I rotate 4 batteries for my 4K and use the Venom ProDuo 80 watt aftermarket charger. The Yuneec charger still works fine but the Venom does 2 batteries at a time, charges, discharges, balances, has "storage" charge setting, and give me good readings of cell conditions. I also use it on RC car batteries and it does seem to be a good buy. I got mine from Amazon and think it was about $119.00 delivered. There are several good units reasonably priced and the HiTech X-4 mentioned above is another that is an excellent choice.