# 60ah + 90ah parallel



## onegreenev (May 18, 2012)

No real issues. As long as you match every pair the same so the whole pack is now 150ah. You may have issues with connections because one cell is a different size but you can parallel a 60ah and a 90ah to get your 150ah cell.


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## arklan (Dec 10, 2012)

i thought the way it worked, it would take power from both batteries equally
so it will take 60ah out of the 60ah battery and also take 60ah out of the 90ah battery so you have 30ah unusuable.
i know it works like this with series but i thought it was like this with paralel too??


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## bwjunkie (Jul 31, 2013)

arklan said:


> i thought the way it worked, it would take power from both batteries equally
> so it will take 60ah out of the 60ah battery and also take 60ah out of the 90ah battery so you have 30ah unusuable.
> i know it works like this with series but i thought it was like this with paralel too??


I'm pretty sure it drains the 60ah faster, so both are always at the same voltage, but the 60ah also experiences a 30% higher rate of discharge C.


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## bwjunkie (Jul 31, 2013)

onegreenev said:


> No real issues. As long as you match every pair the same so the whole pack is now 150ah. You may have issues with connections because one cell is a different size but you can parallel a 60ah and a 90ah to get your 150ah cell.


How is that different from having two complete 144v packs, one is 60ah and one is 90ah, and the you just parallel those two packs? 

josh


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## Tomdb (Jan 28, 2013)

The current sharing (And thus the power) all depends on the resistance of the cells (voltage drop underload). The pack which sags the hardest delivers the least power (if your using the same model of cells the 90ah will deliver more because of its lover resistance).

This is true for a setup where you first parallel a 90ah and a 60ah and then place them in series, AND also for if you have a pack of serial 90ah cells and then a pack of serial 60ah cells in parallel. This means there is no difference, however in practice maintaining a good balance paralleling first is desired and this also reduces the amount of voltages that should be monitored.


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## electricpolo (Mar 15, 2011)

*Re: 60ah + 90ah parallel WILL NOT WORK*

Yes, you will have a 150Ah pack but *THIS WILL NOT WORK* the way you think it will! 
This arrangement will kill the 60Ah cells very fast. 
For example, take a recommended continuous discharge rate of 3C. 
For the 90Ah cells, this is 270A but for the 60Ah cells it is only 180A.
When your controller asks for 270A, the 90Ah cells are discharging at 3C but the 60Ah cells are discharging at 4.5C. The 60Ah cells would discharge more than twice as fast because at 4.5C they would sag a lot more than the 90Ah cells at 3C. You will be overstressing the 60Ah cells at this discharge rate. 
You would need some sort of current limiting which is simply not practical.
You will also need some method of disconnecting the 60Ah cells out of the traction circuit when they reach their lower limit. Also not practical.

You would also have problems charging this configuration as the 60Ah cells would be fully charged much faster. Some method of disconnecting them from the 90Ah cells would be needed as the 60Ah cells would be finished but the 90Ah cells would still be in the CC charge mode.

Even having two packs of different Ah ratings will not work very well either. An associate of mine tried to do this very thing. 
Add a 40Ah pack to his 90Ah pack. He had ongoing charging problems and the 40Ah pack would discharge very quickly because of the imbalance in pack Ah sizes.
He abandoned the split pack idea within a month or two of doing it.
Even combining the same size cells in parallel only sort of works. Each pair of cells needs to be as close in internal resistance as possible otherwise you will end up with cells that are all over the place & you also have twice the number of high current connections.

For the most reliable setup with the longest life, use the biggest cells you can afford. 
Only use cells with the same Ah rating and only connect them in series. 
Forget parallel/series combinations of same Ah rating cells - not worth the trouble. Just get bigger cells in the first place.
Don't even bother with parallel/series combinations of different Ah cells - you WILL kill the smaller cells very fast.


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## Tomdb (Jan 28, 2013)

If you have the cells collected parallel, one wont sag more then the other, unless u have very long connections.


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## electricpolo (Mar 15, 2011)

Sorry to say but you are wrong.
If you have a 60Ah cell & a 90Ah cell connected in parallel and you draw say 150A from the cells, the 60Ah will sag lots more than the 90Ah cell.
When both cells are fully charged it won't be very noticable but it won't take long long before you see the difference between the 2 cells.
The length of the cable is irrelevant. Try it if you don't believe me and see for yourself.


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## bwjunkie (Jul 31, 2013)

*Re: 60ah + 90ah parallel WILL NOT WORK*



electricpolo said:


> When your controller asks for 270A, the 90Ah cells are discharging at 3C but the 60Ah cells are discharging at 4.5C.


Aren't the 60ah discharging at 4.5 anyway regardless of the presence of a 90ah? 

-josh


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## MN Driver (Sep 29, 2009)

electricpolo said:


> Sorry to say but you are wrong.
> If you have a 60Ah cell & a 90Ah cell connected in parallel and you draw say 150A from the cells, the 60Ah will sag lots more than the 90Ah cell.
> When both cells are fully charged it won't be very noticable but it won't take long long before you see the difference between the 2 cells.
> The length of the cable is irrelevant. Try it if you don't believe me and see for yourself.


Actually this in incorrect. If they are wired in parallel, they have no choice but to be at the same voltage. So let's say this theoretical 150Ah pack is being drawn at say 2C and the voltage sags to say 3.15volts from a resting voltage of 3.3 volts. Since they are wired in parallel they are BOTH at the 3.15v. What will happen if the cells were mostly charged is more current will be drawn from the 90Ah cells because they will flow more current when being pulled down to that voltage. Basically to get the answer to this you would need to load both the 60Ah and 90Ah cells to whatever current they need to be pulled from them to get them to 3.15v and you can figure out how the current will be drawn from them when they are together.

The reality is, this can work but the two cells in parallel will not necessarily be at the same state of charge outside of their full charged voltage or their fully discharged voltage. If you want to be safe with this decide the max C rate you would draw from the smallest cell, say you want 8C max. The 60Ah cell at 8C would be 480 amps, if you limit your current to that C rate you decided on, especially when you are getting towards empty you aren't going to stress the 60Ah cells more than you otherwise would with a similar load.


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## MN Driver (Sep 29, 2009)

On another note, if you are okay with 144Ah, you could parallel two of the new CALB CAM series 72Ah cells together. ..or wait until the 80Ah version is available and if you need more than 144Ah, you could get 160Ah. Those new cells are smaller and lighter for the same capacity as the previous generation of CALB cells and probably more energy dense than most of the large format LiFePO4. I'm not aware of any LiFePO4 that can pack more energy with less weight or volume at this point and in the past 90Ah cells have been less energy dense than other sizes for most brands.


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## dougingraham (Jul 26, 2011)

bwjunkie said:


> I understand 2x 60ah cells can be connect in parallel to yield 120ah cell.
> 
> What about one 60ah and one 90ah to achieve 150ah? Does this produce an issue with charging/discharging?
> 
> Josh


No issues other than the physical interconnects due to the differences in the sizes of the cells.

The cells will share the load in a manner appropriate to their size and likewise will charge exactly the way you would hope assuming the same type of cell (LiFe paralleled with LiFe).

This works because the weaker cell would sag more under the load than the stronger cell but because they are paralleled it cannot (sag more). So instead it sources less current than the stronger cell. When charging the weaker cell would have a higher voltage but because it is paralleled it cant so it accepts less current.

Now you could have an issue if the smaller cell is significantly stiffer than the large cell. This could happen if say you were using new 60AH CALB CA series paralleled with 2007 Thunderskys and you are pulling more than the burst current of the smaller cell for extended periods. If the average current is less than the continuous rating of the smaller cell there will still be no long term issue. Most of the time you will not have any issues.

Best Wishes!


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## rmay635703 (Oct 23, 2008)

electricpolo said:


> Sorry to say but you are wrong.
> If you have a 60Ah cell & a 90Ah cell connected in parallel and you draw say 150A from the cells, the 60Ah will sag lots more than the 90Ah cell.
> When both cells are fully charged it won't be very noticable but it won't take long long before you see the difference between the 2 cells.
> The length of the cable is irrelevant. Try it if you don't believe me and see for yourself.


I have and you are very wrong.

I have done what you are describing with fla, lifepo4 and mixes thereof for years.

Both cells end up at about the same voltage during discharge because it is not possible to do otherwise so long as you parallel each cell.

If each cell is connected in parallel (yes it requires more interconnects) you can choose which cell is hit the hardest by interconnect sizing and primary location.

If your primary wiring links all the 90ahr and the 60ahr are buddied off using a lighter gage of cable I guarantee under heavy amps the 90ahr will sit at a slightly lower voltage than the 60ahr but both will recover to the same voltage.

Each cell only contributes what it can based off of voltage, if a 60ahr cell is less capable it sags slightly below the 90 which levels the discharge rate between the two.

Balancing currents then come into play once the load stops so you loose a small amount of efficiency

But it does not behave in any sort of destructive way as you describe.

I can't completely discharge a 60ahr cell while the 90ahr cell is sitting in parallel and full unless I discharge to the point that both are empty.

Cheers
Ryan


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## pgt400 (Jul 12, 2008)

Yes you can connect them in parallel. They will be at the SAME voltage (Dah)....so impossible for 1 to over charge or over discharge. The 60 will match the 90's charge rate.


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## Nabla_Operator (Aug 5, 2011)

In this story you can take the internal resistance of the cells into account, to explain how much current is going through each of the two cells (being connected in parallel). The 90Ah takes a larger share of the total current, because the resistance is less. It is less, because it (physically) contains more "pouches" in parallel, or it has larger pouches than the 60 Ah cell.


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## joewaters (Nov 17, 2013)

I'm new here, but I thought this discussion was within my realm of understanding, so I could give my two cents worth.
The scenario is that parallel sets of 60ah and 90ah batteries will sag the 60's harder than the 90's because the current will be greater.
My simplification is to liken it to jump starting a motor in an ICE. When I connect the 12v potent battery to the system with a 'dead' 12v battery, the dead battery draws current off the charged one, as well as the starter (once the discharged battery builds enough to help for those who are willing to wait) and the car starts. This combination is a parallel connection where the higher ah battery feeds both the lower ah battery and the load. I've never seen the discharged battery fail because of the jumpstart. And the load will only draw the maximum current available from each battery, regardless of nearby cells.
The difference I see would be in a charging circuit that feeds current into a cell beyond its capacity, because it expected all the cells to accept the same fill.
Please critique this position, as I have much to learn.


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## Nabla_Operator (Aug 5, 2011)

Two resistors in parallel: 
0.0006 Ohm and 0.001 Ohm (comparable to the 60 and 90 Ah battery: see here)

There will be more current thru the smaller resistor (= the big cell). And for certain both voltage drops are equal, because they are hardwire connected.


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## green_EV 2000 (May 2, 2014)

dear:


Not DO this the case, so the battery very quickly scrapped if you to do this , if you want to 150AH, you can find a single monomer is 150AH or two 75AH, 3 pcs 50AH can, but must be the same for each cell capacity .


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## dougingraham (Jul 26, 2011)

Jack_winston said:


> Not DO this the case, so the battery very quickly scrapped if you to do this , if you want to 150AH, you can find a single monomer is 150AH or two 75AH, 3 pcs 50AH can, but must be the same for each cell capacity .


Jack, as mentioned previously in this thread there is no real issue with paralleling cells of differing capacities as long as the chemistry types are in the same family. The currents share exactly the way you want them to due to the fact that they are in parallel and behave the same way as parallel resistances do. A cell is simply a bunch of paralleled anode/cathode pairs connected in parallel inside the plastic case. There is no difference between paralleling them inside the plastic case or outside the plastic case.


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## bwjunkie (Jul 31, 2013)

After putting batteries together 60+60, I'm finding this topic is more complicated than the surface level theory of "voltages are forced to be the same if they are parrellel" which I now believe is NOT true. They will eventually equalize but it takes TIME and can be quite a lengthy time, hours or days, depending on the internal resistance and SOC difference. .1v difference going into a 2ohm battery takes how long to transfer a 5ah difference? Meanwhile the charger pushes lots of energy into both cells and the higher voltage battery will go above it's limits alone and get damaged unless (in my case) the BMS prematurely shuts the charger off.

I am not knowledgale on the subject, but thinking it through based on my initial real-world data. I'm postulating, that energy energy cannot transfer quick enough across the parrellel circuit. I could be wrong, lets hope so. 

Comments?
Josh


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## Hollie Maea (Dec 9, 2009)

bwjunkie said:


> Comments?
> Josh


You seem to be confusing voltage and state of charge. Given a sufficient interconnect, two batteries in parallel WILL have the same voltage. That's just ohm's law.


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## gunnarhs (Apr 24, 2012)

bwjunkie said:


> I understand 2x 60ah cells can be connect in parallel to yield 120ah cell.
> 
> What about one 60ah and one 90ah to achieve 150ah? Does this produce an issue with charging/discharging?
> 
> Josh


Normally assuming that your 60Ah and 90Ah are from the same producer/pack type/cell structure this should theoretically not be a problem,.
When using you can assume that the 3,2V 60Ah pack contains 6 parallel cells and the 3,2V 90Ah contains 9 parallel cells of THE SAME KIND.
So you have 15 parallel cells , the same as if you would use one 150Ah pack from the same company/pack/cell type.
Here you can see some formulas and charge/discharge/balance-times for different chemistry.
http://liionbms.com/php/wp_parallel_balance.php.
According to it you just have to ensure that the cells have the same initial/optimal SOC when paralleling the first time, after that they should stay balanced. 

However (and that is my experience at least with Thundersky cells) have I run into a problem if I use more than 20 cells parallel. Regardless of if 3 100 Ah or two 150 Ah (both 30 cells). But I had a discussion with other users (see Tesla battery thread ) and they have not had this problems, at least not with other cell types.

Addition: I would though buy the 150 Ah cells if I had the choice and place in the car. Just a gut feeling (for.example regarding temperature measurement of cell pack, same line of production ), not any good arguments here


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## bwjunkie (Jul 31, 2013)

Hollie Maea said:


> You seem to be confusing voltage and state of charge. Given a sufficient interconnect, two batteries in parallel WILL have the same voltage. That's just ohm's law.


The busbar circuit will have approximately only 1 voltage measurement. But the battery resistance internally creates larger picture with more complex circuit where many voltages can be present. Or no?


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## Hollie Maea (Dec 9, 2009)

bwjunkie said:


> The busbar circuit will have approximately only 1 voltage measurement. But the battery resistance internally creates larger picture with more complex circuit where many voltages can be present. Or no?


The voltage is at the terminals. The cell resistance is internal. The ONLY way you can have a voltage difference between the cells at the terminals is for current to be flowing from one cell to the other. And the voltage difference will depend on the resistance of the interconnect (which is extremely low) and the current flowing across.

Let's say your cell is connected by a bus bar equivalent to 2/0 wire, one foot long. That's a resistance of 0.078 milliohms. With contact resistance, let's round it up to 100 micro ohms. So to have a voltage difference of just one millivolt would require ten amps to be flowing from one cell to the other.

But wait: in order for that current to flow through the bus bar, causing the voltage difference, it also has to flow through the two cells, from the higher one to the lower one. But the cells have internal resistance much higher than the busbar, so the flowing current pulls down the high one and pulls up the low one, which slows down the current until it reaches the equilibrium. Current will flow between the two until they are both at a state of charge that corresponds to the shared voltage between them. But as long as your interconnect resistance is significantly lower than the cell resistance (that should always be true), there cannot be a significant voltage difference between the two. Now the state of charge may or may not be equal when the voltage is equal. If it is not, then as they diverge more current will flow to keep the voltage equal. But they will always be aggressively self balancing in terms of voltage. And if they are the same chemistry, the SOC should also stay closely balanced, and you should have no problems with this arrangement.


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## dougingraham (Jul 26, 2011)

bwjunkie said:


> The busbar circuit will have approximately only 1 voltage measurement. But the battery resistance internally creates larger picture with more complex circuit where many voltages can be present. Or no?


Every connection is a resistor. There are no zero resistance interconnects. If you place two cells in parallel there will be some current flow between the cells across the interconnects. Because of this paralleled cells with a different state of charge will cause current to flow between the cells until the voltages equalize. How long will this take? In reality it will never happen because as the voltage difference diminishes so does the current flow. But there will be a point at which they are close enough as makes no difference.

You can speed this equalization process up by charging and discharging the block of paralleled cells. During charge the cell with the lower voltage will accept more of the charge than the cell with the higher voltage because the voltage difference is greater. During discharge the cell with the higher voltage will provide more current than the cell with the lower voltage. All the while there will still be current flowing between the cells in the correct direction to bring the cells into balance.

One thing you need to take into account is trying to equally load the paralleled cells. Lets say you paralleled 20 cells in order to make a larger capacity cell. Something like Tesla does only in smaller scale. I will use a capital I to represent a single cell. The ++++ symbols are the positive bus bar and the ----- symbols are the negative bus bar.


++++++++++++++++++++
IIIIIIIIIIIIIIIIIIII
--------------------


If you connect the charger to the left end what will happen is that the cells on the left side will tend to accept more charge even if the cells on the far right are at a lower state of charge. This is because of all the resistance of the bus bars between the cells. A load has a similar effect in that it will discharge the cells closest to the load more than the ones far away. The correct way to do this is to connect the positive lead on one end and the negative lead on the opposite end. Every cell will see the same charge and discharge current from the outside assuming the resistance of the connection to that cell is the same as all the others. The above configuration where you have a linear string is the worst case when paralleling. Keep the resistance of the interconnects in mind when paralleling cells.


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## bwjunkie (Jul 31, 2013)

okay thanks guys, all 3 responses make sense to me and I am perhaps wrongly analyzing the unexpected data I was seeing upon the first charge.
Perhaps since my 2p sections were not well equalized to begin with as gun advised would be wise.


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