Re: 6 volt question
You don't generally wire batteries in parallel unless they have a high internal resistance. Flashlight batteries have high internal resistance, but lead-acid batteries have low internal resistance. If you wire two of them in parallel, and one has an actual voltage of 13.1 V and the other has an actual voltage of 13.0 V, you have a low resistance path with a 0.1 V potential difference. This will result in a high current charging the low voltage battery which will last until either the voltages equalise or the wire melts. Very hard on the batteries. This is why jumper cables are so thick - to handle the high current.
Putting two 6 V batteries in series gives you about twice the storage capacity of a single 12V battery, given each battery has the same weight. So you can run that 12 V light bulb for twice as long. It won't burn any brighter than with a single 12 V battery.
2009 17' Plan B