Not sure about lithium batteries if using them, but with lead-acid, AGM, etc you can simply use a standard battery charger that has enough current to keep up hooked to the battery. Older battery chargers simply kept the output voltage at 13.8V so that a lead-acid battery would not be overcharged. Even if the charger can't keep up fully, all that will happen is a discharge on the battery while doing your testing and then it will charge it back up. I have an old 6A and a 10A charger that are like that.