I picked up a new U1252B DMM last week. It was manufactured in 2015. After unwrapping the included NiMH 8.4V 250mAh battery, I checked how much power was in it using the DMM's bar graph battery level checker and it was around 60%. So, I plugged in the included charger and let it do its thing.
The charging system in the DMM has a maximum charge time limit of 220 minutes. The battery should be full before that time and the meter should indicate when charging has been completed.
However, in my case, the DMM never indicated it was done. Instead, as the countdown timer got down to about half an hour remaining, I noticed that the case of the meter around the battery compartment was quite warm to the touch. Taking the battery cover off and touching the battery itself, it was hot. Not good.
In order to see if this might have just been an anomaly with a battery that has been in storage for a year, I let the meter run the battery down until its battery icon started flashing. The battery level showed 0% on the bar graph. Checking the voltage on the battery terminals, it read about 6.5V. That seems rather low. With 7 cells inside an 8.4V battery, 1V per cell would be 7V for "empty".
Anyway, I plugged the charger back in, with my bench DMM reading the current to the battery. When the charger engaged, the battery terminals started off at about 8.5V. Charging current was about 156mA.
Over the course of a couple of hours, the current slowly dropped to about 148mA and the battery came up to 11V. That's when the battery started to heat up again. I pulled the plug on the charger before it overheated the battery again.
The documentation isn't very clear, but it seems that the bar graph is supposed to indicate the state of charge as it's charging. In my case, the bar was always at 76% from empty to the point the battery started to heat up. After removing the charger and letting the battery settle for half an hour, checking the battery level showed 100% on the bar graph.
So, what's going on?
Bad battery? The DMM's pre-charge test passes. The battery holds a good charge, requiring many hours to fully drain it by running the DMM. However, I didn't measure its actual capacity.
Bad charger in the DMM? Could it be failing to correctly sense the state of charge and overcharging the battery as a result?
The meter is under warranty, of course, but I like to have a reasonably good sense of what a problem is before sending anything (electronics, car, myself) in for service.
Searching the interwebs didn't turn up any info about known charging issues like this with the U1251B or U1252B. The U1253B had a charging problem, but it was undercharging and a firmware update was released to correct the battery charging time. I could not find a service note for the U1252B about charging the battery.
Has anyone encountered this behavior with their U125xA or U125xB, or even some other battery charger to shed some light on what's going on?