Car Stereos: 12v to 24v, 12v batteries, 12 volt

Question
hi sir
i have a radio system that i want to install in a car with normal 12 v battery
this system is 24v and for this i will install 2 (12v) batteries in series
but the problem is how i will charge those two batteries while not effecting the car battery or drain it

i thought about installing another alternator but i found it hard to install in the car
another solution i thought about is to install an inverter (12/24v) for charging the 2 additional batteries and to install a protection that disconect the inverter when main battery voltage drop below 12 v ,but for this solution the two batteries may not charge if the protection disconect the inverter also the protection may be not reliable,also i am not sure if it is ok to charge 24v from 12v battery using inverter

please advice and if ther is better idea

thank you for your time and waiting for your advice

Answer
I would not change a thing.  If you want to power a 24 volt radio system in your car, I would buy a power inverter to step up your car's 12 volt system to 24 volts just for the radio.  Then, you do not have to change anything else.  Hope this helps, Scott