includes some old and some new of the same battery.
AGM batteries are not all they are cracked up to be. They are really just a regular flooded cell type battery but with a gel rather than liquid electrolyte and no vent (except an emergency pressure release, and it if pops the battery will dry out rapidly). They have matting to hold the gel to the plates. Some may have a different plate material makeup. The big advantage is the no‑vent requirement, and a tip-proof design. These two features are what allows electric wheel chairs powered by them in the passenger cabin of an airliner.
The best bang for the buck in deep cycle batteries are wet-cell golf cart batteries, commonly called "GC2" batteries (they are also available in AGM, but at a higher cost). They usually come as 6 volt 200-225 amp-hour (at a 20 hour rate) batteries and can be series-paralleled for higher voltage and higher amp-hour capacity. They are available at most Costco and Sams Club stores, are very tough, can withstand lots of vibration and abuse. They provide very good life if properly managed and maintained. Trojan is one of the main manufacturers and their literature says that for best life to never discharge below 12 volts or below 50% state of charge.
You do need to vent liquid filled batteries to outside air. On boats when the batteries are mounted in the engine compartment they have natural venting from engine air vents. You don't want them in a sealed box. Placing these type batteries in an equipment cabinet may be fine for a long time as long as they are just being floated. But when the equipment goes down and a heavy draw is placed on the batteries they will gas. Also during charging they will gas, sometimes heavily (and the gas has acid fumes that do a number on the equipment in the cabinet). If there is not much air movement for the gas to escape the cabinet an arc from a relay can set off the trapped gases. This is the same reason that you always want to turn off a battery charger before disconnecting the charger leads from the battery. I have seen a couple of batteries explode when someone disconnected the charger from the battery first. Hydrogen and oxygen makes a nice bomb!
The Recreational Vehicle community has a number of useful bits of 12 volt technology that may be of interest to the amateur radio operator. Motor home RVs have a "chassis" battery for the vehicle systems and a separate "house" battery for the living space. One interesting piece of battery technology is called a BMS (Battery Monitoring System) - a shunt-based battery monitor that displays house battery "gas gauge" - the instantateous current state of charge.
One representative unit is the Victron BMV-700 (the link opens in a new window). From the data sheet: "The BMV-700 is a high precision battery monitor. The essential function of a battery monitor is to calculate ampere hours consumed and the state of charge of a battery. Ampere hours consumed are calculated by integrating the current flowing in or out of the battery."
These devices are made up of a millivolt shunt (typically 50mv at 100, 500, 1000, 2000 or 6000 amps) that is placed between the negative terminal of the battery and chassis ground. Current going into (charging) the battery results in a positive millivolt value, current flowing out (discharging) results in a negative value.
Starting with a fully charged battery, once the unit is calibrated the unit reads the millivolt value over time and can read amp-hours of charge and amp-hours of discharge and can calculate and display a real-time percentage of charge. The better ones have a temperature sensor that can be attached to the battery since the available energy is affected by battery temperature.
Display options (depending on the manufacturer and model) include some combination of a local display, bluetooth (to a local cellphone) or a USB cable connected to a local laptop.