I've been using a Noco Genius multi bank charger on my E15 for the last few years, and just bought a new Noco Genius Gen3 for my E16 that will be sitting in the shed all winter. I also have an E20 with a dismantled stock charger and an older set of golf car batteries in it, and I bought 3 Batteryminder 1510 charger/maintainer/desulfators for it in hopes that the batteries won't deteriorate any further, and perhaps improve, over the winter. All my ElecTraks have regular golf cart batteries, but I haven't had to add water to the E15 since replacing the stock charger with the Noco charger. On the NOCO website, they say the charger is suitable for wet, gel, AGM, and maintenance free batteries.
Jim, in western Wisconsin