I've been charging NiMH aaa and aa sizes and 18650 lithium an RCR123A batteries. It's a nice system and seems to work just fine. They do warn to charge aaa in pairs in slots 1,3 or 2,4 as the will charge a single aaa at too fast of a rate. Other than that warning, it's plug and play for any single battery. I would say it's not a super fast charger, but I think that also makes it safe. I think it indicates 1.5 watts charging rate per slot. So if you've got a 9 watt-hour 18650, it might take a while to go from zero to full. Quicker for smaller watt-hour batteries like an aa or aaa NiMH. There is a shutoff circuit but I'm not sure it's overly smart. It indicates it will shutoff if the circuit is active for 20 hours. It specifically says it is an "overtime prevention". I'm guessing it relies on a slow charge rate not to damage a battery if it overcharges for 20 hours. But it also says it automatically stops charging when complete. So I'm scratching my head a little bit wondering exactly how the protection works. I like the fact you can charge individual batteries and that it works for so many varieties of batteries. Very convenient single item to charge all of one's batteries. I am satisfied and I do recommend it. I recently charged a 18650 lithium and the charger circuitry did get a little warm, in the top of the unit (still well under 120F I estimate). The battery did not get warm. Not a concern, but one might want to make sure the device has some air circulation to it and is not resting on a blanket or something.