# Mix cell capacities in *parallel*?



## Maury Markowitz (Jul 6, 2012)

Hi everyone, compete noob here - well noob to battery tech anyway, if you have any PV questions, I'm your guy.

I understand that the capacity of a string of cells in series is the capacity of the worst cell in the string. So if I wire a 60 and a 100 Ah together, I have two 60s. Right?

But does a BMS change this? When the shunt on one of the cells fires, can't the rest keep charging?

I *suspect* that when the shunt kicks in the voltage drops and the charger kicks out. But maybe the shunt holds the voltage high?

Can someone explain this shot-by-shot for me?

Ok, so now let's say I wire those two together in parallel... What happens in this case when the 60 fills? I *suspect* that current will drop, but does the charger care. Can the 100 Ah cell keep charging?


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## Ziggythewiz (May 16, 2010)

No, you have a 100AH cell and a 60AH cell. You can only use up to 60AH (less for safety margin). What a BMS will do depends on the BMS. 

If you were to try and pull more than 60AH from the string, the 100AH cell would push the 60 into reversal, destroying it.

In parallel, they fill as one. The voltage on the 60 can't go up the curve until the 100 gets there also, so if the 60 were to fill first, any current going in would go to the 100.


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## Qer (May 7, 2008)

Maury Markowitz said:


> But does a BMS change this? When the shunt on one of the cells fires, can't the rest keep charging?


You can never drain a pack of more Ah than the weakest cell. Normally, when you have a pack of, say, a string of 60 Ah all of them will be slightly different (for example 57, 59, 61, 60... Ah) and the weakest cell will be the one that determine when the pack is drained. If you keep pulling current from the pack, the weakest cell will be completely drained and will, eventually, be reversed and get damaged.

When you parallel cells (like two 60 Ah) you get twice the Ah. It's not uncommon to build racing packs in a grid of serial and parallel cells to get the voltage and Ah you desire, but usually you always use the same cell size in all the parallel and serial cells to keep the pack balanced.

Theoretically you should be able to parallel a 60 and a 100 Ah cell to get a combined cell on 160 Ah, but I'm not sure if that's a brilliant idea in real life. I'll let others fill in on that. 

And BMS can never change this. A BMS can possibly help you getting more power out of the pack (this is a topic of much debate though, bordering on religion  ), but it can't magically coerce a pack to squeeze out some extra Ah just out of the blue by adding a BMS.


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## Maury Markowitz (Jul 6, 2012)

Qer said:


> Theoretically you should be able to parallel a 60 and a 100 Ah cell to get a combined cell on 160 Ah, but I'm not sure if that's a brilliant idea in real life. I'll let others fill in on that.


Which is the real-world example I was actually curious about. My idea is to package up 16 40's in a box, and 16 100's in another. Box 1 would be 48V/2000Wh, and Box 2 would be 48V/5000Ah. They could buy Box 1 or 2 in any (small) number, and wire them in parallel to get close to the capacity they want.



Qer said:


> And BMS can never change this. A BMS can possibly help you getting more power out of the pack (this is a topic of much debate though, bordering on religion  ), but it can't magically coerce a pack to squeeze out some extra Ah just out of the blue by adding a BMS.


If I put a 100 and a 60 together I have a total of 160 Ah worth of electron holes. But from what you've said above, I can only use 120 of them. Is that right?

Isn't the *whole idea* of the BMS to let me use those missing 40 Ah?

Edit: OHHH, wait, I read your post wrong. Am I correct in this interpretation:

If you take two fully charged cells and wire them in series, one a 60 Ah and the other 100 Ah, then *during discharge* the 60 will empty and then you have to stop.

So the problem isn't on the charging side, and thus the BMS can't help you. The problem is on the discharge side, and the BMS doesn't do anything then.


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## jeremyjs (Sep 22, 2010)

You can put a 60 and a 100 in parallel and then put them in series with other 60 and 100 pairs 
+60 parallel 100 -+ 60 parallel 100 -+ 60 parallel 100-
This would give you a 160ah pack

or you could do 2 series strings one of 60 and one of 100.
-60+-60+-60+-60+-60+
parallel at the end of a string with the same number of cells per string
-100+-100+-100+-100+-100+
this would give you 2 packs 1- 60ah and 1-100ah for a total of 160ah useable capacity when the strings are paralleled

You can't put 60's and 100's in series like +60-+100-+60-+100-. This would give you 60ah of useable capacity. I mean you can do it, butover charging and discharging will destroy the 60 ah cells if you use over or charge over 60ah. Your best bet is to use the actual cell size you want or at least whatever sizes are available that are close to what you want though. Making all of the extra connections does nothing but make things more complicated and difficult.


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## Maury Markowitz (Jul 6, 2012)

jeremyjs said:


> or you could do 2 series strings one of 60 and one of 100.
> -60+-60+-60+-60+-60+
> parallel at the end of a string with the same number of cells per string
> -100+-100+-100+-100+-100+
> this would give you 2 packs 1- 60ah and 1-100ah for a total of 160ah useable capacity when the strings are paralleled


Ok, this is precisely what I'm trying to do.

Our inverters take in 48V, so I want to make up packs of 16 cells for ~48V. Sometimes we're going to want to get whatever max capacity we can fit in a box, so the 100's, or even 180's will be the way to go. But for a whole lot of applications, all we need is 48V, the amount of Ah is actually almost an afterthought. So for these roles we need a smaller system, maybe 40's.

But I just want to be sure that when they start with a 100 pack, and decide they need a *little* more power we can add one of the 40's. And from what you're saying here, they can.


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## jeremyjs (Sep 22, 2010)

Maury Markowitz said:


> Ok, this is precisely what I'm trying to do.
> 
> Our inverters take in 48V, so I want to make up packs of 16 cells for ~48V. Sometimes we're going to want to get whatever max capacity we can fit in a box, so the 100's, or even 180's will be the way to go. But for a whole lot of applications, all we need is 48V, the amount of Ah is actually almost an afterthought. So for these roles we need a smaller system, maybe 40's.
> 
> But I just want to be sure that when they start with a 100 pack, and decide they need a *little* more power we can add one of the 40's. And from what you're saying here, they can.


Just as a disclaimer I've never personally done this, but that is the way it works. The parallel strings self equalize in voltage. So long as there isn't a lot of resistance between the two strings there shouldn't be a problem.


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## frodus (Apr 12, 2008)

The issue of this whole thing, is that the 60Ah and 100Ah cells will very likely have different internal resistances from eachother. So no matter what you do, you'll have unbalanced current coming out of them. Now once they're at rest, the one with lower capacity will get charged by the one with higher capacity (not neccessarily the 100Ah cell), but during discharge, if it's being discharged continuously, you may never have a chance for current to flow into the other battery that needs it.

IR determines which cell in the parallel group gives a bulk of the discharge current, and it could be the same, or it could be unequal (more likely).


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## PStechPaul (May 1, 2012)

Theoretically, you should be able to add a diode to each string of cells, and connect the cathodes together for discharging. But you will lose about a volt and at 100 amps that's 100 watts. But it will equalize the discharge because as one pack is depleted (or sags under load) its voltage will drop and the other pack will start contributing. But for charging you would probably need to use separate chargers for each pack.

It should be possible to design a more efficient battery pack sharing circuit, but when currents exceed 100 amps and voltages may be 144 volts or more, the components become expensive and complexity and cost go up.

A better solution may be to monitor the individual pack currents and make sure that they are within specification, and maybe trip an alarm or activate the BMS to bypass a weak cell if that can take care of the problem.

It may be better to standardize on 60 A-Hr cells and build larger packs with several identical packs in parallel. Then you only need to look for the difference in charge or discharge current and do something about it if it exceeds a certain amount.


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## Maury Markowitz (Jul 6, 2012)

Thanks for all the info guys, this is great stuff.


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## jeremyjs (Sep 22, 2010)

frodus said:


> The issue of this whole thing, is that the 60Ah and 100Ah cells will very likely have different internal resistances from eachother. So no matter what you do, you'll have unbalanced current coming out of them. Now once they're at rest, the one with lower capacity will get charged by the one with higher capacity (not neccessarily the 100Ah cell), but during discharge, if it's being discharged continuously, you may never have a chance for current to flow into the other battery that needs it.
> 
> IR determines which cell in the parallel group gives a bulk of the discharge current, and it could be the same, or it could be unequal (more likely).


I'm curious how this seems to work just fine in the battery itself? All it basically is are hundreds, if not thousands, of very thin batteries stacked together, bolted together on the lower part of the terminal, and a casing added. Is there something in this arrangement that's in any way fundamentally different than bolting the cells together through the top of the same terminal. All of the lamination's also have some variation in thickness, IR, etc; which is why we end up with batteries of slightly different capacity. Is there something internal to the cells that I'm not understanding that makes it work?


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## Ziggythewiz (May 16, 2010)

I think it would work fine in most scenarios. The only one I could see an issue for is running full throttle for long enough to empty the smaller cells, so they have no chance to balance. I'm sure it's not 100% efficient to pass the energy back and forth, but I haven't measured that yet.


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## Elithion (Oct 6, 2009)

Maury Markowitz said:


> Ok, so now let's say I wire those two together in parallel... What happens in this case when the 60 fills? I *suspect* that current will drop, but does the charger care. Can the 100 Ah cell keep charging?


I just happen to be scheduled to give a presentation on this very topic at the Battery Power Conference, in a few days: "Connecting Batteries in Parallel: Unexpected Effects And Solutions".

Assuming the 2 cells are of the same type (say, both CALB cells), because they are directly in parallel, their SOC will be identical. Yes, their series resistance will be different, but their specific resistance (measured in mOhm-Ah) will be identical because that is a fixed characteristic of the cell chemistry and construction. Therefore, as current flows into the pair, it will divide unequally, but exactly in the correct ratio so that the SOC levels of both cells remain the same as the cells charge and discharge. In any case, should the currents not divide appropriately, resulting in differing SOC dynamically, as soon as the current stops, there will be current between the 2 cells, until the SOC levels are equalized.

Something that may help you visualize this is to realize that inside a prismatic cell there are many pouches in parallel. 









Let's suppose that inside a 160 Ah prismatic cell there are 16 pouches in parallel.*
If you were to take 10 of these pouches cells and put them in a a different case, and the other six in an other case, you'd end up with a 100 Ah cell and a 60 Ah cell. If you connected the 2 together, you'd end up with a 160 Ah battery, which is electrically no different from the original 160 Ah cell. Just like you would not doubt that the pouches in the original cell are all at the same SOC, you should not doubt that the two sets would also be at the same SOC.

DO PARALLEL CELLS directly, do not parallel strings.

DO NOT DIRECTLY PARALLEL cells or batteries of different chemistries! Bad stuff can happen if you don't know what you're doing. (Ref: Fire in Prius with PHEV pack directly connected to the stock pack.)

DO NOT PARALLEL Li-ion cells or batteries unless their voltages are pretty much equal! Damage will occur to interconnections and possibly to the cells themselves, especially for high power cells, and especially for cells other than LiFePO4.

*That is a simplification.


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## Ziggythewiz (May 16, 2010)

Elithion said:


> DO PARALLEL CELLS directly, do not parallel strings.
> 
> DO NOT DIRECTLY PARALLEL cells or batteries of different chemistries! Bad stuff can happen if you don't know what you're doing. (Ref: Fire in Prius with PHEV pack directly connected to the stock pack.)


Sure, if you have to buy a BMS you're better doing parallel->series. Also, if you're using 20AH or smaller you're better off going that route. However, for anything larger you're not getting any benefit (one bad cell would kill his buddy) and have greater complexity.

My floodies and LiFePO4s get along quite well.


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