Very Simple String-Capacity Question

[The Mighty Volt]

I have 10 Cells.

The capacity of 9 of those cells is a proven 2.0Ah at 1c discharge.

I have a 10th and final cell of a proven 1.7Ah at 1c discharge.

I then parallel all 10 cells into a 1s10p string.

Question: What is the final capacity of my string? Is it 19.7Ah {at 1c} or is it 17.0Ah {at 1c}

Does the answer depend on the position of the lower-capacity cell in the string?

Thanks.

 

[999zip999]

I seems funny how one weak cell has on bring down the level of a hole string. Can't tell you the numbers but has a great weight on the math.

 

[neptronix]

In parallel, it does not matter what the capacity is, thus 9 x 2 = 18 + 1.7 = 19.7AH.

In serial, it matters since you need to stop discharging immediately when that one cell hits LVC. So if these were all in series, you'd have ~1.7 ah and 10S.

 

[SamTexas]

For parallel configuration the final capacity is the sum of all individual cells. So 19.7Ah.

 

[Spacey]

I believe the other strong cells will charge up the weaker cell somewhat to bring it into line.

 

[The Mighty Volt]

I believe the other strong cells will charge up the weaker cell somewhat to bring it into line.

You see this is where I get confused, and where I have been confusing myself....the max capacity of a cell is the max capacity of a cell.

The other cells can charge it but the most they can charge it.....is to 1.7Ah!!!

 

[The Mighty Volt]

For parallel configuration the final capacity is the sum of all individual cells. So 19.7Ah.

Thanks......I hope thats the case!! More Ah is better.

I was concerned that the capacity of one cell could become the capacity of all cells.

I do know that the capacity of ONE STRING is the capacity of ALL STRINGS.

So a loss which is avoided in a particular string would re-surface across several strings.

In otherwords, the capacity of the pack is the capacity of the weakest string, multiplied by the number of strings in parallel.

So, if we have one string that is 19.7Ah and 7 other strings that are 20Ah, and we series all the strings, we end up with a 24v {8s} 19.7Ah pack.

 

[SamTexas]

Thanks......I hope thats the case!!

You're welcome. Yes, that's the case.

So, if we have one string that is 19.7Ah and 7 other strings that are 20Ah, and we series all the strings, we end up with a 24v {8s} 19.7Ah pack.

Correct. In series configuration, the capacity of the weakest cell is the capacity of the whole pack.

BTW, what kind of battery are you using? 8s is 26.4 (LifePo4) or 29.6 (LiCo).

 

[dogman dan]

Yes , correct.

 

[The Mighty Volt]

Thanks......I hope thats the case!!

You're welcome. Yes, that's the case.

So, if we have one string that is 19.7Ah and 7 other strings that are 20Ah, and we series all the strings, we end up with a 24v {8s} 19.7Ah pack.

Correct. In series configuration, the capacity of the weakest cell is the capacity of the whole pack.

BTW, what kind of battery are you using? 8s is 26.4 (LifePo4) or 29.6 (LiCo).

12s LiFePo4 of spurious/unknown C rate, as many in parallel as I can afford, with another 12 of genuine M1 A123 on the other side of my bike, for a finished 72v pack.

I'll probably go 15p with the "Unknowns" {1.1Ah cells} and 8s with the genuine M1's, the plan being to use only 10Ah, to keep everything sweet.

 

[The Mighty Volt]

Yes , correct.

Cheers!

 

[John in CR]

Thanks......I hope thats the case!!

You're welcome. Yes, that's the case.

So, if we have one string that is 19.7Ah and 7 other strings that are 20Ah, and we series all the strings, we end up with a 24v {8s} 19.7Ah pack.

Correct. In series configuration, the capacity of the weakest cell is the capacity of the whole pack.

BTW, what kind of battery are you using? 8s is 26.4 (LifePo4) or 29.6 (LiCo).

12s LiFePo4 of spurious/unknown C rate, as many in parallel as I can afford, with another 12 of genuine M1 A123 on the other side of my bike, for a finished 72v pack.

I'll probably go 15p with the "Unknowns" {1.1Ah cells} and 8s with the genuine M1's, the plan being to use only 10Ah, to keep everything sweet.

Everything was fine until you brought up combining different cell types in series. While you can do it, your M1 cells will go to waste, because if you try to draw high power, then you will kill the other pack. Also, 1p of M1's, whether it's 8s or 12s won't do much of anything in terms of range, because that's just 2.3ah.

 

[The Mighty Volt]

Hi John, that "s" is meant to be a "p". 12s8p

One battery mounted on one side is A123 M1, 12s8p

One battery mounted on the other side is LifePo4, 12s16p {18650's}

Batteries are then series-joined to form a 72v 16Ah nominal pack.

Desired Ap is 80-90amps.

The LiFePo4 appear to be 15c capable. M1 speaks for itself, especially at 8p.

Its not 1c rated stuff.

I can't foresee any issues there, but if you can, I would be sure grateful to hear of them.

Critically, my LiFePo4 sub-pack is itself capable of a relatively high-discharge, and considering the setup {20" 5305} the demand for high amps rarely lasts long.

 

[John in CR]

Since you're building your own packs, what you want to do is make the two 36V packs identical. Put 4p of your A123's in one and 4p in the other. The testing I did today proved how well it will work. Regarding your other Lifepo4, it's very unlikely to be 15C, and the "unknown" cheap cells you mentioned are definitely not 15C. Combining different types of cells that have the same voltage requirements in parallel in one pack is no problem, but combining different types in series is problematic. It will be very difficult to match the power capacity and virtually impossible to perfectly match the storage capacity, so at the very least it will be wasteful.

OTOH if you build the two packs identically, with a blend of the M1's and lower power cells in each, then you'll effectively have 2 high power 36V packs. The wiring will still be the same, since both need balance taps, and the physical structure combining them is simple. That's because the cell lengths are the same....FYI 18650's are 65mm long as are 26650's, which is the M1 size. 18mm and 26mm are the respective diameters.

BTW, Combining 15 1C cells in parallel doesn't give you a 15C pack. It's still a 1C pack, and if they're cheapie cells you'll be lucky to get 15A continuous out of them, and added in series with an A123 pack then both will end up limited to that same 15A max.

 

[The Mighty Volt]

The cells are Ebay A123's which I tested , an 8s1p string gave up 800mah during testing with a geared hub with discharges of up to 15A bursts {10seconds} and 5A constant. I also discharged them using the Turnigy at 5A and charged them at 7A.


However, I am listening to you.....

You reckon that if I mix and match, 50-50, that I will be far better off?? Its still possible, the strings are still out on my worktop, I can still do it.

And yes, you are correct, the cells are both 65mm high so there won't be any major difference there.

It should be fun arranging them in a sub-pack though, as they won't sit flush anymore!

 

[theRealFury]

Hi John, that "s" is meant to be a "p". 12s8p

One battery mounted on one side is A123 M1, 12s8p

One battery mounted on the other side is LifePo4, 12s16p {18650's}

Batteries are then series-joined to form a 72v 16Ah nominal pack.

Desired Ap is 80-90amps.

The LiFePo4 appear to be 15c capable. M1 speaks for itself, especially at 8p.

Its not 1c rated stuff.

I can't foresee any issues there, but if you can, I would be sure grateful to hear of them.

Critically, my LiFePo4 sub-pack is itself capable of a relatively high-discharge, and considering the setup {20" 5305} the demand for high amps rarely lasts long.

I really dont think this is a good idea, for a strong cell to help a weak cell it would need to be in a parallel connection with it, not in a series connection. In the setup you describe all that will happen when you hit the throttle is your weak unknown brand lifepo4 (which is probably low C rate) will sag like double J's on a granny while the Legit A123 M1's will just match what ever current the other pack can put out. This would surely kill the unknown cells pretty quickly while the A123 will be twiddling its thumbs???

For your idea to work you would need to build each pack as 72v half the AH rating then parallel the 2 packs together so that the A123 can support the lower quality cells under high load when they sag which will drain the A123 more than the unknown cells, then when under light/no load the unknown cells will be higher voltage therefore supplying more current / feeding back into the A123 pack.

 

[John in CR]

Lower power cells can deliver more current and be charged quicker than the manufacturer recommends. eg I've pulled 80A from a 15ah Ping pack while bypassing the BMS. Lithium batteries have no problem killing themselves following your orders. Doing so reduces their life.

My only concern with the combining arrangement is premature death of a cheapie taking it's adopted brothers in parallel with it. Hit them with some cycles before pack assembly to weed out bad cells.

I realize you'll be anxious to get flying on the pack, but extra care and time to build a really solid pack can easily mean the difference between a pack that has problems in days or weeks, or one that runs trouble free for years.

 

[The Mighty Volt]

Lower power cells can deliver more current and be charged quicker than the manufacturer recommends. eg I've pulled 80A from a 15ah Ping pack while bypassing the BMS. Lithium batteries have no problem killing themselves following your orders. Doing so reduces their life.

My only concern with the combining arrangement is premature death of a cheapie taking it's adopted brothers in parallel with it. Hit them with some cycles before pack assembly to weed out bad cells.

I realize you'll be anxious to get flying on the pack, but extra care and time to build a really solid pack can easily mean the difference between a pack that has problems in days or weeks, or one that runs trouble free for years.

I hear you, I hear you What I will probably do is break the packs down now and mix them. I am still going to go with 36v 14Ah for each pack and then series then.

I should have 4A123 with 6 of the 18650, per string. 12 of those strings then in series, and 2 of those sub-packs in parallel.

Its going to a PITA getting strings to sit down on each other now, even with the A123 all together and at one end of a string, I will need some sort of a filler for the gaps, something like a hollow cardboard tube would do.

What I will do is build up the 36v sub-pack, add the balance taps {two sixes or an 8 with a 4??} and then get cracking with those hub-motor discharges, a few of them and see whats what after 8Ah or hard work should be telling, a few charges and see how they come back up.

Cheers.

 

[amberwolf]

Definitely if you put all of the good type of cell in one pack, and all of the bad type in a separate pack, then series them, you just turned the good pack into the same capability as the worst of the bad pack.


If you put half the good cells in parallel with half the bad cells, in one pack, then the same in the other pack, and *then* series those, you have a reasonably workable scheme that will still let the good cells do what they are good at, and the bad cells do whatever they can manage.

Even just putting all the good cells in one pack and bad cells in the other, and paralleling the packs at pack level instead of cell level will still work way better than seriesing them.

 

[The Mighty Volt]

Definitely if you put all of the good type of cell in one pack, and all of the bad type in a separate pack, then series them, you just turned the good pack into the same capability as the worst of the bad pack.


If you put half the good cells in parallel with half the bad cells, in one pack, then the same in the other pack, and *then* series those, you have a reasonably workable scheme that will still let the good cells do what they are good at, and the bad cells do whatever they can manage.

Even just putting all the good cells in one pack and bad cells in the other, and paralleling the packs at pack level instead of cell level will still work way better than seriesing them.

Okay so my new plan is as follows:

Parallel the cells at cell level in two 36v 14Ah sub-packs- 12 strings of cells per sub-pack, each string made up out of 6 18650 and 4 A123, for a nominal Ah of 14

Then I series both sub-packs into a 72v pack.

Is this what I need to do?

Cheers.

 

[amberwolf]

Sounds like a better plan than before.

Just remember that AFAICR, the A123 have a slightly different HVC and LVC voltage than typical LiFePO4 cells, and make sure your charging/discharging limits take that into account.

 

[auraslip]

When the weaker cell get's hit for current it will sag lower than the other cells. Since its voltage is lower it will provide less current, and in practice it's voltage will be identical to the rest of the cells.

As you discharge it's voltage will again drop, and it won't provide current until it's voltage is the same as the other cells. Again, in practice this means the voltage of all cells in the group will be identical.

This is why a group of parallel cells i functionally and mathematically the same as ONE cell.

 

[John in CR]

Auraslip,

I did the test and the only time the low power cells had an identical voltage as the A123 group was just before I pushed the button to start the test. They definitely do not act like one single cell.

 

[theRealFury]

Auraslip,

I did the test and the only time the low power cells had an identical voltage as the A123 group was just before I pushed the button to start the test. They definitely do not act like one single cell.

Yes i can confirm that with other chemistries as well, weak cell will sag to the point where the strong cell will give most of the current, then when load drops the weaker cell voltage rises above that of the now deeper discarged strong cell so the weak cell transfers charge back into the strong one. It would therefore be a good idea to make sure the cell level connections can take some current also as they may see tens of amps transferring through them if you run under load for long enough.

 

[Erogo]

"String" seems like a bad word to use for paralleled cells. A string would normally imply series cells. Like a string of sausages. That and the fact that you had to say

Hi John, that "s" is meant to be a "p". 12s8p

makes it seem like you're confused between parallel and series! Be careful!

Eric