I'm looking for the same solution. Currently I'm using a 35ah lead acid that sits externally behind the machine. I put a plug in the back. The best solution I could find was this
, its not cheap, but if the user does get 5x the life then it more than pays. 50% less weight is nice. I thought about building my own pack from LiFePO4 cells, but they don't naturally add up to 12v, which is why the 12v batteries have circuitry to bring it down. Also they need to be protected from overcharge and discharge. So a specific 12v battery is the best solution, there just aren't many different ones available. A123 Systems is the top of the line cell, but I can't find any12v from them.