# 2s2p vs 2p2s for batteries



## darkman0101 (Oct 12, 2009)

Is there any difference?

I'm guessing that it would be better to match batteries in serial and then parallel the serial pairs. My thinking is that this would then prevent reverse charging of the batteries that are in serial (as they are "matched" in their capacities).
Should one of the parallel pairs then become discharged before the other, would I get reverse charging from the higher capacity pair???

Colour me confused.


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## darkman0101 (Oct 12, 2009)

I should mention that the batteries are actually all the same capacity 18650s from KD (came in sets of two). I've bought them at different times though, so when I'm using them as 2s1p I try and keep the same two together as a pair.


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## TakeTheActive (Oct 12, 2009)

darkman0101 said:


> *2s2p vs 2p2s*
> 
> Is there any difference?


Draw it out on paper. :thinking:


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## RyanA (Oct 12, 2009)

Edit: NVM. For safety I'd say just use batteries of the same age/capacity if possible. Mixing and matching could lead to trouble. Not sure about the reverse charging. Someone else probably knows about this in more depth though. The 2p2s thing is kinda funky though. It had me wondering who the heck was out there hawking 2p2s batteries to the masses.


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## Marduke (Oct 12, 2009)

Do you mean something like this?







Sorry for the crude sketch, hurried sketch on a laptop trackpad...


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## Mr Happy (Oct 12, 2009)

I think I would go with the 2p-2s arrangement (right hand picture).

Putting Li-ion cells in parallel helps to keep them balanced, and it means the whole pack can be charged with just a single balancing tap in the middle.


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## RyanA (Oct 12, 2009)

I think this is what hes asking:

He has 2 sets of 2 18650 batteries. One set older and one set newer. He's worried if it is safer to match batteries of like age in parallel or in series.

Edit: I dunno, I'm lost.


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## Linger (Oct 12, 2009)

Neither is safer :shakehead
*All batteries in a pack must be matched*. Safe assembly is %100. Half of safe assembly is unsafe assembly. Matching half can also be read as:
deliberately mis-matching half of the pack



1)Mixing new + used is a poor plan regardless of the combination used.
2)there is a certain amount of risk using using discounted batteries at all. Placing a number of them in the pack together multiplies that risk.
3)Will these be used in an application which will near or exceed the safe discharge limits of the cells?

I vote no. Depending on a the cheapest protection circuit available (only the cheapest used in the cheapest cells) to stabilize an inherently unsafe configuration may result in vent with flames.

A questionable source in a bad assembly (new+old) in a demanding application? 

Maybe read battery university before going any further. Please.


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## 45/70 (Oct 13, 2009)

I agree with Linger. You really shouldn't mix different age cells to make a "pack", especially with Li-Ions.

Just the same, I've run a 2p4s "pack" in a [email protected] mod with older NiMH's (all from the same batch, age etc.). After checking cell capacity on an analyzer, I match the pairs with the highest capacity cell of the 8 with the lowest capacity, the second highest with the second lowest, and by then, for the last two pairs, they are so close it doesn't make much difference. While I've never run the light down until it's "dead", the cells are all pretty close after discharge.

I don't think I'd ever try it with Li-Ions though!

Dave


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## LuxLuthor (Oct 13, 2009)

Darkman, it depends on how old one set is as compared to the other. If there are only a dozen more cycles on the first set, then I would keep old together in parallel, and new together in parallel, following Mr. Happys suggestion.

It also depends on if these are protected, and type of chemistry. Certainly from KD doesn't bode well. So if they are unprotected, lithium cobalt from KD, significantly aged as sets, then some of Linger's concerns are worth noting. Checking charged, middle of run, and ending voltages of all cells with voltmeter will give you a better idea how this is working. The main problems are when you charge ahead and keep using in either 2s2p/2p2s setup willy nilly.


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## darkman0101 (Oct 13, 2009)

Thanks for the sketch Marduke - those are the two configurations that I am talking about.

Batts will be bought at the same time from the same source and will be the same "type" (brand and capacity). However, seeing as I won't know if they come from differing batches from the manufacturer, I was wondering if it would be a good idea to keep them paired - and if so, in what configuration.

Maybe it wont be such a big deal with such a small pack - I've read that the ebike folks tend to wire in parallel then serial, but they are creating things like 46.2v 17ah packs).


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