NiMH battery pack charger

Question

Hello,

About 10 years ago, all the major semiconductor manufacturers offered IC for battery management. Now looking at Texas Instruments, Maxim, Microchip and can't find a single device for charging a pack of 6 NiMH cells (industrial AAA cells) - 7.2 V, 600 - 750 mAh. The packs have an integrated NTC for temperature monitoring.

I have two use cases:

1) just a charger that will be able to charge them within ca. 2 hours (0.5 - 1 C with correct charge detection).

2) charger that will be able to charge the batteries and power a target device (ca. 50 - 100 mA power consumption) - that would be OK with a slow 0.1 C (a even a bit less). The slow charging alone could be easily done via LM317 etc. in constant current mode, but I have no idea how to handle the power consumption of the target device.

Any ideas? I would prefer either some relatively simple solution like LM317 or some specialized charger IC, but preferably with some ready-made evaluation board available.

Thank you,

David

shabaz · Accepted Answer

Hi David,

I think it could be worth making one from scratch in that situation : ( As you say, the charger IC options are limited. Since it has integrated NTC, that's a nice way to (fairly) quickly detect that the charging is complete, and have some hardware watchdog, and some overall timeout too. So, having a charger that stops at temp threshold and also delta T, and also has a worst-case timeout to stop charging it too. A nice project for a small microcontroller : ) The scenario 2 is harder. For the Smart Doorbell project chime module ( Smart Doorbell System – Part 6 – Chime Module Complete Circuit Design ), I decided to trickle charge instead (but that's a much lower current than 0.1C) and switch it off if the device hasn't been used in a long time. An alternative for scenario 2 could be to charge the device using the same algorithm (i.e. T, delta T and time), but power the circuit separately when the charger is operating, i.e. design for the battery to be automatically disconnected from the load. When the charger is unplugged, the battery is automatically connected again. That's actually really easy to do if your target circuit can also function at a slightly higher voltage than the battery will ever get charged at, because then you can use diodes to perform an OR function. Or use an ideal diode IC (AD has this, and maybe TI too). The AD chip is specifically designed for this scenario I think, i.e. switch off battery when the charger is plugged in (but in your case, you can do that but also charge the battery at the same time).

Gough Lui · Answer

You could try the MAX713 as well - https://www.maximintegrated.com/en/products/power/battery-management/MAX713.html - Gough

koudelad · Answer

And thanks shabaz for mentioning the ideal diode circuit, I haven't heard of it yet.

luislabmo · Answer

Have you explored the Maxim DS2715BZ+ Maxim DS2715BZ+ ?. It can handle up to 10 NiMH batteries and has FAST-CHARGE up to 2C, monitors voltage, temperature and time. In the datasheet there are couple of examples with a load that my be worth exploring. Luis

NiMH battery pack charger

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