I have a rechargeable battery pack that is fully charged, the batteries aren't heating up and the charge controller won't shut off. Tick, Tick, Boom?

Question

UPDATED AGAIN Sorry folks. I kind of jumped the gun with this question. I was patient and let the charger do its thing. The voltage increase flattened out as the rate of temp change increased. The temp never made 50°C, it topped off at just under 45°C then shut down. Seems like I have a functioning charger setup after all.

UPDATED Deleted unnecessary question.

Three AA size LSD NiMH batteries in series are connected to Maxim Integrated's DS2715 Charge Controller Evaluation Kit. NiMH batteries are all 1.2V nominal, and as I understand 1.0V minimum 1.4V maximum. With a maximum of 1.4V the three cells should be fully charged at 4.2V.

The charge controller uses cell temperature (from a thermistor) to determine charge status. At 50°C the controller should stop charging. My problem is that the battery pack is fully charged at over 4.2V, yet the temperature of the cells is only 32°C. How far can I let this go? At the rate of change in temp I'm seeing, the rig will blow up in my face before the cells reach 50°C.

Answer 1

I usually find when I am having problems with something like this it is helpful to go back through the datasheet/manual/quickstart and look for assumptions I have made that aren't necessarily true. I often have looked at a half-dozen datasheets for similar products by the time I am actually testing something so it can be easy to skim past some little detail later causes me problems.

How long have you been charging it? The datasheet mentions it will enter a time-out of 70 or 130 minutes depending on charge rate. Could you be in this time-out? Check out this state-machine diagram:

The IC stops charging based on dT/dt, rather than T. When fully charged, the energy is mostly going into heat rather than being stored via chemical reaction in the cell. It also has an absolute cut out temperature, current and voltage so I am not too concerned about explosion (I also read that these cells typically are made to vent before exploding, like the scored backs of electrolytic caps). Another thing you might check, the manual says to make sure your thermistor has good thermal conductivity to the cell pack. You can verify this manually with an oscilloscope or meter by probing the drop across the thermistor when the battery is hot vs when it is cool.

I think you may also need to provide a little more information:

What is the original condition of the cells you are charging? What charge rate are you using? Are you able to measure the actual current/power draw at the cells or the input to the dev board? You say they are at 1.4v, is that measured while being charged?

Are the leds showing anything?