Can you see your batteries? You might want to compare the CCA rating (Cold Cranking Amps) to the unit you're looking at, more amps is better than less. I think what you're referring to is amps not volts. The more amps you have the more current (assuming the resistance is the same) you get at the same voltage.
You can also get a 3/5/8 in 1 unit that includes an internal battery and jump cables but also can have power adapters for 110/240 volt appliances, USB, 12v charging etc some of which can be charged from a solar panel or a pull string. We have one that has a tyre pump/low volume compressor, jumper kit, USB power, 12v power, 240v power, a torch and a pull string for emergency charging, it has come in handy quite a few times.
Similar to this one - http://www.northerntool.com/shop/too...5488_200395488