Audio Systems: 12v to 24v, battery isolator, 12v batteries

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
two ways  very simple and cost eficient
one will be diodes anything with 100 amps or more , they are under 10.00
this will let you connect two batteries to the alternator and charge at 12 volts,  both will charge no problem
you can wire it with the diodes to get 24 volts ,

the diode will let you add anythig but is not visible in the other side of the diode

search and read about diodes so you can understand how the work and isolates circuits

the othe way is a battery isolator  or relay they work the same way

battery isolator  have diodes inside but it is made for the aplication about $50.00 to $100.00
the relays are mechanical

http://i61.photobucket.com/albums/h51/40oldtimer19/Electrics/12-24Vwiring.jpg

the isolator will go to the 12 v and 24 v wires

http://solar-catalog.com/images/batter2.jpg

http://www.powerstream.com/battery-isolator.htm