| > Charging three Li-po cells in series needs only 12.6 volts. Where did
| > you come up with 18 volts?
|
| I have a 2-cell charger and a 3-cell charger. The 2-cell runs from 12v
| and would need more than 8.4v (2x4.2v) to charge fully in a reasonable
| time.
Actually, no. It should need a tad over 8.4 volts, as your charger
should never put more than 8.4 volts into the Li-poly battery.
You may put 1.6 volts per cell into NiCd or NiMH cells for a fast
charge, but you never exceed that 4.2 volt figure for Li-poly cells.
This does mean that your charge rate will taper off when it's almost
full, but that's just fine.
| Li-ion/po cells are nominally 3.7v and reach 4.2v at full charge.
| So 3 cells in series requires more than 12.6v.
Only a tad more. If it needs a lot more, it's because the charger
isn't very efficient or something. It's not putting more than 12.7
volts or so into the battery if it's smart.
| My 3-cell charger specifies 15-18v input and charges at 0.8A. 12v
| wouldn't do it. If I get a pack with more cells in series then I
| will need to use an even higher input voltage. What I'm saying is:
| think ahead to when you may use bigger packs when you buy charging
| equipment.
Get a Triton. It can charge up to 24 NiCd cells in series even with
only 12.5 volts of input.
| So, as for your question, I came up with ~18v from the instructions for
| my 3-cell charger (~ means approximately). It is related to number
| of cells-in-series -more cells means more input voltage required and your
| input voltage must be greater than number of cells times 4.2v to charge
| them fully.
That reasoning is true for NiCd/NiMH. It is not for Li-Poly. With
4.2 volts, you can get a Li-poly cell around 95% charged. With 4.3
volts or so, you can get to 100%. Throw more voltage at the battery,
and you can get it to puff up all pretty like :)

Signature
Doug McLaren, dougmc@frenzy.com
Are you going to come quietly, or do I have to use earplugs?
--from The Goon Show
Jim Lilly - 10 Jun 2004 07:54 GMT
Doug,
> Throw more voltage at the battery,
> and you can get it to puff up all pretty like :)
LOL!
Same effect if you drain a LiPo cell below it's minimum:-(
--
Jim L.
https://sourceforge.net/projects/virtual-access/
Using - Virtual Access(OLR), ZAP 4.5, & WinXP Pro w/SP1