I use a smaller 50Ah AGM battery for charging at the track. Allows me to charge a few of my 2s batts for 1/10th racing. The best ones are the deep cycles like that used in 3 wheel scooters, electric wheelchairs as they are true deep cycles. You can usually find them used for only $20 or so, and still have enough capacity to last the day for me. Of course, it is all going to depend on the amount of charging you want to do. They say to try not to discharge a lead acid deep cycle (includes AGM) below 50%, which usually equates to 2V/cell unloaded.