2 chargers vs one?

[Garry]

On one of the RC car forums I'm on, theres currently a thread on charging 2 packs of six NiCD or NiMH cells at once. The obvious answers are either one charger capable of charging both the packs separatley, or two stand-alone chargers. However, one member thinks its fine to use one charger that'll handle more than 12 cells, just wire up the two packs in series.

I've tried to explain that this is dangerous as the packs have been discharged at different rates, and would probably lead to overcharging/venting, and possible explosion.

Can someone point me to an article, guide or post to support the 2 chargers vs one theory?

[Chrislong]

OK,

Think of it like this.

You have two empty glasses infront of you, one pint glass and one half pint (i.e. 4300's v2 2000's MAH).

Fill the glass's up seperate, so you can stop filling them when the water reaches the top.

Fill them up at the same time, the half pint glass will overflow before the pint glass is full.

An electronic techy here mentioned to me that water flow is a good way to think about electrical flow. The difference in this instance, is that half pint glass won't explode like cells will as the Delta peak will be miles out.

Chris

[Col]

All true, no doubt Chris, but does the same still apply if charging 2 identical types of cells (I have 4 packs of Dans old 3700's and have wondered if it's possible to charge 2 packs at the same time from the same charger)

[Jony]

If the packs are very well matched, then you'll be fine. After you can charge all the cells in one pack at a time.

But I'd be very cautious - it likely that the packs are no longer that closely matched, even if they were the same when new. If they're a long way out you might run into trouble. If you have a fancy enough charger that you can identify the voltage(s) at which the cells peak, then you could compare the peak voltages of the packs. If they're peaking at near the same voltage and taking the same capacity you might get away with it.

[Col]

Cheers Jony - it all makes sense. I'm not ever gonna charge 2 packs at once though, just wondered if it was possible!

[mark christopher]

dont want to point out the obvious if a manufacture says thier charger can do 12 cells it has to do it.
if both packs are the same capacity/type ie gp 3700 and they are discharged and equalised the same you will have no more trouble than charging 6 cells in one pack

[Jony]

Originally Posted by mark christopher

dont want to point out the obvious if a manufacture says thier charger can do 12 cells it has to do it.
if both packs are the same capacity/type ie gp 3700 and they are discharged and equalised the same you will have no more trouble than charging 6 cells in one pack

I'd really say be very careful if you're going to try this. While what you say is kind of true, it's not exactly true. Even when the packs are new all gp3700s don't have the same capacity/peak voltage. And it gets worse as the cells wear out. Imagine you take two packs that were identical when new. Run one pack for a few hundred cycles at high current and it'll lose capacity (the cells wearing out). Now if you try and charge those two packs together the worn one will overcharge before the new one is full.

Like I said, if the mismatch is small, you'll be ok. After all, cheapo unmatched packs don't explode when you charge them. But if the mismatch is bigger you're inviting trouble. Unless you have the tools to know whether the packs are well matched I'd say don't bother trying.

[mark christopher]

Originally Posted by Jony

I'd really say be very careful if you're going to try this. While what you say is kind of true, it's not exactly true. Even when the packs are new all gp3700s don't have the same capacity/peak voltage. And it gets worse as the cells wear out. Imagine you take two packs that were identical when new. Run one pack for a few hundred cycles at high current and it'll lose capacity (the cells wearing out). Now if you try and charge those two packs together the worn one will overcharge before the new one is full.

Like I said, if the mismatch is small, you'll be ok. After all, cheapo unmatched packs don't explode when you charge them. But if the mismatch is bigger you're inviting trouble. Unless you have the tools to know whether the packs are well matched I'd say don't bother trying.

yes it is there is as much chance of one cell expolding etc in a 6 cell pack as in a 12/18 etc pack
boats use far more cells in one pack thean we do!

[Jony]

Originally Posted by mark christopher

yes it is there is as much chance of one cell expolding etc in a 6 cell pack as in a 12/18 etc pack
boats use far more cells in one pack thean we do!

Sorry to keep the argument alive - I'm not trying to just prove a point, but I don't want anybody to read some bad advice on a forum and then damage their cells, their mum's kitchen table, or themselves

I still don't agree Mark. There's certainly no problem with charging a 12/18 cell pack. But that's not the same as charging two (or three) 6-cell packs. The difference is, in the 12 cell pack all the cells have been charged and discharged together each and every time, so they are much more likely to still be in balance. Two 6-cell packs that have had different discharge cycles on them are much less likely to be in balance with each other, and so there's a risk of overcharging one of the packs.

Anyway, my advice to anyone reading is don't believe either me or Mark Unless you feel like you understand the issues well enough to make your own mind up, then I'd say just stay on the safe side and charge one at a time!!!

[Chrislong]

Im sure anybody reading this realises, they follow advise at their own risk. A fair amount of common sense must be applied, as with anything.

[sim]

Also, the more cells in a pack, the higher chance of there being a big difference between the highest capacity and lowest capacity cells. You can do it, but it's not as good for them as charging them 6-cells at a time.

So...what you need to do is buy two Spintec ICC chargers... and then... okay, just kidding.