On Wed, 18 Nov 2009, Sean Breheny wrote:

> Some battery technologies are very efficient. I think that Li Ion is
> >90% efficient. Wikipedia gives Li Poly efficiency at 99%.

This is amazing. I obviously haven't been keeping up with developments. Do 
you have any references other than Wikipedia?

> Capacitors
> can be just as good, but it depends on the charging technique. If you
> charge a capacitor through a resistor from a voltage source, the
> efficiency cannot be better than 50%. You really need to charge them
> using a switching-type power supply to get close to the optimal
> efficiency.
> 
> The major advantage of supercaps as I see it is their very long cycle
> life. They can routinely do several million full charge/discharge
> cycles. Even the best battery technologies in this regard can only do
> perhaps 10 or 20 thousand full charge/discharge cycles.

I guess you're not talking about LiPo here. Last time I looked (about 2yrs 
ago) good LiPo were rated at between 100 and 200 cycles and you need to be 
extremely carefull about over discarge or they die completely.

> 
> The best supercaps out there have roughly 10% of the energy density
> per volume of lead acid batteries. Energy density per mass is probably
> more like 20% of lead acid due to the lower density of the capacitor.
> Supercaps are way more expensive than any typical kind of battery on a
> per energy basis (energy for 1 full charge) but taking into account
> the cycle life, the cost per total energy throughput is probably a
> little less for supercaps than for batteries.

But again, aren't lead acid batteries only about 60% efficent? Ok, LiPo 
might be much more efficent but their energy density decreases 
dramatically with each cycle (several % - can't remember the exact figures 
off the top of my head).

Regards
Sergio Masci
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