Paralleling different capacity packs

[Beachcruzer]

Okay, I think I know the answer to this one in theory. Wondering if anybody has real-world experience doing this. . . .

Say I want to parallel 6s lipo packs of different capacities, 5ah and 8ah to make a 13ah pack. Same number of cells, same C-rate, harness wires and connectors are identical. The only difference is the amp-hour capacity. Do I have anything to worry about?

Related question: What if I want to run an oddball combination in series, say 5s and 6s to make a hypothetical 11s pack. Now am I going to see smoke?

 

[wesnewell]

Doesn't hurt a thing to parallel different ah packs. Neither does it hurt to series different cell packs of the same type.

 

[amigafan2003]

Doesn't hurt a thing to parallel different ah packs. Neither does it hurt to series different cell packs of the same type.

But you need to bear in mind if you parallel a 5ah pack and an 8ah pack then you'll have an effective 10ah pack, NOT a 13ah pack.

This is becuase the 5ah pack will be exhausted first, leaving 3ah left of the 8ah pack. If you continued to draw current to try to use the 3ah of the 8ah pack you'll pull the voltage really low on the 5ah pack and damage it.


Also, if you parallel a couple of packs make sure they are both at the same level of charge - if you have one pack at 3.8v and the other at 4.2v then the higher pack will try to charge the lower pack - and with lipos that can happen at 30c+ - poof!

 

[Sunder]

Doesn't hurt a thing to parallel different ah packs. Neither does it hurt to series different cell packs of the same type.

But you need to bear in mind if you parallel a 5ah pack and an 8ah pack then you'll have an effective 10ah pack, NOT a 13ah pack.

This is becuase the 5ah pack will be exhausted first, leaving 3ah left of the 8ah pack. If you continued to draw current to try to use the 3ah of the 8ah pack you'll pull the voltage really low on the 5ah pack and damage it.


Also, if you parallel a couple of packs make sure they are both at the same level of charge - if you have one pack at 3.8v and the other at 4.2v then the higher pack will try to charge the lower pack - and with lipos that can happen at 30c+ - poof!

I was actually wondering what would happen if you connected every CELL with like 14 awg wire... in theory it should re balance - as the 5ah cell gets low, it gets topped up from the 8ah cell. As long as you weren't continuously drawing high amps, surely the pack would have time to re balance? as you said, it can happen at 30c+ so as long as the balancing wires held up, you'd be golden.

 

[dogman dan]

No, it will be a 13 ah pack, but at some point the 8 ah pack will have to start sending power to recharge the 5 ah pack, and supply power to the motor. So the discharge c rate will be slightly higher on the 8 ah pack.

 

[Beachcruzer]

Okay, now I'm more confused than when I asked. The theory as I understood is that the packs should stay in sync, because the 8ah cells will supply more power than the 5ah ones. C-rate is a function of capacity, right? So the power draw should come out of the combined pack at an 8:5 ratio.

Not interested in being the guinea pig on this one. Just thinking about how to fit more battery into a confined space.

 

[dnmun]

they will stay in sync because the voltage of each pack is identical. the pack with more capacity will always provide more current during discharge. ideally this would be 80/50 so the lower capacity pack will not totally discharge before the larger one. in theory. eh?

 

[Beachcruzer]

Here's what I mean. Say I parallel the 8ah and 5ah packs, plug them in and go for a ride. When my controller is pulling 13 amps, 5 amps are coming from the 5ah pack, and 8 amps are coming from the 8ah pack. Both packs are discharging at 1c.

(Actually that's a hypothetical to make the math easier. I'd want to series four of those oddball pairs together for a 24s 13 ah pack, and pull a lot more than 13 amps. Am I right on the theory or is something gonna get smoked?)

 

[Beachcruzer]

they will stay in sync because the voltage of each pack is identical. the pack with more capacity will always provide more current during discharge. ideally this would be 80/50 so the lower capacity pack will not totally discharge before the larger one. in theory. eh?

Haha, yeah, in theory.

 

[sangesf]

Here we go again... LOL

The best way to test this theory is to take two cells..

8Ah cell
5Ah cell

Parallel them... Then draw 10a for an hour. When done, pull the cells apart and see what their voltages are...
If the theory that the 8Ah cell gave more current during discharge, they will have the EXACT same voltage.
If they don't then obviously it didn't.
(p.s. I have done this with SLAs and they definitely did NOT have the same voltage.)

LiPo may work differently... Also if they don't, maybe different c-rated cells may make up the difference..

My "general statement" I usually make is this...
"If widely different AH'd cells did magically throw current around in a paralleled configuration, why don't battery makers take 1000mAh cells and mix in some 5000mAH cells and make packs that way, instead of closely matching the AH of each cell?"

If I had some spare small format cylindrical lifepo4's I would test it and as I don't (or haven't yet) played with LiPo.

 

[dnmun]

they will always be the same voltage if they are connected in parallel.

that was my point, that the 8Ah pack would have to deliver more juice to keep the voltage identical to the smaller 5Ah pack.

 

[sangesf]

they will always be the same voltage if they are connected in parallel.

that was my point, that the 8Ah pack would have to deliver more juice to keep the voltage identical to the smaller 5Ah pack.

You didn't notice I said pull the cells apart from each other and THEN measure their voltages... They will always have the same voltage while still connected.. (on my SLA tests it was an average voltage of the two).

The other test after that could be to draw from the 5ah pack and see what happens..

Hey Nep! Take a couple of cells and run the tests for us!
(you just love making then graphs!).

 

[dogman dan]

Don't forget, we are talking lipo here. I bet any difference in the packs voltage would be evened out by sitting for just a very short time. The high pack would charge the low one much quicker than lead would.

But I am making some assumpitons, like that the controller is not taking much more than half the c rate out of the packs. This extra capacity for discharge is going to be why the 8 ah pack can still keep up with feeding the motor and feeding some back into the 5 ah pack. As the 5 ah pack gets to a lower voltage, it's resistance will increase, and more and more will get pulled out of the 8 ah pack.

There is however, no telling how much the 5 ah pack would sag along the way, especially at the end of the discharge. So no, it's far from an ideal setup, and not one you would plan to do.

But if I ended up with such a set of batteries, I'd paralell them in a heartbeat, then discharge them to no more than 10 ah used. You'd want to leave that 30% in reserve anyway right? At the very worst, you'd be hammering one pack to 100% dod while the 8 ah ones got tlc.

 

[sangesf]

Well, yeah, if you pulled .5c from the packs, that would work..

The OP sounds like he he wants to pull 5C....
He could do this tho...
Pull the packs apart and go 24s,2p 8ah and parallel 24s,3p 5Ah until you get 24s 15/16Ah pack.
(the 1ah difference shouldn't cause any problems).

 

[dnmun]

what is the reason that the packs will shift in voltage when you disconnect them?

where is the logic?

i think the OP has a better handle on reality here.

 

[Beachcruzer]

Well if I had anything besides 5ah packs right now I'd run the test, but I'm not curious enough to buy some just to find out. I'm contemplating a build where the batteries would be in the 'gas tank' area of a stretched Worksman Newsboy, and the measuring tape tells me that an 8x5 13ah pack is the absolute most lipo I can squeeze into that space. If that's still the case after I cut and bend the metal, maybe I'll give it a shot.

 

[wesnewell]

You don't need to run any test. When you parallel the cells, they equalize, and they will stay equalized until they are unparalleled and drained separately. If you parallel a 20C 5ah pack and a 20C 8ah pack, you have a 13ah 20C pack. It's that simple. Meaning it will be rated for a 260A draw. Realistically, you probably wouldn't want to go over 130A.

 

[sangesf]

Someone run a test and prove me wrong...
Like I said, I don't have any LiPo to try to test with...
My tests with SLAs have shown me that paralleling two widely different AH'd batts didn't draw current "unevenly"..
And like I said before... LiPo may be different... I dont know..
Does anyone run a 10Ah LiPo battery paralleled with a 5Ah LiPo and get 15Ah out of it?
How many of you run crazy uneven packs?
There is a lot of conjecture, but as of yet, no proof.. We need some.

P.S. I don't know if it's the best idea to have LiPo built into a frame of a bike with no way of getting to it easily..

 

[Beachcruzer]

P.S. I don't know if it's the best idea to have LiPo built into a frame of a bike with no way of getting to it easily..

The lipo's not built in to the frame. The battery box is built to fit between the dual top tubes of the Worksman frame, where the gas tank on a gasser would be.

 

[neptronix]

The amount of misinfo in this thread is totally frustrating.. of course you are confused, beachcruzer..

Packs in parallel will equalize each other's voltage as quickly as they can - this is one reason why i tell people to not connect batteries in parallel that have highly differing voltages..
Packs in parallel will provide whatever fraction of the current they can as well.
Even the most brain dead RC airplane flyer knows this; so they parallel all sorts of weird mAH configurations to jam the most watt-hours they can into their little planes.

The only thing that would prevent perfect current sharing + voltage sync from happening is if you had a weak parallel link that couldn't transfer the current fast enough, for example if you connected your batteries in series with 20AWG wire and were pulling a few amps - the pack closest to the current draw would drain quicker and the 20AWG wire would act like a resistor and gradually trickle the current out... then, and only then, would there be a difference in voltage between the paralleled batteries.

I wonder how y'all think that an EV car battery pack, with dozens to hundreds of cells in parallel, manage to survive more than a few cycles? proper parallel current sharing!

 

[dogman dan]

Those sla's were trying to equalize in voltage, they just were pokey about it for some reason. Lipo will equalize fast, if it can't because of the wiring, the wiring goes poof.

The lead would have equalized in voltage if you gave it some time. This is what goes on inside a pack with paralelled equal capacity cells all the time. The weak cell with low capacity gets charged by the other cells after getting pulled down. This is why a 20 ah battery lasts more cycles than a 10 ah, more capacity helping any weak cells.

You don't see mismatched cells in any EV, because they design it so there is no particular reason to use different sizes.