Battery wiring for DIY speaker question
Chargers normally supply current pulses and thus would cause severe noise ripple in series with a battery and perhaps create havoc if the amp has some boost regulator.

What is the max voltage rating on  the ojačevalnik (amp.) 

Normally 12~14.2V is supplied to auto audio units with the Amp using a step-up voltage converter with a differential output drive to get max power at the lowest voltage.