Replace old cell with new cell

[pmbdk]

This is a hypothetical question: I have a 40 kWh battery (96s2p). At some point in time, one of the cells may go bad. From what I can read from many sources is that it is a bad idea to replace an old module in an old pack with a completely new module. A module is 2s2p, i.e. I just need to change one series connected module

What I don't understand is why? Assuming the Leaf has a passive BMS, the "only" thing it does (in this respect, ignoring all other safety features!) it to bleed off residual charge when cell voltages gets too high.

Assuming all the old modules have a capacity of 50% of the new module and all modules (48 of them as far as I know) start off at the same voltage, then we just need to consider two scenarios:

1. Discharging: Here the old cells will discharge faster compared to the new cell as the capacity is smaller. But that only means that the new cell is not utilized fully.
2. Charging: Here the old cells will charge faster compared to the new cell as the capacity is smaller. But again: That only means that the new cell is not utilized fully.

So what am I missing?

I am pretty sure the Leaf does not have an active BMS, as in that case the arguments would be slightly different.

EDIT: Changed cell to module

 

[raideris]

This is a hypothetical question: I have a 40 kWh battery (96s2p). At some point in time, one of the cells may go bad. From what I can read from many sources is that it is a bad idea to replace a cell in an old pack with a completely new cell.

What I don't understand is why? Assuming the Leaf has a passive BMS, the "only" thing it does (in this respect, ignoring all other safety features!) it to bleed off residual charge when cell voltages gets too high.

Assuming all the old cells have a capacity of 50% of the new cell and all cells start off at the same voltage, then we just need to consider two scenarios:

1. Discharging: Here the old cells will discharge faster compared to the new cell as the capacity is smaller. But that only means that the new cell is not utilized fully.
2. Charging: Here the old cells will charge faster compared to the new cell as the capacity is smaller. But again: That only means that the new cell is not utilized fully.

So what am I missing?

I am pretty sure the Leaf does not have an active BMS basketball stars unblocked, as in that case the arguments would be slightly different.

Voltage Differences: While your hypothetical scenario assumes all cells start at the same voltage, that won't be the case in reality. Batteries naturally self-discharge at different rates, and the new cell with higher capacity will likely hold most of the charge initially. This creates a voltage imbalance during discharge, with the new cell having higher voltage than the old ones.

Charge Cycles and Internal Resistance: Even if you start with balanced voltages, the old and new cells have different numbers of charge cycles and internal resistance. The new cell will cycle less and have lower internal resistance, meaning it will charge faster and reach full capacity sooner than the older cells. This again creates an imbalance, causing stress on the older cells as they are forced to "catch up" during charging.

Balancing Issues: Passive BMS systems like the Leaf's lack the ability to actively balance individual cells. This means the entire pack will be limited by the slowest-charging old cell, leading to wasted energy in the new cell that reaches full capacity earlier. Additionally, the voltage discrepancies can put unwanted stress on the BMS itself, potentially shortening its lifespan.

Safety Concerns: The biggest concern is safety. Lithium-ion batteries are notorious for their instability, and any imbalance can lead to thermal runaway, fire, or explosion. The voltage and charging differences you described can increase the risk of these events, especially with older cells that may be nearing the end of their lifespan.

Remember, the cost of a single cell replacement might seem attractive initially, but the potential damage and safety hazards outweigh the savings in the long run. Playing it safe by replacing the entire pack or using matched cells is the better option for both performance and peace of mind.

 

[pmbdk]

Let's take them one at a time. Please note that when I speak of a cell, I actually mean a module (i.e. a 2p2s module). This means that I am simply taking one of the series connected modules out and replacing it with another. I agree that you should _never_ replace a new cell in parallel with an old cell, as this will certainly lead to problems!

1. Voltage Differences. Yes, the new cell will obviously discharge, self-discharge and charge differently. And this will create a voltage imbalance during both charge and discharge. But so what? Just because voltage differences in a pack is in general an indication of a worn battery, it does not mean that a voltage difference in general is bad. I can't see why this should be of any concern.
2. Charge Cycles and Internal Resistance: It is true that the old cells will charge up faster than the new ones. But I don't agree on the argument "they are forced to catch-up". None of the cells are forced to do anything. They are simply being charged by the same current as all the rest of the cells, including the new ones.
3. Balancing Issues: I am pretty sure the Leaf BMS is able to bleed off residual charge on each 2p coupled cells. The entire pack will certainly be limited by the "slowest" cell. But that has nothing to do with the new cell (module), as nothing is wasted on the new module. This is just how old packs works.
4. Safety concern. Imbalance - when only talking about series connection - will not pose any safety issues. If we are talking about parallel connected old/new cells, this is true.

I think the main issue here is that people don't really understand the difference between series and parallel connected batteries when talking about replacing cells, and then it's easier to just say: Hey, don't do that, use an old cell instead of a new.

 

[pmbdk]

In the above I am assuming that the BMS will stop (or limit) charging whenever _one_ of the cells in the pack reach full SoC. Additionally I am assuming that the BMS will also stop discharging whenever the lower cut-off voltage is reached on a cell level.

I don't know much about the Leaf BMS, but I would assume that this is something all BMS'es (at least for EVs) do.

 

[cornbinder89]

Cell replace isn't for the casual weekend mechanic, but with the proper tools and PPE is done all the time. New vs. used, is likely to be a moot point because it is hard to buy new cells and even harder to buy a single new cell or module. Used cells are being used for replacements all the time (See Green Tec or others that specialize in EV batteries). Each module is two cells in parallel and two packs of 2 are placed in series to make one module. The BMS is connected to the center jct and that way can "look" at each "1/2" of the module.
Seams to me for all the work and cost involved and the fact the pack, as you say is going to be limited to the "weakest link" it wouldn't be the balancing but the costs/benefit that would tip the scale to using used, but matched cell for replacement of one module.
I have a Maxwell super capacitor ESM that the caps are monitored and charge balanced in the same way, and they to can explode if there is a large imbalance, but no thermal run-away. Still the power contained is enough to require caution working around.

 

[pmbdk]

True, it is hard to find these modules consistently. It is however interesting that the Leaf seems to be _so_ much simpler to do advanced battery maintenance on than (say) Tesla. This is most likely due to the missing active thermal management (which is of course also bad); but given my own (cold) location I would rather have a HV voltage battery with ease of access than an active thermal management with liquid cooling.

I wonder how hard it would be (if possible at all) to make a module in the same form-factor and electrical interfaces but based on commercial NMC lithium batteries in 18650 or 21700 format.

 

[Stanton]

True, it is hard to find these modules consistently. It is however interesting that the Leaf seems to be _so_ much simpler to do advanced battery maintenance on than (say) Tesla. This is most likely due to the missing active thermal management (which is of course also bad); but given my own (cold) location I would rather have a HV voltage battery with ease of access than an active thermal management with liquid cooling.

Very true.
I have replaced a (bad) cell/module in my 40kWh pack, and all I can say is don't over-think it (just the labor is hard enough). The real problem is finding a "donor" module (Nissan won't be any help); I found mine on eBay, and it's always preferred to get the "best available" (which depends on the knowledge/accuracy of the seller). If you are only replacing one (or two) modules, then it doesn't hurt for them to be "better" than the general pack. The trick is balancing them (outside of the pack) to be just slightly ahead of the overall pack; that way instead of determining when the pack turtles (weak cell), they determine when there is a full charge. Essentially, you want to eliminate all the problems with low SOC.

 

[cornbinder89]

The Leaf Battery was designed with servicing in mind, It constructed such that what you propose is possible. That said, the actual availability of modules and places to do the work were never constructed. I could foresee that the designers imagined dealerships servicing batteries at the modular level, but that the corp never followed up on that.
The replacement of the cells with a spiral format cell (i think this is what you are proposing) will not be feasible, the reason the leaf cells are flat is to increase the surface area exposed for the "passive cooling" of the cells and the wound cells would have no way to rid themselves of the heat generated.

 

[pmbdk]

Some random ramblings below...

I found the module dimensions online, see 2018 Nissan Leaf battery real specs - PushEVs . For the 40 kWh pack the modules seems to be 300x222x68 mm. They contain two units of 2s2p pouch cells connected in series. Thus there should be 24 modules in a 40 kWh pack (in total 24 modules x 2 units/module x 2s2p cells/unit = 96s2p cell configuration).

Each module is thus 1666 Wh.

Just for the fun of it, I tried to put in as many 21700 standard NMC lithium cells in a box of the same dimensions of the module, see below (only one layer is shown; there are room for three layers). Each layer has 36 21700 cells. So in total 108 cells. Assuming 5000 mAh per cell and 3.65 nominal voltage, we get 1971 Wh or 18% better than the original module!

I have probably made an error somewhere. But if my calculations are correct and the batteries could be cooled as "well" as the original, it would be possible to make you own module.

Probably not worth it, but still... Found someone on ebay selling modules for £180/module. Just buying the 100x21700 cells alone would be something like £300... LOL...

 

[cornbinder89]

You're missing the point, the flat pack cells can be cooled passively precisely because they are flat, a big surface area for cell capacity. If you took them and rolled them up the same cell could no longer be cooled passively. Your round cells have no way to dissipate the heat. They require and are provided active cooling when used in vehicles (spiral cells)

 

[pmbdk]

I think you didn’t read my “and the batteries could be cooled as "well" as the original”… But of course it can be done. And the surface area of the round cells are larger than the pouch cells, that’s not the problem. The problem is how to get the heat away.

 

[cornbinder89]

I think you didn’t read my “and the batteries could be cooled as "well" as the original”… But of course it can be done. And the surface area of the round cells are larger than the pouch cells, that’s not the problem. The problem is how to get the heat away.

No the surface area of round cells is not bigger, it is smaller and becasue it is wrapped up the inside of the cell will not cool at all.
The only reason I am making an issue is I don't want your car to go up in flames.
The Leaf battery is made like a 70"s Polaroid SX-70 battery, with a large flat surface, any point in the cell is the same distance to the outside as any other point in the cell, there are two flat plates (top and bottom) of the cell to disperse the heat.
Round cells will not cool at all in the center of the three tier rows. You are begging for the thermal run away.
There is a reason that all round cell vehicle battery packs are all actively cooled.
Please consult with an engineer before attempting this.

 

[pmbdk]

Well, I am actually an engineer but that does not really matter. The surface area of the round cells is larger than the surface area of the pouch cells. The surface area of the 108 21700 cells is 498744 mm^2 (not counting the flat ends) whereas the surface area of the 8 pouch cells is 451008 mm^2.

But again, it doesn't matter what the surface area is: The problem is heat-transfer. If there is only air around the cells, it will effectively act as an insulator. That is why pouch cells are easier to use, since it's easy to put those pouch cells up against large pieces of metal, which can transfer the heat away. Of course you can use cylindrical batteries - Tesla has been doing that for a long time - but those serpentine cooling coils they use for dissipating the heat are probably out of the question for DIY type battery making.

 

[Ingbert]

Very true.
I have replaced a (bad) cell/module in my 40kWh pack, and all I can say is don't over-think it (just the labor is hard enough). The real problem is finding a "donor" module (Nissan won't be any help); I found mine on eBay, and it's always preferred to get the "best available" (which depends on the knowledge/accuracy of the seller). If you are only replacing one (or two) modules, then it doesn't hurt for them to be "better" than the general pack. The trick is balancing them (outside of the pack) to be just slightly ahead of the overall pack; that way instead of determining when the pack turtles (weak cell), they determine when there is a full charge. Essentially, you want to eliminate all the problems with low SOC.

Hi. At what voltage difference between the cells is it relevant to think about measures. Should you measure at SOC 90% or 20%

 

[rsbrax]

Cell replace isn't for the casual weekend mechanic, but with the proper tools and PPE is done all the time. New vs. used, is likely to be a moot point because it is hard to buy new cells and even harder to buy a single new cell or module. Used cells are being used for replacements all the time (See Green Tec or others that specialize in EV batteries). Each module is two cells in parallel and two packs of 2 are placed in series to make one module. The BMS is connected to the center jct and that way can "look" at each "1/2" of the module.
Seams to me for all the work and cost involved and the fact the pack, as you say is going to be limited to the "weakest link" it wouldn't be the balancing but the costs/benefit that would tip the scale to using used, but matched cell for replacement of one module.
I have a Maxwell super capacitor ESM that the caps are monitored and charge balanced in the same way, and they to can explode if there is a large imbalance, but no thermal run-away. Still the power contained is enough to require caution working around.

I see you mention Green Tec. The nearest Green Tec shop is 80 miles from me. The nearest Leaf Repair Network shop is 300 miles from me. My 2012 Leaf battery has 51% SOH and I want to upgrade to a 40kWh battery. My concern is which shop to choose for this in the event of a bad cell or other problem with a 40kWh battery especially after the warranty expires. Can you comment? (I would prefer not start a new thread at this time.)

 

[cornbinder89]

I have no experience with either. There is a big difference, however, With Green Tec you are getting a used, rebuilt with better cells but not new battery. They can do all sizes but if you opt to upgrade to a 40Kwh battery, you pay dearly. If you choose the LRN you are getting NEW cells and they only make the 40Kwh.
If I needed to repair a 24 Kwh battery to sell a car, I would choose a rebuilt. If I wanted to keep the car, the price difference for the upgrade to a 40 Kwh pack isn't that great either way. I'd want to talk to some people who had it done from either supplier to see if it met their expectations. In the end the choice is yours.

 

[cornbinder89]

Well, I am actually an engineer but that does not really matter. The surface area of the round cells is larger than the surface area of the pouch cells. The surface area of the 108 21700 cells is 498744 mm^2 (not counting the flat ends) whereas the surface area of the 8 pouch cells is 451008 mm^2.

But again, it doesn't matter what the surface area is: The problem is heat-transfer. If there is only air around the cells, it will effectively act as an insulator. That is why pouch cells are easier to use, since it's easy to put those pouch cells up against large pieces of metal, which can transfer the heat away. Of course you can use cylindrical batteries - Tesla has been doing that for a long time - but those serpentine cooling coils they use for dissipating the heat are probably out of the question for DIY type battery making.

Ok, I guess I should have put it a different way, the surface area exposed for cooling purposes is much greater on the flat cell than a spiral cell I will admit the area for power generation is greater on the spiral is larger.
What prevent a thermal run a way is the cooling area on passive cooled cells.
Your last paragraph is exactly my point, that when used they have to be actively cooled. Given the Leafs passive cooling, you are begging for a thermal run away event using round cells. it does seam in the end you do see the issue, ignore the problems at you own expense. (and anything that may be around your car.

 

[Stanton]

Hi. At what voltage difference between the cells is it relevant to think about measures. Should you measure at SOC 90% or 20%

The question really is: at what point does the weak cell impact your driving/safety? Weak/bad cell problems typically manifest themselves at low SOC, and how low depends on the magnitude of the voltage delta. I noticed problems once I got beyond a 100mv delta, so I can't imagine driving the car with a 200-300mV delta. Of course, that assumes you even can/want to make the repair; it's not cheap, and you definitely don't want to do it twice...so make sure you have identified all the problem modules (there are 8 cells/module in the 40kWh and 62kWh packs).

 

[Stanton]

I see you mention Green Tec. The nearest Green Tec shop is 80 miles from me. The nearest Leaf Repair Network shop is 300 miles from me. My 2012 Leaf battery has 51% SOH and I want to upgrade to a 40kWh battery. My concern is which shop to choose for this in the event of a bad cell or other problem with a 40kWh battery especially after the warranty expires. Can you comment? (I would prefer not start a new thread at this time.)

If you buy a "turn key" replacement 40kWh pack, then they should warranty the work. I'll give you a tip though: weak/bad cells typically manifest themselves within 10k miles (mine did much less than that), so it helps to have this information on the replacement pack. For example, I thought I found the holy grail when I installed a 40kWh pack with less than 1k miles...only to have to replace a weak cell/module after a year or so. It's all good now, but it wasn't easy.

 

[cornbinder89]

Both the places do warranty their products, I don't know if that includes labor and I would look closely before assuming.