CD V-700 Geiger Counter
On six different occasions, while doing operational checks on a recently acquired Victoreen CD V-700 Model 6B Geiger Counter, the two batteries which supply the audio pulse and metering circuit became so hot that I couldn't hold them.
For the life of me, I couldn't identify a circuit problem that could be causing the batteries to heat up. Then it happened that only one of the two batteries (they're in series) got hot. That meant that either the battery shorted internally or something physically close to it did the deed.
It turned out that there were two contributing factors to the hot battery problem. (1) The battery retainers in this unit were deformed - see diagram below - (probably to make tighter battery contacts) such that, where their sharp edges came in contact with the batteries at their negative ends, it was cutting into the plastic insulation that surrounds much of the the batteries. (2) That insulating coat is paper-thin and the positive and negative parts of the battery exteriors are very close to each other at the edge of the negative terminal.
When the retainer cut completely through the insulation, (usually after some kind of jostle to the meter) its angle with respect to the battery was such that it shorted the positive metal cylindrical housing to the negative end-piece. Sizzle, sizzle!
The scanned image below shows one of the batteries after its hot episode. The red arrow shows where the insulation had been cut through. (The peeling back of the insulation was caused by the heating.)
One time hot battery from CD V-700
The red arrow in the following image shows the part of the deformed battery retainer clip which cut through the battery insulation.
(a) Properly shaped battery retainer.