Minimum mAh per Cell for DIY Powerwall

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caosmen

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Hi Guys,

i am harvesting since 3 Month cells for a first goal of 20 kwh.

My Question is what minimum amount of mA per Cell would you choose to take cells or dont use them since its alot of work and the powerwall should run a very long time.

My first E bike packs spend me alot of 2600 mAhor more cells.

Right now i am getting like 600 cells with just 1800 mAh.

My first idea was a range 2400 mAhor more to go into my Powerwall. But i dont know if that is to high or ok.

Your Opinions will help me alot.

Thanks for reply and have good christmas days guys

Nils
 
I try to use 2500-3000 mah.

Doesnt matter really, you just loose density if you use smaller capacity cells and use more material packaging cells into batteries.

I am more concerned with how healthy the batteries are. If I have a ton of healthy 1800mah cells, I would use them. If those cells were originally 3000 mah cells, I would think twice as they have lost a lot of capacity from new.
 
For me - its a matter of physical space. My battery/pack design is maxed at65 cell spotsand I need 130ah per pack - so 130/2 = 65 cells. When I use 2600mah cells then 130/2.6 = 50cells which fit in 65 spot pack design. If I went 1800 it would be 130/1.8 = 72cells.. and so I couldn't build a pack with 130ah to match all the others.

If the original cell has spec of 1800mah and the tested at 1800mah - then GREAT. That's better than a cell that has a 3000mah spec and tests 2400mah (for example).
 
Thanks alot guys.


Space is not a real problem, i could add with 2400 mAh around 60 kwh in my cellar room.

But sounds like i better just use Cells that lost 10-max 15 % from a new cell.



CGR18650CG(2200 mAh) i have right now around 500 with 1850-1950 mAhtested. Looks like you guys would use them too.
NCR18650PF 2900 mAhalso around 400 right now and less then 5 % lost.

thanks alot guys. will harvest more and more read as mutch as i can.
 
Its mAh/Ah not mA.

A = Current
Ah = Capacity.
 
caosmen said:
My Question is what minimum amount of mA per Cell would you choose to take cells or dont use them since its alot of work and the powerwall should run a very long time.
Click to expand...

I look at age, design capacity, degradation, etc. For example, most of the cells I am using are 2600mAh. If it tests at 2199mAh or below, I put them in the recycle box. But if I get a batch of cells that were 2200mAh rated, I'll generally take them down to 2000mAh. I haven't used anything below 2000mAh regardless of factory spec as those tend to be very very old and just don't feel they're worth my time.
 
Harvest all cells you have. Then you start adding the cells from the highest bucket and fill upp with lower untill all packs have equal amount of each type of cell.
Then you dont really decide. You just build.

Note that you can of course sort so the 2xxx bucket have y amount of cells to cope with 90% of the total needed cells.

Dont ever build your packs untill you have all cells tested and sorted because you will end up with uneven packs! You can for instance get a really bad batch of last cells and then you suddenly have a pack of crappiness installed.
 
Thanks for all replys. i will make a new thread now with my current setup.

daromer said:
Dont ever build your packs untill you have all cells tested and sorted because you will end up with uneven packs! You can for instance get a really bad batch of last cells and then you suddenly have a pack of crappiness installed.
Click to expand...

My goal is a 20 kwh battery. But right now i get every month 1000 cells for free to test and check. all cells are coming from "dead" e bike batterys so i could end up here with 10000 tested batterys and then start with packs.

So you advice to better get my goal done and then starting the packs.

Ok will do that. since i am a beginner.

thanks alot
caos
 
Get enough cells first before assemble. Then you can ensure that you have even spread of cells in each pack and hopefully get packs that lasts longer.
 
I'd like to jump on this old thread because I think it relates to my question that I have in planning stage.

I'm planning 24p packs in a 7s string. Taking inspiration from here, though I'm planning 7s instead of 14s. Setup should be similar. I love this idea because of it's ability to add capacity one 7s string at a time as I get more 18650s.

Question is this: in this hypothetical setup that I'm planning, when I add capacity, what are the mAh requirements, if any? I know that voltage needs to be matched up incredibly close before adding a new string. I know that adding capacity could increase draw requirements, so the fuse requirements to the main bus need to be considered. Just wondering about capacity for now. An example:

I have 24p 7s string. Each cell averages 2000 mAh. This gives me 48Ah for this 7s (let's assume I built the packs nice and balanced with pack builder)
I have collected more batteries and want to add another row. All these new batteries are better, they average 2200mAh per cell. If I make a 7s string from these better batteries, I get 52.8Ah.
Can I add this new string to the main powerwall without issues? I'll have just turned a 24p7s pack into a 48p7s pack. What are the balancing difficulties that I just introduced to the build, which my BMS has to deal with? (I am going to select and purchase a good, high rated BMS that will be overkill for the first string, but should be able to keep up as I add more capacity)

My goals are to create a physical battery storage layout that lets me easily add capacity as I gather more batteries. Easily, in that I can pop them into the shelf without disassembling anything crazy, and easily, in that I don't have to risk massive sparks or fire damage each time I add a row. Thanks for the input!
 
harrisonpatm said:
I'd like to jump on this old thread because I think it relates to my question that I have in planning stage.

I'm planning 24p packs in a 7s string. Taking inspiration from here, though I'm planning 7s instead of 14s. Setup should be similar. I love this idea because of it's ability to add capacity one 7s string at a time as I get more 18650s.

Question is this: in this hypothetical setup that I'm planning, when I add capacity, what are the mAh requirements, if any? I know that voltage needs to be matched up incredibly close before adding a new string. I know that adding capacity could increase draw requirements, so the fuse requirements to the main bus need to be considered. Just wondering about capacity for now. An example:

I have 24p 7s string. Each cell averages 2000 mAh. This gives me 48Ah for this 7s (let's assume I built the packs nice and balanced with pack builder)
I have collected more batteries and want to add another row. All these new batteries are better, they average 2200mAh per cell. If I make a 7s string from these better batteries, I get 52.8Ah.Can I add this new string to the main powerwall without issues?
Click to expand...
If you have 2 separate 7s batteries, then it's better if all 14 packs of each battery have close to the same ah. 24cells * 2000mah = 48,000mah. 48,000mah / 2200mah = 21.8cell - e.g. 22cell packs would match up for battery #2. Keeping each pack 'very close' will ease things.

If you are paralleling each pack for a larger, single 7s, then sure, 24 * 2200mah = 52,800 + 48,000 (original pack) = 100,800mah per pack. Again, it's best if overall, each pack in series has the same total mah.

harrisonpatm said:
I'll have just turned a 24p7s pack into a 48p7s pack. What are the balancing difficulties that I just introduced to the build, which my BMS has to deal with? (I am going to select and purchase a good, high rated BMS that will be overkill for the first string, but should be able to keep up as I add more capacity)
Click to expand...
Yes, you may be able to use the original BMS. The only issue with larger and larger packs is that the BMS will take longer and longer to balance as the BMS balance current is 'fixed' but the pack it's balancing is now twice as big / takes twice as much power to balance.

At 7s48p - I wouldn't worry to much and just try the original BMS. Once you get to 200p then it will be a consideration.

Also, once balanced, healthy packs don't need a lot of balance. You can brute-force balance them by paralleling them before re-arranging into 7s and then a lighter BMS may be able to take if from there.

harrisonpatm said:
My goals are to create a physical battery storage layout that lets me easily add capacity as I gather more batteries. Easily, in that I can pop them into the shelf without disassembling anything crazy, and easily, in that I don't have to risk massive sparks or fire damage each time I add a row. Thanks for the input!
Click to expand...
I also started this way - but over time I built up to 14s100p batteries of 48v@260ah each as my standard/max 'battery' size. This is about 130lbs. Once I put them in operation, I found that I didn't need to fuss with them - the oldest has been sitting on the shelf for 3 years now.
 
OffGridInTheCity said:
Yes, you may be able to use the original BMS. The only issue with larger and larger packs is that the BMS will take longer and longer to balance as the BMS balance current is 'fixed' but the pack it's balancing is now twice as big / takes twice as much power to balance.
Click to expand...
I was thinking about getting a BMS that can handle a huge 7s bank. Then when I put my first 7s24p, the BMS will be a Godzilla compared to puny chimpanzee it's hooked up to manage. As I build and add each string, the chimp gets bigger until it becomes a huge King Kong that's more evenly matched to the Godzilla BMS. Also, it's an investment. Expensive, high quality BMS now with a small pack will last longer than a small BMS and a battery bank that quickly outgrows it, necesitating purchasing another BMS.

But to confirm your earlier response. Are you saying that once I build my first string of 24p packs at a certain capacity, say 48mAh, I have to keep each string within that capacity? Within a certain range? Because your suggestion of using 22 higher capacity cells for the second string to match the lower capacity string that came before it... doesn't IR come more into play then? And individual cell draw?

I want to know if I should scrap this idea now before I should even start it. Only selecting cells in the future within a certain range isn't a dealbreaker, just a difficulty that needs to be considered.
 
harrisonpatm said:
I was thinking about getting a BMS that can handle a huge 7s bank. Then when I put my first 7s24p, the BMS will be a Godzilla compared to puny chimpanzee it's hooked up to manage. As I build and add each string, the chimp gets bigger until it becomes a huge King Kong that's more evenly matched to the Godzilla BMS. Also, it's an investment. Expensive, high quality BMS now with a small pack will last longer than a small BMS and a battery bank that quickly outgrows it, necesitating purchasing another BMS.

But to confirm your earlier response. Are you saying that once I build my first string of 24p packs at a certain capacity, say 48mAh, I have to keep each string within that capacity? Within a certain range? Because your suggestion of using 22 higher capacity cells for the second string to match the lower capacity string that came before it...
Click to expand...
Yes - this is advised. I'd suggest reading some articles on google with search terms "Batteries in series and parallel" such as this - https://batteryguy.com/kb/knowledge-base/connecting-batteries-in-parallel/

Case #1
If you do battery 1 = 24v@48ah and parallel in battery 2 = 24v@58ah - then it's not 'the best' because the batteries are not the same power. 48mah vs 53mah is not a big deal, but in principle you want them to as close as you can.
1646077887115.png



Case #2
If you do 1 battery (instead of 2 batteries) with each 48 + 53 paralleled together - then this is good as each pack = 48 + 53 = same.
1646077914856.png



You can use 1 BMS either way, but in the upper case its not so good since some packs are 48mah and others 53mah. 48mah vs 53mah (specifically) is not a huge deal but if this gets wider or all over the place its not so good.

---------------------------------
This is why I make my packs big enough to handle 130cells / be all the same physical size. But some packs have 120cells (only a few empty spots) and some have 90 cells (a lot of empty slots) but every pack = 260ah. I leave a few empty slots because I find that I need to add few cells here and there in about 10% of the packs to get them to be 100% in balance with the others.

Ultimately, unless you have perfectly matched cells with perfect IR across every single pack, there's going to be a bit of fuzziness with a lot of frankenstien (different cell types) packs playing nice. Here's a example of my 84 packs - notice that blue bars (voltage of each pack) is not quite 'perfect' - a couple pop-up and some are a little low :)
1646077208855.png



harrisonpatm said:
doesn't IR come more into play then?
Click to expand...
The better the IR the less balance issues - but this is not the main thing about matching capacity.


harrisonpatm said:
And individual cell draw?
Click to expand...
Yes - but you should design your load to stay <500ma/cell for laptop cells. At this level it's not a huge deal.


Bottom Line - At 7s24p and 48ah vs 53ah the issues are small. This discussion is about what can happen if you grow and start getting some at 48ah and others at 70ah or you get 48 packs or 96 in 1 battery bank etc.... :)
 
Last edited:
OffGridInTheCity said:
Case #2
If you do 1 battery (instead of 2 batteries) with each 48 + 53 paralleled together - then this is good as each pack = 48 + 53 = same.
1646077914856.png



You can use 1 BMS either way, but in the upper case its not so good since some packs are 48mah and others 53mah. 48mah vs 53mah (specifically) is not a huge deal but if this gets wider or all over the place its not so good.
Click to expand...
Hey, are you saying this is a better situation? Because this is exactly what I was planning, maybe I had just not described it as best as I can. Case #1 was not my plan. In my imaginary scenario, I will have this top row of 48ah done. Then when I want to add more, I will add the second row of 53ah packs. The connections points will be the same as what you've drawn, 8 vertical lines. I will accomplish this by installing busbars not just for positive and negative, but a busbar in between each pack column, 8 total. When I add the 7 packs at 53ah each, the connections will not be to each other, but to the busbar on either side of the pack. Even the BMS balance leads will be connected to each series busbar, so when I add 7 new packs, all at the same capacity, they will also be connected to the BMS.

If you're saying case #2 is ideal, then my takeaway is that my plan doesn't suck. Anyone else have holes to poke in my plan so I know what to fix before it's broken?
 
harrisonpatm said:
Hey, are you saying this is a better situation? Because this is exactly what I was planning, maybe I had just not described it as best as I can. Case #1 was not my plan. In my imaginary scenario, I will have this top row of 48ah done. Then when I want to add more, I will add the second row of 53ah packs. The connections points will be the same as what you've drawn, 8 vertical lines. I will accomplish this by installing busbars not just for positive and negative, but a busbar in between each pack column, 8 total. When I add the 7 packs at 53ah each, the connections will not be to each other, but to the busbar on either side of the pack. Even the BMS balance leads will be connected to each series busbar, so when I add 7 new packs, all at the same capacity, they will also be connected to the BMS.

If you're saying case #2 is ideal, then my takeaway is that my plan doesn't suck. Anyone else have holes to poke in my plan so I know what to fix before it's broken?
Click to expand...
Yes case #2 should work well as long as you make sure each series of 7 new packs are balanced with each-other capacity/IR wise. And like you said make sure voltage are close when you combine everything. There will be some self prioritizing under high load/charge situations where the higher cap/lower IR pack will take more of the total needed load but as long as you have good sized wires and aren't pushing the cells to hard everything should run nice.

Personally I do case #1 which I would say can not use the same bms because then the bms wire would be serving as the connecting pairs wire but it is too small for that. For my I have bigger 100p packs so Case#1 is easier to monitor and maintain as I build up to a planned 400-800p 7s. A key thing for case #1 is using an advanced bms like DIY BMS or Batrium which can monitor each parallel pack independently.
 
With Case #2, all packs that are added to the primary should be equal in capacity. So in a 7s configuration, if the first 7 packs are 25Ah, and you add another 7 packs, they can be any capacity, but the whole of the added packs needs to be same. Such that, the next 7 packs, if one is 10Ah, then all 7 packs needs to be 10Ah. Because the added capacity needs to be equal along the whole string. Otherwise it's far to easy to get imbalancing.

Pack | String
| #1 | #2
----------------
1 | 20Ah | 10Ah
2 | 20Ah | 10Ah
3 | 20Ah | 10Ah
4 | 20Ah | 10Ah
5 | 20Ah | 10Ah
6 | 20Ah | 10Ah
7 | 20Ah | 10Ah

So you wouldn't want something like this:
Pack | String
| #1 | #2
----------------
1 | 20Ah | 5Ah
2 | 20Ah | 11Ah
3 | 20Ah | 8Ah
4 | 20Ah | 9Ah
5 | 20Ah | 9Ah
6 | 20Ah | 10Ah
7 | 20Ah | 10Ah

And of course, the cell "count" for each pack needs to be the same in each string, too. That doesn't mean that String 1 Pack 1 and String 2 Pack 1 needs to be the same. But each pack for each String needs to be the same.
Even if the packs will be connected in parallel, each added batch of packs could be considered an additional string. It's just that you only need 1 BMS to handle them all.
Just make sure that the voltages are within 100mV or less before hooking them together. 4.2 * 100mV = 0.042 Amps
 
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Korishan said:
With Case #2, all packs that are added to the primary should be equal in capacity. So in a 7s configuration, if the first 7 packs are 25Ah, and you add another 7 packs, they can be any capacity, but the whole of the added packs needs to be same. Such that, the next 7 packs, if one is 10Ah, then all 7 packs needs to be 10Ah. Because the added capacity needs to be equal along the whole string. Otherwise it's far to easy to get imbalancing.

Pack | String
| #1 | #2
----------------
1 | 20Ah | 10Ah
2 | 20Ah | 10Ah
3 | 20Ah | 10Ah
4 | 20Ah | 10Ah
5 | 20Ah | 10Ah
6 | 20Ah | 10Ah
7 | 20Ah | 10Ah

So you wouldn't want something like this:
Pack | String
| #1 | #2
----------------
1 | 20Ah | 5Ah
2 | 20Ah | 11Ah
3 | 20Ah | 8Ah
4 | 20Ah | 9Ah
5 | 20Ah | 9Ah
6 | 20Ah | 10Ah
7 | 20Ah | 10Ah

And of course, the cell "count" for each pack needs to be the same in each string, too. That doesn't mean that String 1 Pack 1 and String 2 Pack 1 needs to be the same. But each pack for each String needs to be the same.
Even if the packs will be connected in parallel, each added batch of packs could be considered an additional string. It's just that you only need 1 BMS to handle them all.
Just make sure that the voltages are within 100mV or less before hooking them together. 4.2 * 100mV = 0.042 Amps
Click to expand...
Excellent excellent excellent. These are all my thoughts as well, and it's lovely to have it confirmed. I appreciate everybody's additions.
 
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