# NiMh switch-mode charger for 6-cell NiMh

I've built a charger for my batter (NiMh 6-cell 7.2v) using MAX712 IC.

The problem I'm facing is that it's in a linear mode which requires either a huge heatsink or a heatsink with a fan (9W). See my previous question - MAX712 charging circuit - diode gets too hot

The charger has to be a part of a robot, so size does matter.

I was looking at using MAX712 in switch-mode, which is possible, but requires substantial work:
https://www.maximintegrated.com/en/app-notes/index.mvp/id/4496

Someone suggested to use DS2715 instead which is switch-mode natively.
This charger uses temperature change (Dt/dt) for detecting charge termination instead of voltage drop that MAX712 uses. My battery doesn't have a thermistor built-in.

So my questions:
1. Can I just glue thermistor on top of the battery pack so it would allow me to use DS2715?
2. What is a better method - voltage drop detection or temperature change (Dt/dt)?
3. Any other battery charging ICs I should consider?

Cheers, Leonti

• What's your DC input voltage? How many mA? Sounds like your DC in is too much higher than the charge voltage. May 15, 2017 at 3:41
• My input is 15v, 1A current charge. Input has to be at least (1.9v * 6) + 2v = 13.4v according to datasheet, so 15v is pretty close. Even if I manage to get a power source down to 13.4 I will still have 7.4W to dissipate which is still a lot. May 15, 2017 at 4:31
• If you look at most commercial laptop battery packs, they do indeed just use some form of temperature sensing device glued to the battery. I would still like to point out that you shuold be cautious doing this tho, and advise you to verify the performance with a second thermometer during testing. I do believe that temperature (or if my memory serves me right, the change $\frac{dT}{dt}$) is a better way of checking charging, but I'm not confident enough to put it in a answer. May 15, 2017 at 6:30
• @JorenVaes yes, I meant temperature change, not the temperature itself. Thanks for pointing it out. May 15, 2017 at 7:03

Any other battery charging ICs I should consider?

This one is worth a look. I'm not saying it is better, it is a switcher and appears simple.

The TI BQ24115 appears to be capable and not too complex with minimal external components given the feature set.

For NiMH you would need the "System-Controlled" (i.e. host controller) version. It cannot do NiMH as a stand alone charger.

• Integrated Power FET