NiMH internal charger

Hello. Over the years i've been building little amplifiers for friends. I generally use 8x AA NiMH batteries to power them. It's important to me that the user not have to remove the batteries in order to charge them. It's also important to me that they be able to use the amp while the batteries charge. In the past i've always built my own battery charging circuit using the Maxim MAX712 or MAX713 (i can't remember which atm). They work great, and deliver exactly the functionality i want. However, i do it all on perfboard and i really hate the process of building the chargers. People are always asking me to make them an amp, and i find myself turning them down simply because i don't feel like making another charging circuit.

I've experimented with using a couple different external chargers. I've used the Tenergy Smart NiMH charger, and also the type where the charging circuit is built right into the wall-pack. Both of these work just fine for charging the amp, but neither allows the amp to be used while charging is in process. Also, they charge considerably slower than the Mixim chip.

So, does anyone know of a pre-built charging circuit that would suit my requirements? Perhaps i'm just not using the correct search terms, but i'm finding nothing of the sort available. Alternatively, if i can't find something pre-made, does anyone know of a printed PCB for the Mixim chip (or another NiMH charging chip really, i'm not that picky..) that i could use?

Thank you for your time and any advice you can offer!
-Scott

You wish to charge batteries in the device. You want the supply to provide sufficient current so that batteries draw power, or the amplifier, or both simultaneously.

The easy way I think of is to set an adjustable regulator IC to 11.04 V. Feed everything from it. That voltage is based on my measurements of a NIMH at 1.38 V in rest state, 24 hours after being taken off the charger. Therefore I believe it should do no harm for a cell to be held to that voltage indefinitely. However you can select a lower voltage. Nominal level is 1.25 or 1.2.

Add a resistor if you wish to limit charge current into the battery. Add a steering diode so current can flow freely out of the battery.

This is not necessarily the optimal method but it ought to make construction easier. I could be wrong.

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For an adjustable regulator IC, I was thinking of the 317. For it to deliver 11.04 V to 8 AA batteries, you must feed it at least 13V. Maximum current is 1.5A.