Well I'm on the recieving end of a hard lesson. I ordered a 9.6V 2200 mAh NiMH battery, but it has 16 cells in it. My Super Brain 969 is rated to charge only up to 8 cells per channel. For now it appears to be charged up and I've fired a few rounds through my gun with it. It works great and all, but here's the other hard lesson, that sucker won't fit into my ICS M4A1's hand guards very well. I have to take one of my boot bands and wrap it around the hand guards to make sure it doesn't pop open and spill the batteries out.
Now I have to spend around $60 to order a G&P 2000mAh 8 cell NiMH battery all the way from Hawaii. I'm not even sure if it'll make it in by Saturday.
Let this be a lesson, look before you leap, I saw a picture of the battery and could've counted that it had more cells than my battery charger was rated to handle.