LiPo or NiMh
 

I have two 7-cell packs(spc IB4200 matched and zapped) versus two 2s packs(TrakPower) Please explain to an electrical newb why the NiMh at 16.8 volts is not as good as the Lipos at 14.8 volts?

I am trying to decide if I should spend another $200 on the two lipo packs or just use my NiMh packs?

The NiMHs drop their voltage under load more than lipos do, especially when comparing to the flightpowers.

At 100A, FP cells maintain a 3.6v/cell voltage (IIRC), while NiMH can drop to 0.9-1v or less. So, at 100A, the FP pack is at 14.4v, but the NiMHs can be as low as 12.6-14v.

This dropped voltage also creates heat inside the batteries, which reduces their life.

You should also take into account the weight savings you'll have. 2 X Trakpower packs are 19oz, 14 - IB4200's are >32oz...big weight savings with the Trakpowers

That's true. Less weight is less load on the motor and less current draw and more runtime.

Thanks all!

It seems that you also get better runtime with lipo as well (not talking about capacity spec's). A couple of guys at racing just switched from 12 cells nimh to 2 trackpowers (same time as I went to 5s lipo), they got a pretty good power increase and now they use about 1400mah in a 5-6 minute race and I use about 1300mah with my 5s setup. Before when we were using nimh it wasn't uncommon for one of us to start loosing power near the end of the race (3300's up to 4200's). I think usually we were using close to 2400mah or something like that.

Lipos numbers?
 

Understanding the numbers for Lipos? ( 2300mah 5S 20C )There are so many models, types and capacity I know the 2300mah is the capacity but what does the 5S or 20C stand for?

5s stands for the number of cells wired up in series. each li-po cell has a nominal voltage of 3.7V, hence 5 cells in series is 5 x 3.7 which is 18.5v.
the 20c stands for the discharge rate that the pack can handle continously. 1C is equal to the capacity of the battery, hence in your case 1C is 2300mAh.
that means the battery can handle a continuous discharge rate of 20 x 2300mA = 46000mA or 46 Amps of load.