Re: LiPo's in parallel?

The last time I got into parallel cells-- needed added resistors would work
if they had the right positive temp coefficent.
series diodes plus a small value resistor can be a viable alternate solution
if the few tenths of a volt drop across the diodes is acceptable.


"RogerN" <regor@xxxxxxxxxxx> wrote in message
news:BrydnV20KeU9ynLVnZ2dnUVZ_uidnZ2d@xxxxxxxxxxxxxxxx
Point well taken and I would prefer to error on the side of safety but...
My charger doesn't have a separate setting for 15C LiPo's vs 25C LiPo's.
Since they all charge to the same 4.20V/cell, and all are recommended to
not discharge below 3V/cell, and they all charge from the same algorithm,
shouldn't they work in parallel if you're not pushing the limit? I mean
if the maximum recommended charge current is 1C, you don't want to exceed
that but as long as you stay within the recommended bounderies I would
think it would be OK. It's not like paralleling LiPo's with LiIon or some
other chemistry that has a different charge algorithm.

To match them, what I was thinking on doing is after use is balance
charge/discharge them all to the same voltage, then parallel them with a
small resistance and a fuse between each connection. Then they could all
be held at storage voltage or all be charged together. If one cell went
bad, it would blow fuses at perhaps 2C and not be able to electrically
damage the other packs. The fire might damage the other packs though! :-)

But I do think the better solution would be to connect all packs to a
multi-port charger and charge them independantly. That way you wouldn't
have to worry about getting them to the same voltage before connecting
them.

RogerN

"The Natural Philosopher" <a@xxx> wrote in message
news:1223628702.17808.0@xxxxxxxxxxxxxxxxxxxxxxxxx

Yup. The thing here is that as long as the packs are of the same
chemistry/construction style and from the same manufacturer, *even if the
cells are of differing capacity* you can connect two packs in parallel
*once you are sure they are at an equal state of charge*.

If you are not sure, don't risk it.

Now once they ARE in parallel, you can charge and discharge as a single
pack.

But the moment you use them independently you MUST go through the above
procedure all over again to make sure the state of charge is the same.

The reason for this is that *as long as the cells are identical chemistry
and construction style*, there is a completely unique relationship
between voltage and charge state. So they will distribute charge and
current correctly, and stay in balance. This is of course how large
capacity packs are made up: From identical cells in parallel..

What is distinctly dangerous, in order of increasing insanity, is mixing
cells of different quality - say 15C packs versus 25C cells - from the
same manufacturer, mixing cells from different manufacturers, or
connecting cells in parallel when at a different state of charge.

You MAY be lucky. Do you feel lucky today?




.

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