How Can I Charge Series NiMH Batteries In-Circuit?

[Jdo300]

Hello, I am designing a circuit that will be running off of two AA rechargeable (1.2V 2500mAH NiMH) batteries that are stacked in series, and I am looking into a way to charge the batteries in-circuit rather than having to take them out and charge them externally. I did a very brief search online and see that there are all kinds of different battery charge management ICs out there but I want to know if there are any ICs out there than can handle charging two cells in series that may not necessarily be at the same state of charge.

I saw many chips that handle single cells but when it comes to series cells, I've been hearing that it is not a good idea to charge them unless you are certain that they are both discharged to the same extent, which may not always be the case in my application (the user may put in batteries that are not both at the same charge state). To mitigate this, I was thinking that I could design a circuit that would switch the two series batteries in parallel when charging but this has complexities of it's own to deal with so I'm wondering if anyone knows of any simple solutions to this problem? I know I can't be the first person with this problem so what do you all think?

Thanks,
Jason O

 

[vk6kro]

We had a similar thread a few days ago:

https://www.physicsforums.com/showthread.php?t=361219

Some of that might be interesting to you.

I put a mechanical battery clock across the battery in that discharger and it gives a readout of battery discharge time by when it stops working.

I have some NiMH batteries in a digital camera. They were bought new together and always used in the same pairs and charged the same way, in series.

I made a charger using a micro (a Picaxe) that checks for 20 identical voltage readings in a 20 minute period as the criterion for the batteries being fully charged. This seems to be a valid test as the batteries seem to last very well in the camera. And I haven't blown any up yet.

It avoids the situation of testing for a rise in temperature which some chargers use. I feel that the damage has already been done at that stage.

And it is heaps better than chargers that don't cut off at all.

 

[Mike_In_Plano]

Hello Jason,

Mike here. Most of the new charger chips are targeted at single cell Lithium batteries. Putting out three volts, they function well as a single cell.

For NiCad an NiMH, you don't get much bang for the buck, voltage-wise, so they're usually stacked in series. That's not much of a problem though. They can take a little over charging - especially the NiMH.

Dallas semi used to make some app specific chips for NiMH. Essentially, the key, always, is to know when to stop charging. You can:
1. Keep track of how many ma-Hrs you've drawn and charge back 120% of that.
2. Charge quickly and terminate when the batteries get warm (hydrogen recombines = heat)
3. Charge quickly, monitor the battery voltage, and terminate when the voltage decreases by so many mv of the peak (I think it was 10-20mv).

The simplist of these is to simply terminate on temperature. It doesn't get the longest life out of the batteries, but it's much better than trickle charging. I used a pair of NTC thermisters one for ambient and one ofr battery. That way you get a good termination temperature regardless of your outside ambient.

If you use a micro, it's probably good to take a couple more things into account, such as the run time since last charge, and the current charging time. That way, you can avoid charging when it's not needed and terminate on time if something crazy is going on.

Best of Luck,

- Mike

 

[famousken]

You could use two dedicated single cell controller Ics run from dedicated power supplies, this is how Li-Po batteries are charged. Having only two cells means you would only need three connections to the batteries, so its not too complex.

 

[Jdo300]

Hi Everyone,

Thank you all for the great suggestions. After a more thorough search for some charge controller ICs, I finally decided to go with the DS2712. It is designed for monitoring 1 or two NiMH cells in series and also offers a built-in PWM current limit feature which is convenient to keep unnecessary power and heat dissipation to a minimum. Now my next problem is power management. I'm planning on enabling this circuit I'm designing to be charged from a USB port so I know the current must be limited to 500mA. I'm working on designing a circuit that would allow the full 500mA to be used by the charger when the rest of the circuitry is turned off, but to limit the current proportionally when the circuit is running so that I can still charge the battery but not go over the max current.

God Bless,
Jason O