Need more juice for stereo???

I guess you are right about golf carts, however in the arena of many toys added to cars, buses, trucks it doesn't fly. If you go down to a West Marine today they sell 6 volt batteries to be used in series with boats with that the demand for power. If you charge batteries in parallel thru a switch one never gets charged up if you don't change over time and again, or without the switch.
I have set up 12 -24 volts series on diesel applications that start on 24 and switch back to 12 still to this day. As I said before cheap is cheap it won't be long it will catch up and go around. The older boats out there were never set up for all today toy's, I have two.
I thought this was a learning forum rather than I got one better than you, my mistake.

     OK, sorry I had to butt in again. For SURE, batteries added in series add the voltage and not the current. Say you have 2 6V 100amp hour batteries hooked in series. You would get 12V at 100 amp hours of capacity. 2 12V 50 amp hour batteries in parallel would give you 12V at 100 amp hours. If you look at size of a 6V 100amp hour battery, and a 12V 50 amp hour battery they are usually identical. 12V at 50 amp hours is the same power as 6V at 100 amp hours. Power here is referred to as watts. Adding 2 6V 100 amp hour batteries in series gives you twice the power since doubling the voltage. You cant double the current as you would be getting 4 times the power. This I am 100% SURE of, and wouldnt say so if I wasnt.

    Now where I am not 100% sure, is why they still hook 2 6v batts in series instead of 2 12V in parallel. This I think is due to the fact that golf carts and starting large diesels require tons of current. 6 volt batteries will offer less of a voltage drop internally due to the larger cells. 12V batteries have twice as many cells as a 6 volt in the same physical size causing the cells to be smalller and not able to handle the large current as well.

    Hope you all had a great 4th!! Still had fireworks going on the lake tonight!!

                                   Jeff...

No, i couldnt leave the topic alone either.
Cost vs. amp hours is at issue too. A 12 Volt Optima Blue Top runs about $150 and is rated 55 AH, 2 would be $300 and 110AH/12volt. A six volt 200 AH golf cart batt. is about $190 so 2 would be $380 but you get 12 volts/200 AH. Thats almost twice as long as the Blue Top Optima. So run the stereo on the golf cart batt and keep a good 12 volt w/ cca to start and a combiner for trouble free charging.
Of course you will need a larger boat to hold all those heavy batteries.