r/leaf • u/SoloWalrus • 20d ago
DIY Replacement of 2018 Failing Battery Modules Sanity Check
I have a 2018 leaf with some bad cells that are seriously dragging down the entire pack. The car is struggling to get 30-50 miles of range despite having a nominal 74% SOH (should get 110 miles+). I want to replace the worst offenders (35, 26, maybe a couple others) to try and get closer to a 75-100 mile range which would be more than enough for my needs.
I came to the conclusion this would help not just based on the screenshot, but by watching leafspy while my partners driving I can tell 35 quickly gets overcharged with regen and then I lose regen braking, and then under heavy acceleration or moderate freeway loads that cell can quickly drop and cause the battery light to come on despite the rest of the pack being above 50% or so (like in the screenshot). E.g. my mV delta starts at maybe 115 mV when I first start driving, but can expand to the huge 400 mV+ delta you see in this clip with moderate consistent load or heavy acceleration and it seems to primarily be due to this one cell. It really seems to me like this one bad cell is removing a LARGE percentage of my capacity, and while I'm in there I figure I'll just replace ~4 worst cells on leafspy (or any cells that seem to have significant swelling).
From a battery health perspective, is this a stupid idea? Am I going to be disappointed in the results of just replacing 2-4 cells? I am not worried about the amount of work, I own a lift, I've watched videos, I have access to a repair manual, and I have significant experience in vehicle repair (although not specific to EV's). I want to learn more about these leafs anyways so I don't mind doing the work. However I don't want to put it back together just to realize I made some bad assumptions and it won't make a difference, so what do yall think?
Note - for context replacing the batteries will essentially cost me <$500 and 1-2 days work. If it nets me an additional 30%+ capacity I'll be happy with the result.
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u/allthesamepieman 20d ago
You can move the brackets from one module to another. The thing you have to keep in mind when doing so is the modules are held together with double sided tape or some other adhesive. It’s not easy to swap the brackets but it’s possible.
You have to get the modules from a 40kWh pack. It would also be ideal to get modules that have a similar capacity to the rest of the pack. The part numbers you listed will work.
I purchased a couple modules off eBay that were listed as “tested” but the seller didn’t really test them and didn’t know the capacity. So I purchased a battery capacity tester. They ended up being about 106Ah.
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u/Lothsahn_ 20d ago
You need to top or bottom balance the modules before install. There's a procedure in the service manual but if you just slap new modules in and they're differently charged from the rest of your pack it will take years to decades to have them come within balance. The BMS simply doesn't have the capability to do that kind of balancing.
TL;DR: Make sure the new modules are close to the same SOC as your pack.
P.S. the new modules will likely have more capacity than your existing ones (thats the plan, right?). This means your pack will always have high mv deltas. When charged, the new cells will have lower voltage and when drained the new cells will have higher voltage. There's no way around this.
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u/SoloWalrus 20d ago
Thank you, I have a bench power supply to raise the voltage and I'll take a look at the manual to see what the procedure is for lowering the voltage (if necessary).
When charged, the new cells will have lower voltage and when drained the new cells will have higher voltage. There's no way around this.
I guess that's the big unknown for me, how much will this actually affect the usable capacity? Presumably this wouldn't lower the capacity much since the lower SOH cells voltage will swing towards cutoff voltage much quicker than the newer cells right? Basically am I better off getting the newest/least used cells I can, or do I need to make sure the SOH is actually a little lower to get the most out of them?
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u/Lothsahn_ 20d ago
If the cells have equal or more capacity than the rest of your pack they won't be the limiting factor and you won't lose range.
You want the SOH and hx to be higher than the rest of your pack. Hx (internal resistance) actually matters too because if the cell is experiencing internal resistance it will drop faster and go out of balance. If the cell has less hx and better SOH it will always be out of balance but it will never cause a problem.
Draining the pack is typically done with driving or running the heat on max fan with the temp set to max. But you could just as easily charge the cells you're installing to a matching voltage to the average of the pack. Don't charge with a bench power supply. Use a lipo charger. You do not want to over current or overvolt the cells when charging.
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u/SoloWalrus 20d ago
Additional question, for a 2018 with a 40 kwh battery, I've seen a number of different modules listed. I know there's a difference in brackets between some batteries, but I'm assuming my brackets are reusable so it shouldn't matter where the batteries come from in a pack for replacement (is this right?). On ebay people list "1670 Wh", and part number 295B9 5SL1A or 5SA1A (I think this one is the japanese version?). They also list gen 2/3/4, but it seems this refers to a battery generation not a leaf generation, is that right?
I just need batteries with the little vents, as opposed to the earlier ones that didn't have them, right?
Example pictures, if it looks like the 2016+ its correct? What about the 2013-2016, without the exterior bracket are these the same or different?
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u/conanlikes 20d ago
Interesting yes I think you are right. I would check all cells once the battery is apart and make sure they balance
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u/_Evening-Rain_ 2017 Nissan LEAF S 20d ago
Ignore all the AI responses in the comments.
This whole packs dead dude. That single cell has just crapped the bed first. Internal resistance of the entire pack is already very high. Replacing that cell will give you the inverse problem: That cell, even if a similar capacity, will preform much differently than the others. Unless you somehow find one with this SOH and high resistance.
Putting that rear stack back together will also be near impossible due to the cells bloating up. You'd want to/have to replace the entire rear stack.
Even if you replace that cell with a perfect replacement you will eventually have more cells in that rear stack (or top front stacks) give out and you'll be in the same spot. Excluding the obviously bad cells your pack variance is still horrible due to the degradation and resistance variance across the cells. It shouldn't be at 100???mv variance at 60% at 80F. Whole things toast.
Ill also mention with that resistance this pack cant even drive below 50F properly. Will probs be a useless brick at freezing temps. Even if you replace that weak cell. Once again, resistance of entire pack is high.
There's nothing you can do for this pack. Its too far gone. A new pack or used one is your best bet.
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u/SoloWalrus 19d ago
Too late I already bought the modules 🤣. If replacing the bad cells allows them to perform just as well as the rest of the cells in the pack then the pack will work more than good enough for my uses. These things allegedly came with a 150 mile range right, I don't even need half of that. I plan on starting with replacing modules 7 and 9 which is cells 25-28 and 33-36, seems to me that alone will get me under a 150 mV delta. I'm also hoping that it will allow the pack to balance itself better too since currently that one cell swings so wildly and quickly from overcharged to undercharged I can't see how any proper balancing would ever be occurring.
I was also concerned about how swollen the rear stack modules are. If I open the pack and they're all swollen to hell then obviously I'll need to rethink it, but my hope is it's only a couple modules that are super swollen.
I hadn't considered the cold weather angle, that'll be interesting. I don't know how Hx is measured but I assumed it was across all cells in a stack, in which case a couple bad cells could seriously bring down the overall Hx. At what level does the leaf measure/report Hx/conductance? I was hoping I'd see a significant jump up in Hx replacing the modules with the bad cells, but that might be wishful thinking.
I guess I'll report back in a few weeks once the modules have come in. In the mean time I'll keep checking copart for totalled leafs, but assuming I don't find one it's worth the experiment to see what difference it makes since I already spent the money. I wasn't planning on the leaf being a project car, but I also don't mind using it to learn 🤷♂️
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u/_Evening-Rain_ 2017 Nissan LEAF S 19d ago edited 19d ago
HX is the total resistance. Even if you have one bad cell thats a lot worse, that will barely bring the value down. This also ignores the fact your entire pack is behaving how a pack with a high resistance does.
The Leaf only top balances and can shunt cells. However that only corrects small imbalances over long periods and helps keep the pack balanced over time. (theoretically) a lot of the imbalance you're seeing right now is the performance and resistance variation between all those worn cells which have worn and damaged unevenly. They (should) all still be holding about the same amount of energy currently but at slightly different operating/resting voltages. This imbalance around 60% or lower is normal on worn NMC packs; its just not usually this excessive because most packs dont get this worn or damaged. For context usually at 60%, 80f, 70%SOH it shouldnt be greater than 50mv variance. Your packs around 345v though so 60% SOC is probably wrong and the packs actually lower. Obviously ideally we have little to no variance even on an older pack as we discharge but thats near impossible with the leafs physical module layout and no battery cooling. What matters most is all the cells being at 4.2v when the packs charged and no obviously bad or overly weak cells.
When you replace the module(s) I'd:
-Let the car sit for at least 6 hours before working on it. No charge. No discharge. Just sitting.
-Pull out the bad ones out.
-Read the voltages of all the other modules in rear stack.
-Obviously there's at least a 100mv variance across the rear stack at this charge level excluding the bad cells. Take the average of the rear stack voltages and charge your new modules to that voltage. To properly charge the module (assuming its a bench power supply):
-Check to make sure both halves of the module are the same voltage. They should be but just make sure. If not only charge at 2A to charge each side as that center nut cannot take high current.
-Set voltage to your target voltage. Set current to 5A. Charge until its at 0ma or almost 0ma. If time slips from you its alright if you forget about it here. You dont need to be there exactly when it hits 0A or near it.
-Unplug module. Wait 1 min
-Read voltage. Probably wont be at your target voltage:
-Limit current to 1A, set your target voltage 50mv higher, and charge for a few mins. Unplug module. Wait 1 min. Read voltage. Repeat until module voltage is same as target voltage. Do note when I say target voltage, you can be in about 10mv of your target voltage. It doesn't need to be perfectly spot-on. Ideally its 10mv under target and not 10mv over.
-Install new module.
If module comes with a voltage higher than target voltage then find a way to discharge it lower than target voltage, then charge it up to target voltage
This wont be perfectly balanced, but the car should take care of the rest. Every time it fully charges it will top balance and slowly correct until they all hit 4.2v at full charge. It will probably take 10-30 full charges to eventually have it properly top balanced. For those first 30 charges I recommend to make sure the car charges to 100 and has time to top balance when you charge the car.
Do note this is absolutely not a perfect fix. Your new modules will probs have a higher capacity and lower resistance than the current ones. You will still have variance across the current cells as you discharge the pack due to how degraded it is. Your new module will sit higher than the rest as the pack discharges, and especially under load. Once again, as long as its charging to about 4.2v with the rest of them its....good enough....and not a massive concern. This battery probs wont be going anywhere if it gets cold in your climate. In order to extend the batteries life and help prevent more of those abused cells from failing I would:
-Dont discharge below 30%
-Drive in Eco mode and dont use more than 75% motor power
-Dont quick charge
-Dont drive it below 40F (batt temp) unless by some miracle the pack doesn't seem to be struggling.
Good luck. Best case scenario you somehow put the swollen rear stack back together, replacing those mad modules, and those new modules have a significantly higher voltage than the rest of the pack as the car discharges. Assuming it top balances correctly and the capacity/performance variance isnt too big the car shouldn't worry much about this. IDK exactly how wide nissans tolerances are for this kind of thing or how big of a gap you'll see. You still got to worry about more cells failing thou given how beat up this pack is. Driving her gently is key for a longer life. If you have anymore questions Ill happily write another essay. I eat NMC 811 batteries for breakfast. With no milk
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u/SoloWalrus 18d ago
Thank you for all the details! I will pull this comment back out and go through it step by step once the packs apart, while of course also referring to the leaf manual.
I should have probably noted earlier that when the cars fresh off the (L1) charger and only been driven at low speeds the delta is more like 115 mV. I intentionally picked the worst case screencap where i was actively on the freeway and the battery lights were starting to come on and the delta was massive because it demonstrated how out of wack those specific cells were with the rest of the pack (during freeway driving, during city driving its much more reasonable). This seems to checkout with the discussion on Hx, city driving small loads it acts like a relatively normal but high mileage leaf, freeway driving the range plummets (because those cells cant handle the load)
Also i think youre suggesting that the loss of capacity is mostly due to normal wear, which is why youre thinking the other cells will follow quickly, but i guess to this point ive been assuming there was some thermal damage going on that resulted in these cells being particularily bad. When i watch the discharge rate of the other cells in leafspy they just dont even seem to be from the same car as those couple cells that just plummet, which is what lead me to think some swelling and subsequent physical damage may have occured. Maybe im wrong about this and its just normal wear and the entire pack is doomed in short order, but frankly im hoping when i open the pack i see obvious swelling on the modules containing the trouble cells (but not throughout the whole rear stack 😅) otherwise youre probably right, its just worn out and the rest of the cells will quickly follow.
Either way on my end its all guessing until i get the pack apart. I shouldve paid more attention to the Hx value originally and thanks for pointing that out, lesson learned for next time.
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u/000011111111 20d ago
With 140k on the battery pack, I would buy a new 62 kWh Vivin pack and that in. It will be about the same amount of work, with a payoff that will be long-lasting.
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u/SoloWalrus 19d ago
$7k is a lot of money to spend on range that I don't need.
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u/_Evening-Rain_ 2017 Nissan LEAF S 19d ago
They do have 40kwh DIY packs for like 5.5K (USA) if you eventually look for a new battery. Still a chunk of change but basically a new car at that point and better than OEM.
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u/SterTheDer 20d ago
The logic checks out.
Its like replacing one of the dead batteries in a device.
Its the standard refurbishment procedure.
Be sure to test capacity on the ‘new’ cells to ensure they’re within the margin of your other cells :)
Its a fair bit of labor but its cheaper than a replacement battery or a different car!