Just picked one of these up and I have a question about what is displayed as it charges. I charged a 4200mah 2S last night and when the cycle stopped the mah was only 2754. Does that mean there was 1446mah in the battery when I started? (battery is almost new)

If so how can I check the total mah of a charged battery?

Yes, your battery had a partial charge. To check the total mAh of a battery, you have to run it down and recharge and let the charger tell you how much it put back in. If you run a LiPo down to LVC (Low Voltage Cutoff), which should be no less than 6.4V (I think mine is setup for 6.8V in the ESC), then recharge, you'll get your mAh reading.

Looking at it another way, you replaced 66% of the lipos capacity. Lipo doesn't like being over discharged which is defined as more than 80% of capacity.

So as long as you don't see more than 3360mah going back in on recharging, your lipo should be happy. This should work out to be close to 3.7v/cell 'resting' voltage. Voltage will be a little lower immediately after running, lipo does have some voltage recovery.

Awesome info guys, thanks!