New 40kwh battery on 2018 SL, sudden charge drop at low load

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Jerther

Well-known member
Joined
Sep 26, 2023
Messages
114
Location
Quebec, Canada
Hi!

I just had the battery of my 2018 SL replaced under warranty like two weeks ago, after months of diagnostics (personal and dealer). A few days ago, I was driving in town, really smoothly, at 40% soc, and if I ever pushed the pedal just a bit too far (ie normal acceleration from a traffic light), the soc would suddenly drop, like my bad battery did but on the highway under heavy load.

I had a perfect 0 QC count in that battery. Now it's 2. What a bummer...

A bit more context:

- I had the car sit at 40% for 9 hours at -10c (because the evse failed to charge).
- prior to this incident, I hadn't charge the battery to 100% once, trying to keep it at 80%.

I have since read that the leaf calibrates the battery when it charges to 100% so I have charged the battery to 100% a couple of times, but haven't tried the car at low soc again. I will soon though, and take leaf spy screenshots.

Should I be worried again? Is this unheard of for a new battery? Or is it normal and yet another thing I should know about? Nissan was really nice to quickly give me a free battery but I'm getting tired and this is not fun anymore, if it ever was...
 
Charging to 100% at least once (on a brand new pack) would be helpful, but we need more info to diagnose anything:
1) LeafSpy voltage graphs
2) Temps at your location (cold doesn't help "weak cell" symptoms)
Unfortunately, 40kWh packs (and even 62kWh) are notorious for bad/weak cells/module.
 
OP, this is not normal for a new pack, and you cannot fix it.

I can explain it as either a weak cell, or a high resistance chokepoint. The latter could be an installation error or a manufacturing defect.
To figure this out for yourself, you need Leafspy running while driving so that you can monitor the cell voltages under load.
 
So it was -15C this morning. The car has been sitting unplugged at 83% SOC all night and was last charged two days ago. I remember the battery temperature was somewhere between -5C and -10C.

The 45 km trip went very well at 110 km/h, except for when the SoC reached below 40%. Then SoC started to drop suddenly under heavy load, but not as much as my old battery. I also noticed the problem got less severe as the battery temperature gradually increased.

Here's the worst case, near destination, where I was accelerating, WOT, from 0 to 120 km/h. Battery temperature was around 9C. At that point the car's reported SoC dropped from around 40% to 30% in seconds, but did not drop under 30%. Of course SoC went back up to 40% soon after I lifted the pedal. My old battery would have just given up. This one held better.

1706120734009.png

And here's one when I finally got to work, and parked the car: SoC 40% and battery temperature is 13C (wow, 43% for 45 km, and sudden drops at <40%. It's like having 60 km range on a full charge...)

Also, I realize I had it much worse that time in the top post here where the SoC was 40% AND the battery was cold, maybe 0C.

1706120968366.png

I took other screenshots earlier in the trip on the highway. It did also act up once between 40 and 50% but not so much, I easily could've missed it. I can upload them if necessary, I didn't want to clutter the post.

Any conclusion to be drawn?
 
A brand new battery and a reset BMS means the computer has a lot of learning to do about the capacity. I don't see anything of concern in your screenshots, but if you want to get the BMS to better learn the capacity, wait until the weather gets warmer. You're going to need access to a working L2 and a lot of free time to spend watching LeafSpy. Find a way to deplete the battery down to near 0% at the working L2 location and just run the battery the rest of the way down while watching with LeafSpy until you get it to around 0.5 kWh of capacity left (If possible, the BMS might not be able to do this just yet since it's been reset and you have a brand new battery). Use the heat or AC or both to slowly run out the rest of the capacity. Either the BMS will shutdown the battery before 0.5 kWh of capacity or do it yourself if you reach it that low. Being that's it brand new, the BMS will probably shutdown the battery around 1.0 kWh of capacity or higher, depending on the cell voltages of the weakest cell. If you can reach 0.5 kWh before shutdown, then the battery is fine. If it happens well before it (say 1.0 kWh or higher), the battery just needs more cycles to exercise the new cells. As soon as shutdown happens, plug in the L2 right away to get it charging. If possible, all the way to 100% as that will give the BMS a really good set of data to build for the battery profile.

For context, when my wife bought her 2018 (40 kWh pack), it had a similar issue to what you described. Mainly, it had been sitting at the dealer lot for probably close to half a year doing nothing. Luckly, wasn't left at 0% or 100% charge, I think it was closer to 65% when we did the first test drive. She was concerned (as was I) and it took about 2 "cycles" of this type to get her battery pack to function properly all the way down to depleted. It also fixed her balancing issue on the low end, so that instead of being like 500 mV out of balance (which causes wild range fluxations as the battery approaches beign depleted), it was closer to a typical "healthy" Leaf battery where even all the way to near 0%, the balance would only be off by a 100 mV or less.
 
Oh thank you, that's great news. So how should I plan my trips then? Should I consider 40% the real 0% until summer gets here? I got stranded once with the old battery, I'd like that not to happen again ;)

I do have a L2 station at home. I'll try to do the procedure next summer. I'm a bit worried there's only 2 years left on the battery warranty.

By the way is there anything special about cell #1? It's the lowest on the new battery and was also on the old one.
 
The BMS is trying to be "cautious" because it doesn't know the battery well enough yet to avoid those silly SOC fluctuations that sap the "apparent" range/SOC and causes range anxiety. 😨
LeafSpy can help with that because if you can see for yourself that the battery cells are fine and you can see how much capacity the BMS thinks is left (4th screen, left side right under the graphic that has mini charts for power usage, regen, etc.), then you can ignore it until enough time has passed and the BMS has gathered enough data to build a profile that smooths out those "rough" areas. Lithium-ion batteries don't have a linear discharge curve like a lead-acid battery for example. They have a lot of power in the beginning, slowly "seem" to discharge, then quickly fade away near the end when they become depleted. That's the only way to get a "smooth" discharge of percentage on the Dash for the driver. Otherwise, the layman driver would freak out if they just used straight voltage readings where you start at 100% and it rapidly falls to 80%, then stays steady until around 20% and falls again rapidly.

As far as Cell #1, unless it becomes far out of the sync with the rest and it's a pattern. Meaning for example, in your screenshot, Cell #1 and some Cell in the middle both look to be much lower than the pack. But.. the pack is nearing the low end of depletion, so it's possible they are just the "first" group of cells that are running out of power before the others. It could mean they just have not been "fully" charged yet so when the others are full, they still have some room left, but the BMS cuts off charging too early. But the BMS has to cut-off early because the others are all full and does not want to over-charge them. Seems like a real paradox, how to get full charge when the others are stopping it? The only way is to deeply deplete all of them and charge right back up to full so those other cells get a chance to reach a final full charge. When the Leaf hits full charge (what it thinks is full anyway), from my own memory, I think it only spends about 15 minutes doing a balance. That might not be a enough time to bring those "outline" cells up to full with the rest. That's why LeafSpy is such a great tool, you can keep mental notes on what is going on and you'll know if something seems off.
 
Thanks for the info! I'll keep that in mind in the upcoming weeks.

I understand the discharge curves of pretty much all batteries are similar (alkaline, lead acid, nimh, nicd, etc.). For example:

1706129383282.png

And the BMS tries to make that linear. And a single value for a whole bunch of batteries in the pack. For this specific pack. I'd understand if it took a few cycles for the BMS to figure it out. I really hope that's the case here :)
 
On my way home I was wondering, does the same thing happens on new LEAFs? I mean, surely they don't do that long learning process at the factory, right?
 
On my way home I was wondering, does the same thing happens on new LEAFs? I mean, surely they don't do that long learning process at the factory, right?
I would suspect from the factory, knowing it's a brand new battery, the BMS is probably already pre-loaded with profile data. If you read some of the older topics about dealerships resetting the BMS on bad/degraded Leaf batteries, the same pattern happens to them where the "mileage" seems high and then quickly tanks during a trip, but most non-technical people probably don't really eye the SOC that much, probably focus more on the remaining mileage. Having gaps in the BMS data just means whatever kind of programming Nissan uses, it errors on the side of the battery more than helping range anxiety. Being a new battery, I would lean more towards this than a failing battery, but it's not impossible. I just don't want to be an alarmist about what you see in LeafSpy.
 
Hi Folks, @knightmb a question for you please (or any others that can answer). I have a 2018 LEAF SV with 33000 kms on it. I am in southern Ontario, garage parked every night, and usually charge L2 from 45% to 85% (sometimes 100) at home.

My question is, Do you think the discharge and recharge cycle is recommended for me? I have never done so in the 5.5 years I have had it. 3x L3 charges and no L1 charges to date.

I have no concerns, the BMS seems fine, never any sudden fluxuations in SoC. Only recent concern is driving in the last cold snap at -15C and lost 50% more range than expected, over about 30 kms. I usually get 3-5 km/kW in winter, this was <2 km/kW. This bothers me most as I assume I have about 220 km real range (vs 240 at new) and 2km/kW would be under 80 km full range, that seems a lot.

I have an OBD dongle and LEAF Spy Lite if needed for graphs.

Thanks, Paul
 
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Hi Folks, @knightmb a question for you please (or any others that can answer). I have a 2018 LEAF SV with 33000 kms on it. I am in southern Ontario, garage parked every night, and usually charge L2 from 45% to 85% (sometimes 100) at home.

My question is, Do you think the discharge and recharge cycle is recommended for me? I have never done so in the 5.5 years I have had it. 3x L3 charges and no L1 charges to date.

I have no concerns, the BMS seems fine, never any sudden fluxuations in SoC. Only recent concern is driving in the last cold snap at -15C and lost 50% more range than expected, over about 30 kms. I usually get 3-5 km/kW in winter, this was <2 km/kW. This bothers me most as I assume I have about 220 km real range (vs 240 at new) and 2km/kW would be under 80 km full range, that seems a lot.

I have an OBD dongle and LEAF Spy Lite if needed for graphs.

Thanks, Paul
The Deep Discharge/Recharge cycle is useful for helping the BMS better learn the capacity of your battery. The benefits are on the low end of the battery discharge, better balanced cells mean you can stretch out the 0% further. It's useful for emergencies and getting the cells back into balance if they are far out of sync as the battery is depleted causing issues (early turtle mode, early shutdown, large fluctuation of range while driving, etc.) Doing this has no effect on driving efficiency or how quickly you can charge the battery. I would NOT recommend it for your winter driving range concern.

For winter driving, I always recommend owners check the tire pressure first. If you are driving on flat tires to gain traction in snow or ice (which I myself do at times), it will sap the range a lot. If you are driving on snow tires, that will sap the range a lot too. No way around that unfortunately, as driving slow to negate wind resistance is being greatly over-powered by the tire resistance. The next power saving area is the climate control. The Gen 2 Leaf climate control system actually has a way to re-circulate air for heating when using the floor/windshield heating mode. This can cut power consumption by 50% to 80% because it's not wasting power trying to re-heat freezing air that it brings into the cabin. When we had 0F (-17C) temperatures for days in my area recently, I could see this power saving first hand as switching between the fresh air and re-circulate mode made a 2,000 to 3,000 watt difference between the two as I was driving around during those extreme cold and wind chill days (for us anyway ;)) I'm sure the effects are magnified for our neighbors to the north that have much more extreme winter conditions.
 
Thanks for getting back. So you dont recommend unless a low SoC or balance issue. What range of cell values would you call normal?

And yes I knew about the low tire pressure and the recirculate. I cant recall if that was on in the cold days. I do run with snows, and try for 38+ lbs.
 
I have no concerns, the BMS seems fine, never any sudden fluxuations in SoC. Only recent concern is driving in the last cold snap at -15C and lost 50% more range than expected, over about 30 kms.
You are over-thinking things. Severe cold has a huge negative impact on any Lithium-based battery chemistry.
Enjoy your Leaf and just don't leave it "empty" in extreme cold or "full" in extreme heat.
 
just don't leave it "empty" in extreme cold or "full" in extreme heat.
Oh, interesting. Define extreme? I understand the less time a battery stays at 100% the better, but it's also a function of time and temperature. Under what temperature does it make no sens anymore to do all the annoying stuff to keep the SoC of the LEAF under 80%?
 
Oh, interesting. Define extreme? I understand the less time a battery stays at 100% the better, but it's also a function of time and temperature. Under what temperature does it make no sens anymore to do all the annoying stuff to keep the SoC of the LEAF under 80%?
I would say 0 degrees (anything) is extreme.
Once you get below "room temperature", you don't have to worry about a 100% charge...but I would never leave any EV close to 100% for more than a day.
 
Thanks for getting back. So you dont recommend unless a low SoC or balance issue. What range of cell values would you call normal?

And yes I knew about the low tire pressure and the recirculate. I cant recall if that was on in the cold days. I do run with snows, and try for 38+ lbs.
It's not an exact science because it varies by battery health. A brand new battery should easily be in sync near the depleted state of the battery. A degraded battery might have cells that will never hold as much capacity as the neighbors and it really shows towards the end. Driving a Leaf to 1% SOC is a good example. If you pull up LeafSpy on the first screen where it shows all the cell voltages, you'll see a wide gap between many of them, easily over 200 mV between the highest and lowest. On my wife's 2018 for example (when she bought it long ago), at 1% SOC, she had over a +500 mV gap in cell voltages spread all over the place. It took over +2 deep charge/discharge cycles to bring it down under 100 mV (where it still remains today glad to say) but those deep cycles were not done one after another, I spaced them out by a few days. Now, I only do this deep discharge cycle to my 2020 and her 2018 only about 1 or 2 times a year, mainly just for BMS reasons and to see if any cells appear to be "failing" slowly in some way. It's about time to do this again, so I can give you some more accurate numbers later this week perhaps, at least in regards to old Leafs with one running a 6 year old battery and my own running a 4 year old battery. We both have over +80K miles on our Leafs now, makes for good long-term data. ;)

I had a chance to experiment with tire pressure as we had a rare "entire week" of ice and snow to drive around on. My state is never equipped to handle more than 1 day of snow, so only highways and main roads are cleared, the rest of the roads, you are on your own. 🤣
I normally deflate my front tires to 30 psi, which gives great snow and ice traction. I deflate my rear tires to 36 psi so my back-end isn't fish tailing around everywhere. I was able to determine that 33 psi gave me a good balance between snow traction and road efficiency. Normally at 30 psi, my range is nearly cut in half because the tires just drag so much on the "road" I can almost stop without brakes, lol. At 33 psi, I still have good snow/ice traction and driving on the road hits me for about a 25% reduction in range. At 36 psi, ice and snow traction become very poor (compared to 30 psi), but manageable at very low speeds and the road penalty mostly disappears at about a 10-15% reduction in range.
I would believe that you could probably "tune" your snow tires the same way. Find the maximum psi that still allows safe snow travel but also helps reduce the penalty of range when driving on cleared roads where the snow traction isn't really necessary.
 
Oh, interesting. Define extreme? I understand the less time a battery stays at 100% the better, but it's also a function of time and temperature. Under what temperature does it make no sens anymore to do all the annoying stuff to keep the SoC of the LEAF under 80%?
From the research I've read, 82F is the max temperature for a lithium-ion battery at 100% charge. Once you get over this temperature, you get the capacity decay. Slow of course, but as the temperature increases, so does the decay speed. That's why we all recommend (if possible), don't charge to 100% in warm weather unless you are going to drive out very soon. Say, planned for a long trip for example. Once you get into colder weather, where the battery is always sitting under 70F, or even better 60F, a 100% charge decay will slow down a lot. Once you get down below the 32F range, the decay is almost stopped. Granted, it still exist, but at those temperatures, it might as well almost be zero. As the battery gets colder, the charge decay problem is replaced with the lithium plating problem. As the battery gets really cold, there is a limit to how much power you can feed in to charge or pull out to use. Then you have to watch out for cold damage, granted, it has to be really cold, like -40F(-40C) cold before the damage happens just from being cold and not doing anything, but above those temperatures, you still have to limit how much power is going in and out of the battery. The BMS is suppose to manage this, so the typical end user should never really have to deal with this manually. It means though, if you search around for some youtube videos of people driving a leaf with 0 temperature bars, you'll notice the power output is greatly limited and trying to charge them is also limited. Not to mention at those cold temperatures, the battery warmer is leeching power in the background to keep the battery above a certain temperature threshold to avoid any rapid cold damage.

Anyway to answer your question, usually any temperature below freezing. The extra power is useful to "buffer" the leeching of the battery warmer and reducing charge time since it will be slower. So trying to slow charge a battery at 20% takes a long time, but if the battery was at 40%, it works better for the driver since it takes less time to charge and more buffer for the battery warmer should it kick in for extreme cold temperatures, wind chill, etc. On top of that, the BMS will NOT actually charge to 100% in the cold anyway as the battery capacity is technically "reduced" in the cold and the BMS tries to respect that. If the battery normally can be charged to 56 kWh in warm weather, the BMS might only charge it to 54 kWh in the cold weather because it has those loss calculations already coded into it with respect to battery temperature. The BMS does a lot of work trying to keep the battery healthy, but it never hurts to "help" it along with good charging habits. ;)
 
Dear Jerther. Just my personal opinion. At 3.55V/cell your battery is pretty empty and not at 40% SOC. It would be nice to know at what motor power those voltages were so all over the place. I would say, that this pack is not new but refurbished from used battery cells. but to prove this, you should charge it to some 80% and do the balance test under load from there.
 
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