Dennis,
I don't have the ThunderPower Charger, but I am sure it will cutoff at the right point. Does it have an A123 charge mode? Again I am somewhat unfamiliar with the A123 and what constitutes a full charge. You need to distinguish between nominal voltage, and fully charged voltage. 3.4V might be nominal voltage (but I DON'T know).

Typically charging Lipo's, the first part of the charge goes pretty fast, but then the last 10% goes pretty slow. It certainly isn't linear. I am not positive about the A123's.

A123 cell has charging voltage 3.4V almost all the charging time. It is full when the volatge jups up (it goes very quickly and it allows quick charging) to 3.6 or 3.65V.

BTW if you have 2p pack, you can use 20A for charging, so you have full battery after ~15minutes.

Sorry I do not mean to jack your thread but I have a question I seem unable to get an answer for and do Not want to assume anything. Does the charger tell you the current voltage of the battery ? Is that how you know what the voltage is ? Does the charger also tell you how many amps you added to the battery ?

Jim,
I have two chargers, but I would recommend the FMA CellPro 4s charger (will charge up to a 4s pack at a maximum current of 4 amps. If you need more there are other chargers, but likely to be more expensive. With this charger, you still need a DC power source like a car battery or a supply that plugs into the wall and supplies about 13V.

Anyway when you plug the battery into this charger (you may need an adapter since it plugs into the balancing leads of the battery), it will directly tell you the overall battery voltage, each individual cell voltage, and an estimate of how much capacity is left in the battery. As the battery charges, it tells you the estimated capacity and how much it has put into the pack. Also you can see the individual cell voltages and check to see they are all the same. The cost of the charger alone is $75+S&H, and they offer the AC power unit for another $30. However this setup often goes on sale (that's when I picked mine up), and sometimes is bundled with the AC charger.

So that's how I tell how much juice I have used.

There are other good chargers out there too. I might add at this point that I cannot use my FMA charger on my 2s730 Thunderpower Prolite packs because they (still!) come without balancing leads. I have heard this is because these cells are actually matched at "birth", and so stay closely matched. I can't verify that personally. Anyway, these I charge using my Astro 109 charger through the normal power leads connector. It is a good charger, and it is the first one I bought, but I think the FMA is a bit more useful for someone who plans to fly using packs in the 2-4s range.