Hi Josh,

I have seen a fair number of Lithium chemistry packs which parallel
cells or strings of cells without any protection or effort to balance
across strings. Parallel-connected cells will balance themselves and
if you have strings of cells which are then paralleled, they often add
lower-current (i.e. smaller wire) cross connections among the
cell-cell junctions in the middle of the strings to ensure that each
intermediate node of each string is at the same voltage as its
adjacent intermediate node on all the other strings.

Theoretically, you could have problems if the cells have a negative
temperature coefficient on voltage (so that the hotter cells in a
parallel connection took more of the current). Some chemistries DO
have this characteristic (for example, lead acid), but as far as I
know, Lithium chemistries do not change their voltage much as
temperature changes. Even if you do have this situation (like in lead
acid), you are usually safe because there is usually not much of a
thermal imbalance among the cells in a pack and also, if an imbalance
happens, it will fix itself after charging stops (the more
fully-charged cells in the parallel combination will charge the
less-fully-charged ones).

Some packs will recommend individual fuses for each pack if you
parallel whole packs but that is, I think, more because of the demands
which are being placed on the fuse. It would have to interrupt a much
larger short-circuit current if it were in series with the entire
parallel set of packs.

Sean


On Fri, Feb 4, 2011 at 8:26 PM, Josh Koffman <joshybear@gmail.com> wrote:
> Hi all,
>
> I need to parallel two battery packs to get one with a higher supply.
> These are lithium ion packs, 6 cells in series (so I'll end up with 12
> cells in total). From what I've seen when these packs are build
> commercially, they don't add any protection between the two series
> strings. I could add a couple of diodes to prevent one pack from
> charging the other, but I'm not really keen on the voltage drop. Is
> this even needed? Both packs will be separately balance charged, so in
> theory all the cells should be roughly equivalent.
>
>
> These are 22.2V packs and I'll be drawing just over 10 amps. If I do
> need protection (I mean, other than fuses), I welcome your
> recommendations!
>
> Thanks,
>
> Josh
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