Back to Icom index page Icom Charger Information
Written by Mike Morris WA6ILQ
 


The charger has an interesting design - there are three microswitches that are opened or closed by a 3-way programming insert mounted in the bottom of the battery. They control 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 is old enough that it has one or more shorted cells. The design of the charger has a circuit that measures charging current using four resistors in series-parallel, which together comprise a 1 ohm 1 watt resistor. This 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 R39, R40, R41 and R42 (all of which are 1/4 watt, 1 ohm resistors) develop 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 fine.

HOWEVER... All of the batteries except the penlight pack and the BP-7 (and its commercial equivalent the CM-7) are 9.6 volts, the BP-7 is 12v and has a relay inside that compensates for the BC-35 being a designed to charge 9.6 volt batteries. The BP-7 internally is made up of two separate 6 volt strings, which are placed in in series for use (normally closed 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 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 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 at: his-callsign // at // repeater-builder // dot // com.



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This page originally posted on 14-Oct-2004

Text, artistic layout and hand-coded HTML © Copyright 2004 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.