I'm always looking for a small, cheap, battery powered, rechargeable amplifier that sounds good. For some reason this combination of requirements doesn't match anything on the market.
The Boss Katana Mini caught my eye as it can be found for $100 or less, it sounds quite nice and it has built in delay and a few 'overdrive' settings. It almost meets my needs. Although it features a compartment for 6 AA batteries, it doesn't include any charging circuitry. I bought this amp with my own money to try to hack it to charge some NI-MH batteries.
Inside is quite empty. Huge potential for hacking. We can see that the muscle in this operation is TPA3131 Class D power amplifier IC, delivering 7W to the 4" speaker. Close to a dozen NJM2740 op amps shape and pre-amplify the signal. The delay is provided by the notorious PT2399 karaoke echo chip.
My plan is to implement a trickle charger using an LM317 in constant current mode. I've settled on a value of C/40 or .025C which is the recommended maintenance charge for NIMH chemistry. Ideally we would implement something with smarter charging, but that will be a project for another time.
To get C/40 we need to provide 25mA to the batteries, which results in an R value of about 47 Ohms.
A diode was necessary as there is some circuitry that detects whether battery or dc power is applied. The batteries were trickling backwards through the LM317 and preventing the amp from turning on.
I used batteries from Dollarama, at $4 per pack of two. My ammeter shows the batteries are indeed trickle charging, but it remains to be seen how effective this will be. I will update after a few weeks of operation.