Alfalfameister

I have two packs: one GP3300, and one 3000 mah (6 sub-C cells each).

I figured, if I were just gonna form charge anyway, can I wire them in series and form-charge it at 300 mah (C/10 for the 3000, and C/11 for the 3300)?

I was gonna leave it for 14-16 hours anyway, so does the 10% difference in capacity really matter?

I can just set the Multiplex 5014 (Swallow Advance) to 12 cells.

Any nay-sayers?

richrd

NAY

Yes 10 % is irrelevant but the NAY wins because the point of form is more of balancing than wake up call IMHO. You see in form every one hits it’s peak then drops this puts energy into the next cell it peaks goes to next on so on until all are pulling currents and each an all cells are pulling current. BTW this is not the true technical talk just a way of looking at what is happening.

So series two different packs and now 12 cells trying to get all equal voltage just not the best thing to do if you are really trying to get them all up equal. But they would charge.
Rich

PS this is why series liPO are a really bad thing. They do just the opposite.

Rodney

Alfalfamaster, I see no reason this would not work okay to form charge. I would not use missmatched sizes as a power source (I'm assuming you will not leave the missmatched sizes in series when using them later). What forming does is insure that all cells are charged to their fullest as the cells that reached full charge first now have the oxygen recombining at the positive plate but at a low enough rate to not excessivily raise the internal temperature and pressure while the not-fully-charged are still undergoing the charge chemistry. I'm pretty sure Red or Johnson will chime in here and correct me if I am wrong.

Alfalfameister

Great... two people (Rodney and richrd) whose opinions I value... and they have differing opinions! Ha ha! Will wait for a few to chime in more.

BTW, only reason I want to do this is so I can form-charge two batteries at the same time. And I'm form charging not for the initial charge, but for the balancing (form charge every 10 charges, the other 9 being fast charges).

Nope, no worries there. I don't want one to "dump" earlier (and have the risk of cell reversal).

But now that I think about it, even individual cells in a pack have different (albeit slight) capacities. Say, in a 3000mah pack, cell #1 has 3050, cell #2 has 2988, cell #3 has 3020, etc... Even matched packs have ever-so-slight differences (and they get un-matched over time anyway).

Alfalfameister

Corollary question:

Can I wire two similar packs (I'm reluctant to say identical, but in "theory" they are: they're both GP3300)...

Anyway, can I wire two similar packs in PARALLEL and just form-charge it at 660 mah (or thereabouts) because two packs wired in parallel are 7.2v 6600mah, right?

I don't really plan to do this, but just checking if I COULD do it.

So now, let me repeat/re-state my two questions:

1. Can I wire dis-similar (though not overly so) packs in SERIES and just C/10 charge it as a 12-cell pack?

2. Can I wire similar packs in PARALLEL and C/10 charge (at the COMBINED mah since it's wired in parallel) it as a high-mah 6-cell pack?

gerwen

You can't charge packs in parallel. I remember one of the gurus around here explaining why, but can't remember the specifics of it.

*edit*
Found it, quoted from Red: "Not a good idea charging in parallel. One pack will be charged slightly before the other and as its voltage drops it will demand more current, leaving the other pack under charged."

Alfalfameister

Thanks for that, Gerwen... I kinda knew it wasn't gonna be a good idea to charge in parallel. The "series" thing though is still up in the air for me.

richrd

Hay I agree Ha Ha

Parallel bad, series good.

Parallel is a engeering nightmare and design must have parellel current correction. Bipolar verse Fet long story . Thermal runaway

Don't miss lead my post yes it will work just that 3000's will be charged first then start to over charge waiting for the 3300 to finish is 11% and issue is your personal option what is OK, but it's just a fact that is [X(]....I doubt it would hurt the 3000's
Rich

Alfalfameister

Right...

Well, I think C/10 or C/11 shouldn't really matter then, as people charge between 14 to 16 hours (there's a two hour discrepancy right there, or about 600 mah if 300 mah charge is used).

That means I can do two batts with one charger. I really should get myself a couple of those Sirius Pro Formers sometime.