Recharging battery-powered amp while playing

[Yosefu]

I built a distortion preamp + tda7297 amp with busking in mind. It has 16 AA NiMh batteries plus a 18v regulator (L7818). To recharge the 16 batteries all at once, I'm plugging in a 24v power supply, limiting current with a 50 Ohm resistor in series. Yes, I know this is not the ideal way to treat NiMh batteries, but that's ok for now. The problem is an annoying high-pitched buzz that oscillates in amplitude at around 4 Hz, that makes it impossible to play while recharging.
Why is this happening? Could a diode intersection solve it?
Thanks!

 

[Mooly]

One possible reason could be your 24v supply (SMPS ?) that it is operating in burst mode due to the light loading. The series cells will need more than 24 volts via a 50 ohm to fully recharge them.

As you say, its not an ideal method.

 

[Yosefu]

The cells add up to 1.2*16=19.2, so 24v is 4.8v above their voltage - is that not enough? Should I use a smaller resistance? I don't care shortening the batteries' life span, I'll worry about that later using some microcontroller to detect the voltage drop when they're fully charged. But for now, something that doesn't burn them and lets me play while charging should be more than enough.

The supply is an ALM054, but apparently it isn't marketed outside Spain. The ST-242A seems to have the same characteristics.

 

[Mooly]

The cell voltage goes over 1.2 volts when you charge it. The problem with a resistor is that the current is ill defined. With batteries discharged to 1.00 volts per cell you have 8volts across the 50 ohm and so a charge current of 160ma. Very quickly that voltage rises and the charge current decreases. When the voltage across the cells has reached say 1.35 volts then you have just 48ma charging current. In practice this means the cells will take a very long time to fully charge.

If you charge at higher current then the cell voltage goes even higher, closer to 1.55v when fully charged but you could never reach that point with a 24 volt source.

 

[Yosefu]

Thanks, well explained! I didn't know the voltage of the cells would vary that much upon charging.

Since I only have that power supply and 24v seems to be a standard value, I'll take the resistor value down to ~15. The batteries won't fully charge, but at least that will prevent them from frying in case I forget them plugged.

For the next amp I build, I'll use 12 AA batteries and operate between the range of 12v to 18v, which is fine for the tda7297, and hopefully no longer need the voltage regulator.