[QUOTE=U4edot;4446]Say, for a 72v system could you wire six 12v batteries in series for the motor, and wire the same six 12v batteries in parallel for the 12v accessories?[/QUOTE]
No, that would make a gigantic short circuit. In order to make the series string you’d have to connect the batteries positive terminal to negative terminal, and to make it parallel you’d have to connect it positive to positive and negative to negative. This would be the same thing as taking one battery, and connecting the positive terminal to the negative terminal with a wire. Then you take bets on what will happen first, will the wire melt, or will the battery detonate. Which goes first will depend on the battery charge and the wire diameter. 
What the question I was asking about would not make a parallel connection on a series connected battery, it just takes a tap in the middle to get a lower voltage. It’s still a series connection. You actually wouldn’t try tapping a parallel battery pack in this manner, as it only matters for voltage, not current. Simply speaking, in general terms, you can put a 12V 1 amp load on a 12V 1 amp battery or a 12V 1 million amp battery, the 12V 1 amp load isn’t going to draw more than the 1 amp. Put it on a 120V battery though, regardless of amperage, and the 12V load will fry.
You can wire the batteries in three configurations, in series, in parallel, or a series-parallel. Let’s say you have six 12V 500 amp batteries to connect.
The series connection will tie the six batteries together by connecting the positive terminal on one battery to the negative terminal of the next battery. In the end for your 72V string you will have six 12V batteries connected positive to negative, with the positive pole on the first battery and the negative pole on the last battery not being connected to anything. When ready for use, the load will be connected to these unused terminals. Electrically, this makes one 72V 500 amp battery.
The parallel connection will tie the six batteries together by connecting the positive poles together, and the negative poles together. In this configuration, all poles will be connected to a pair of bus bars. Usually this is done by putting a pair of bus bars (a solid copper bar) alongside the batteries, with a wire that connects the battery to the bus bar. There will be one on the positive side and one on the negative side. The load will have its own connection to the bus bar, and the bus bars themselves are not connected. Electrically, this makes one 12V 3000 amp battery.
The series-parallel would be wiring several series strings to an equal voltage. Since we need several strings, for this one we will use thirty six 12V 500 amp batteries. This would leave one positive pole not tied to a battery in each series string, and one negative pole not tied to a battery in each series string. These untied poles on each string would be tied together over a pair of bus bars, connecting positive to positive and negative to negative, to make the parallel part. Physically, you will have six strings of batteries connected in parallel, each string will electrically be a 72V 500 amp battery, connected in parallel. Electrically, this will make a single 72V 3000 amp battery.