making a recharge circuit for 4x aa batteries

[MattP]

Hello, I'm making a little portable speaker system, and I was wondering if it would be possible to build a recharge circuit into it so that I won't have to take the batteries out to recharge them once they're flat.

The amp runs off 3.7v to 5v (any higher than 5.5v will blow the chip), so I'm using four AA Ni-MH batteries (1.2v), effectively giving 4.8v (I still need to check that they don't hit 5.5v at full charge).

I could just have the batteries accessible via a panel, but it would be really cool to build a recharge circuit into the unit, so whenever I want to top up its batteries I could just plug in a cable to do so. It would need to have an auto-stop function, so that the batteries won't get overcharged.

Is there any simple circuit for doing this? I've searched for a while online but found nothing that I can really understand.

I've also found this on ebay, but I'm not sure if it will work. I asked the seller for more info, but they could give none. Maybe it's just what I need?

http://www.ebay.co.uk/itm/330743141341?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

 

[Harald Kapp]

The one you link is not suitable. It is for Li-ion, a very different chemistry.

You can build your own charger using e.g. one of these chips, or a similar one from other vendors. You may be able to find a kit.
Or buy an off-the shelf charger, dismantle it and put the circuit into your speaker box.

Google "Ni-MH battery charger"

 

[(*steve*)]

Li-Ion may actually be a good battery technology to use. Charging is a lot easier, and if you can use a single cell you don't have to worry about cells getting unbalanced.

 

[MattP]

I'm happy using Li-Ion if it's going to be easier. I just measured the voltage of my fully charged Ni-MH batteries and it's over 5.5v, so I would need to use three of them if I were to use them at all. Would four Li-Ion batteries provide a more consistent voltage?

Regarding the batteries I have currently, after draining them a little, I attached them to my little amp and have been listening to music since about 11am today. The voltage has dropped from 5.25v to 5.20v, so it looks like the thing is going to last for absolutely ages on a single charge. So fast recharge times aren't required at all in light of this.

Making a trickle charger circuit sounds, to me, like it would be the simplest option. As far as I understand things that is, which isn't very far at all.

From that PDF Harald linked to, it seems to say that to make a trickle charger requires simply to have a current limiter between the batteries and a DC current. It seems almost too easy? I've got to be missing something.

 

[BobK]

Here is good info on how to charge NIMH batteries. They are among the most difficult to charge correctly.

http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

Bob