Roughly speaking there are two simple forms of losses. Coulombic efficiency, that is amp hours out is less than amp hours in, and resisteive, that is that you have to charge at a higher voltage than you'll get back.
Used over less than their full range lead acids are pretty much 100% for coulombic, and around 90% for resistive. If you try to deep discharge them, and fully recharge them, the overall efficiency rapidly drops to 75% or worse. Also, the coulombic efficiency drops rapidly as you increase the current, this is called Peukert's effect. Energy density is say 30 Wh/kg
Early Priuss used NiMH, 60 Wh/kg, see
for efficiencies. That article also has NiCd batteries, which would need very careful management in a hybrid.
Actually that's a good document, as it makes clear there is no such thing as one efficiency number for a given battery. say 1.0/1.2 coulombic, and 1.27/1.4 resistive, ie 75% overall
The next step in hybrid batteries is some sort of Lithium Ion based system. 86% resistive, very non linear coulombic efficiency. If you can figure out how to use them these are the best bet.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
