Car lighter plugs are NOT the worlds best power sources

IF you can do a bit of wiring yourself, here is what I would do - Ham radio (and a LOT of other folks who are "in the know") have converted over to a connector called the "Anderson Powerpole" - great connector - used by RC car and airplane folks too - and they configure the connector in what folks call the ARES standard

The normal size we use can handle 30 amps, and one of the good parts is that they are not "gendered" - aka you can use the same connector to draw power, as to charge - all my bug out batteries have them - so I can either put a load (my inverter, my radios) on the battery, or plug my charger there, and charge the battery

Here is my web page on them

http://www.thegallos.com/ppole.htm