Is it better to use Higher or Lower SOH module when replacing bad cells?

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Simo

Member
Joined
Jan 6, 2025
Messages
12
Location
New Zealand
Hi all

I have purchased a 2015 24kWh leaf with what appears to be set of bad cells in one module, although I have ordered a used replacement 30kWh battery for the car, I intend to use the 24 as solar storage connected to a Fronius inverter (using Dalas work).

I'd like to replace the module containing cells 29-32 (number 8?) but finding modules with similar SOH here in NZ is not that easy. The question I have is it better to use a higher or lower SOH replacement and how much deviation is sensible? The best I have found so far is 65% SOH to fit to my pack with 71ish.

Failing that, I will attempt to bring the weak cells up by charging that module separately once the pack is opened. It seems that is not a long term solution in a car but considering that the current draw when used as storage is less it might hold?

Have mercy 😅 I have searched the forum to find the answers in other posts before posting this. TY!

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Hi, just looking at what you are wanting to do, and especially being a 24kwh pack I would suggest that it would be more suitable for a nominal 48v solar storage, you would probably be better off doing that or buying yet another pack in better condition if you want to go hv into the inverter, I am also in New Zealand, happy to talk to you about it if you like
 
Hi all

I have purchased a 2015 24kWh leaf with what appears to be set of bad cells in one module, although I have ordered a used replacement 30kWh battery for the car, I intend to use the 24 as solar storage connected to a Fronius inverter (using Dalas work).

I'd like to replace the module containing cells 29-32 (number 8?) but finding modules with similar SOH here in NZ is not that easy. The question I have is it better to use a higher or lower SOH replacement and how much deviation is sensible? The best I have found so far is 65% SOH to fit to my pack with 71ish.

Failing that, I will attempt to bring the weak cells up by charging that module separately once the pack is opened. It seems that is not a long term solution in a car but considering that the current draw when used as storage is less it might hold?

Have mercy 😅 I have searched the forum to find the answers in other posts before posting this. TY!

View attachment 6442
Just as a matter of interest is that a car that came from Whangarei?
 
Hi, just looking at what you are wanting to do, and especially being a 24kwh pack I would suggest that it would be more suitable for a nominal 48v solar storage, you would probably be better off doing that or buying yet another pack in better condition if you want to go hv into the inverter, I am also in New Zealand, happy to talk to you about it if you like
Honestly, I'm just learning with cheap stuff so it doesn't matter if I break it. The 24kWh pack should be fine as our solar system is only 3kW and grid tied.
 
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I'll comment on 2 separate things; for reference, I've done a module replacement (on my 40 kWh pack).
1) not sure ordering/buying a 30 kWh pack is the best solution, since those are notoriously the worst packs ever manufactured in terms of degradation/longevity. I would even go so far to say you might have better luck "repairing" the 24 kWh pack in the long run (being mindful that modules for the different packs are different).
2) as far as SOH, you always want to get the best you can find, but the real trick with cell/module replacement is preparing/charging the replacement module w.r.t. the whole pack. First, charge the whole pack to something near "full" before cracking it open. Then, charge the new module (each cell pair individually) to just a bit higher than the rest of the pack, so it's always the "strongest" cell/module in the pack (whereas it used to be the "weakest"). The closer you get it the better, but it's a manual job (at least it was for me) and don't worry about getting it perfect; however, realize that whatever voltage differential you end up with w.r.t. the new module will always remain (mine was a but too high ~60mV, but my overall range has held up over time).

Hope that helps; and don't waste any time trying to (individually) charge the old/bad module and re-inserting it...because it will just return to it's old/bad state.
 
I'll comment on 2 separate things; for reference, I've done a module replacement (on my 40 kWh pack).
1) not sure ordering/buying a 30 kWh pack is the best solution, since those are notoriously the worst packs ever manufactured in terms of degradation/longevity. I would even go so far to say you might have better luck "repairing" the 24 kWh pack in the long run (being mindful that modules for the different packs are different).
2) as far as SOH, you always want to get the best you can find, but the real trick with cell/module replacement is preparing/charging the replacement module w.r.t. the whole pack. First, charge the whole pack to something near "full" before cracking it open. Then, charge the new module (each cell pair individually) to just a bit higher than the rest of the pack, so it's always the "strongest" cell/module in the pack (whereas it used to be the "weakest"). The closer you get it the better, but it's a manual job (at least it was for me) and don't worry about getting it perfect; however, realize that whatever voltage differential you end up with w.r.t. the new module will always remain (mine was a but too high ~60mV, but my overall range has held up over time).

Hope that helps; and don't waste any time trying to (individually) charge the old/bad module and re-inserting it...because it will just return to it's old/bad state.
Thank you for the info :) I went with the 30kWh pack as it is a project for learning on, and for the price of 40kWh packs, it's not a lot more for a complete road-legal car!
 
Update - I have managed to locate NOS modules. Although they have probably been sitting for some time and have degraded a bit I'm going to assume they are still some where in the 90s SOH. Taking the advice from above is there likely to be any bad effect using one in a 71% SOH pack? TY :)
 
Update - I have managed to locate NOS modules. Although they have probably been sitting for some time and have degraded a bit I'm going to assume they are still some where in the 90s SOH. Taking the advice from above is there likely to be any bad effect using one in a 71% SOH pack? TY :)
It will be interesting to see what happens, I am picking that under load the older cells will sag in regards to voltage and the new ones won't causing the LBC to see a large mv difference,then the turtle and ev warnings will show,if you drive gently it would probably be fine, if I were doing it I would deliberately run the new cells at a low voltage when manually balancing them in the pack,I would be going with about 75mv lower if I were to try it, very interested to see how it goes, best of luck 🤞 hope it works out
 
Is it better to use Higher or Lower SOH module when replacing bad cells?

Try exactly same.
If you can't , try to have new module that have a better lowest voltage then the rest of them
 
It will be interesting to see what happens, I am picking that under load the older cells will sag in regards to voltage and the new ones won't causing the LBC to see a large mv difference,then the turtle and ev warnings will show,if you drive gently it would probably be fine, if I were doing it I would deliberately run the new cells at a low voltage when manually balancing them in the pack,I would be going with about 75mv lower if I were to try it, very interested to see how it goes, best of luck 🤞 hope it works out
There is a lot of conflicting information around so I'm going to have a go and see what happens, although this pack is lowish kms I do think it has a reasonable chance of holding up OK compared to more a worn one. I might even try a few varying SOH replacement modules (not all at once) as they are cheap enough. Ultimately the pack will end up as storage so I'm not too worried!
 
There is a lot of conflicting information around so I'm going to have a go and see what happens, although this pack is lowish kms I do think it has a reasonable chance of holding up OK compared to more a worn one. I might even try a few varying SOH replacement modules (not all at once) as they are cheap enough. Ultimately the pack will end up as storage so I'm not too worried!
I think that you are approaching this from the right angle, I think that you will have varying degrees of success, and in the end of the experiment using the pack for solar is a great idea too, the sort of thing you are doing is the reason I built my battery trailer so cells can be changed quickly without removing the entire pack from the car as I am planning on doing similar things too
 

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I finally had time to open the pack to check for the bad modules. It looks they they had been replaced with lower SOH previously so that answers my question! Unfortunately I missed out on the new modules so am on the hunt for 2 decent ones!
 

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SoH at a Pack level is not the same as individual cell SoH level.
I recommend to buy modules from front pack not from back pack

If you don't have a intelligent charger / discharge devices need to change them with good luck.

If you have , need to do some cycles at individual cell/module ( to good old ones and "new" ones ) and if you buyd more modules , use that that same like that old good cells/module.
 
SoH at a Pack level is not the same as individual cell SoH level.
I recommend to buy modules from front pack not from back pack

If you don't have a intelligent charger / discharge devices need to change them with good luck.

If you have , need to do some cycles at individual cell/module ( to good old ones and "new" ones ) and if you buyd more modules , use that that same like that old good cells/module.
I purchased new modules as they have become available again thankfully! I have a small battery analyser that can charge and discharge Li-ion cells and test capacity, but it will be very slow. Will post updates as I go :)
 
If I do a rebuild, I charge pack to max and new cells between min/max pack cells.
 
I think that you are approaching this from the right angle, I think that you will have varying degrees of success, and in the end of the experiment using the pack for solar is a great idea too, the sort of thing you are doing is the reason I built my battery trailer so cells can be changed quickly without removing the entire pack from the car as I am planning on doing similar things too
I like the idea of a battery trailer.... self-propelled tho! :D Would be cool to check yours out someday when you have it up and running, any test runs to Whangarei in the future?
 
I like the idea of a battery trailer.... self-propelled tho! :D Would be cool to check yours out someday when you have it up and running, any test runs to Whangarei in the future?
Absolutely, that is probably where I would be doing most of my testing as state highway 14 would be perfect,I just bought a leaf and drove it from one tree point to dargaville with a 4 bar battery, that was a challenge,( multiple charges on the way)this above Car is what I am hoping to use as a test platform to tow the trailer,if you are ever in the dargaville area you are welcome to call in and see what I am up to
 
Hi all

I have purchased a 2015 24kWh leaf with what appears to be set of bad cells in one module, although I have ordered a used replacement 30kWh battery for the car, I intend to use the 24 as solar storage connected to a Fronius inverter (using Dalas work).
What is the process for getting this certified in NZ.
I am interested, have a Solis inverter.
 
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