Icom Charger Information

Back to Icom index page Fixing a design flaw in the Icom
BC-35 Handheld Charger
Written by Mike Morris WA6ILQ

The Icom BC-35 desktop charger is used with the IC-02, IC-03, IC-04, H16 and U16 series of handhelds. This unit has an interesting design - there are three microswitches in the base of the battery pocket. These switches are opened or closed by a 3-way programming insert mounted in the bottom of the BP-2, BP-3, BP-4, BP-5, BP-7 and BP-8 (and the CM-equivalent) battery cases. The switches control both the charging current and the trickle charge current.

This unit has an almost-fatal design flaw that only shows up when used with the 12 volt BP-7 or CM-7 battery pack that has one or more shorted cells. The design of the charger has a circuit that measures charging current using four 1 ohm ¹⁄₄ watt resistors (R39, R40, R41 and R42) which are wired in series-parallel and which together comprise a 1 ohm 1 watt resistor. This composite resistor is in the path of the charging current and functions as a current sense resistor and as a current limiter. When a discharged battery is inserted current starts to flow and the composite resistor develops a voltage drop that is proportional to the current. They are sized for the current level needed to charge a 9.6 volt battery, and they work just fine.

HOWEVER... All of the batteries except the penlight pack and the BP-7 are 9.6 volts. But the BP-7 is 12v and has a relay inside that compensates for the BC-35 being designed to charge 9.6 volt batteries. The BP-7 battery internals are made up of two separate 6 volt strings, which are placed in in series for use (normally closed relay contacts) and in parallel for charging (i.e. when there is voltage across the charging contacts). If your (normally 12v) BP-7 has one or more shorted cells in it the radio still works since it was designed to operate at anything from 9.6v (anything but a BP-7) to 12v (a BP-7). When the BC-35 is used with a BP-7 that has one or more shorted cells (i.e. less than 6 volts) the charger doesn't know any better and pumps more than enough current to cause the four resistors to generate more heat than they can handle. And since the four resistors are very marginal in wattage to start with and are soldered flush against the circuit board they toast the glass-epoxy board substrate.

A while back my friend WB6GSO came to me with a dead charger... Everything seemed to work except that the battery pack wouldn't charge. A bit of disassembly and some looking around revealed those four resistors were badly burned and the board under them was seriously charred. My fix was simple: I took advantage of the board layout that had two of the four resistors at the edge of the circuit board and jumpered out the other two. I then soldered a one ohm, five watt ceramic coated wirewound in place of the ones that were at the edge of the board; the new resistor hung over the edge on its leads in the open airflow. A two watt carbon resistor would have worked, but I used what was in the junk box, and there was plenty of room for the small wirewound.

If your BC-35 is in daily service, make the fix soon, like this weekend. And replace the defective inserts in your BP-7s with new ones. Since WB6GSO lived and worked in the prime coverage area of the repeater he had no indication that his handheld radio was effectively in the BP-8 (low power) mode when the BP-7 with a shorted cell (or two) was in use.

Contact Information:

The author can be contacted here.

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This page originally posted on 14-Oct-2004

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.