MGM controller do I go for amperage or cell count??

[Finnster]

The A123s do peak @ 3.65 on the charger, but fall pretty quickly down to ~3.3v/cell until they dump. Even my 5S packs are about 17.8V (~3.5V/cell) after they come off the charger and rest for a bit. That why I said 7 cells should be just fine w/ the 18, but if you are thinking of leaving headroom for higher voltages later than the 16024 is the safe bet. You should be able to hit 40 tho on the 7S I would think, unless it really is that fat :)