Why so much buffer/reserve kWh?

[Crazypostman]

I made a post a few weeks ago talking about the range not really being up to what it should be. Of course all of the apologist that know more than me came out letting me know how I was wrong it was the temperatures and the HVAC usage. Well today I just wanted to make a post about capacity we're not even going to talk about miles. So the model I have has a 76kWh gross and 70kWh usable battery pack. I know the BMS doesn't know the exact kWh remaining but it's a pretty good guess based on voltages.

So here's where my problem, when I charge the car to 100% and it cuts off the car shows that I have 65.61 kWh remaining. When I drive down to 0% I have 3.34kWh remaining. ( I assume those 3.3kWh are usable still below zero but that's risky so I'm not going to count those as "usable" )

So really from 100% to 0% I only have 61.7 kWh usable.

(I'm using displayed state of charge numbers not BMS. BMS shows 95% when car stops charging full, and 5% when the car shows 0%)

So that leaves 14.3kWh of reserve/buffer.
10.96 if I use those 3.34kWh that are below 0%.

That seems excessive to me! I'm not a battery engineer, but other car manufacturers do fine with 5-7kWh battery protection buffer.

What's the highest kwh that any of y'all have been able to pull out of an Sr?

Okay let the flogging commence!

PS. I actually hope my numbers are wrong somewhere. Or I'm misunderstanding something.

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[RickMachE]

Numerous posts question the accuracy of numbers you find with Car Scanner and other apps.

Batteries have different capacity based on... temperature. Do those readings when it's 75 in your garage.

 

[Crazypostman]

Numerous posts question the accuracy of numbers you find with Car Scanner and other apps.

Batteries have different capacity based on... temperature. Do those readings when it's 75 in your garage.

Yeah I'm looking forward to checking this in June or July. I hope I find a few more kWh

 

[RickMachE]

I have sensors on my garage doors with lithium cells. Dead of winter they screamed low battery. Pulled them last week, they are very strong. Put them back in, no more low battery warning.

 

[SpaceEVDriver]

At what HVB temperatures are these capacities measured? What are the error bars on the measurements you've taken?

Most modern BMS will count coulombs rather than estimate SOC based on voltage. If CarScanner is actually pulling these values from the vehicle's BMS measurement, it is likely close (+- 1%)...IF you occasionally recharge to 100%, which it sounds like you do.

Have you independently measured what is put back into your battery after running it down to 0%? I'm curious what your charging station says. If you don't have a "smart" charging station, maybe get a coulomb counter you can put on the charge cable to track what's provided by the EVSE (it will be higher than what makes it into the battery of course).

 

[Crazypostman]

At what HVB temperatures are these capacities measured? What are the error bars on the measurements you've taken?

Most modern BMS will count coulombs rather than estimate SOC based on voltage. If CarScanner is actually pulling these values from the vehicle's BMS measurement, it is likely close (+- 1%)...IF you occasionally recharge to 100%, which it sounds like you do.

Have you independently measured what is put back into your battery after running it down to 0%? I'm curious what your charging station says. If you don't have a "smart" charging station, maybe get a coulomb counter you can put on the charge cable to track what's provided by the EVSE (it will be higher than what makes it into the battery of course).

I had the full intention of doing a zero to 100% DC fast charge but our stations charged by the minute and after 80% it was charging so slow I gave up at 90%
I do have a mobile EVSE that a company sent me for testing that does have a kilowatt hour added screen. I will run the battery down to zero tomorrow and see exactly how many kilowatt hours are added. Shouldn't be too much heat loss it's only 32 amps.

 

[SpaceEVDriver]

I had the full intention of doing a zero to 100% DC fast charge but our stations charged by the minute and after 80% it was charging so slow I gave up at 90%
I do have a mobile EVSE that a company sent me for testing that does have a kilowatt hour added screen. I will run the battery down to zero tomorrow and see exactly how many kilowatt hours are added. Shouldn't be too much heat loss it's only 32 amps.

I'm sure the numbers aren't going to be wildly different from the ~67 kWh estimated at 100% SOC you're already seeing. I'm just curious if they vary by +-1% or +-10% or something in between. I suspect it'll be around 2-4% 1-sigma with consistent HVB temperatures. But between about 0 degrees C and 25-30 degrees C, you could see up to 30% difference in maximum capacity.

Of course, I wouldn't recommend you do many 0-100% charge cycles. You probably have the results from the charge cycles you've already done.

 

[andrew]

I had to replace a 2013 Nissan Leaf battery. I had lost 25% capacity by the time the warranty was set to expire (5 years), so I got the cost covered. I was much more careful with my second battery. Due to some changes in commute distance… and eventually the pandemic… I was able to charge less frequently and rely on level 1 charging (which I assume generates less heat). I do think I was also driving less, so it’s hard to tell what I did right with the second battery. I sold my leaf after 4 more years, and I had just dropped to 91% capacity.

I have assumed that stressing the battery really does cause a lifespan drop, and I think this buffer is there to keep the battery in good shape for a longer period of time. Nissan didn’t have that luxury with a 24kWh pack, but I am pretty sure that buffer is the reason we have a longer battery warranty, and will likely decrease costly battery replacements once the warranty expires.

 

[SpaceEVDriver]

I had to replace a 2013 Nissan Leaf battery. I had lost 25% capacity by the time the warranty was set to expire (5 years), so I got the cost covered. I was much more careful with my second battery. Due to some changes in commute distance… and eventually the pandemic… I was able to charge less frequently and rely on level 1 charging (which I assume generates less heat). I do think I was also driving less, so it’s hard to tell what I did right with the second battery. I sold my leaf after 4 more years, and I had just dropped to 91% capacity.

I have assumed that stressing the battery really does cause a lifespan drop, and I think this buffer is there to keep the battery in good shape for a longer period of time. Nissan didn’t have that luxury with a 24kWh pack, but I am pretty sure that buffer is the reason we have a longer battery warranty, and will likely decrease costly battery replacements once the warranty expires.

There's a major difference between the Leaf and most other EVs. The Leaf doesn't have active high voltage battery temperature management. It relies on passive features. That means it can not manage the HVB temperature when charging, as one important example (nor when driving, etc).

 

[chuckles]

Battery life is dramatically impacted by temp and discharge depth:

Personally, I'd much rather have 12% buffer on both sides (I have a 100kwh, 88 usable) in exchange for a battery that lasts (much) longer.

(edit: math is hard)

 

[Crazypostman]

Battery life is dramatically impacted by temp and discharge depth:

Personally, I'd much rather have 22% buffer on both sides (I have a 100kwh, 88 usable) in exchange for a battery that lasts (much) longer.

This will probably come off negatively, but I I'm really not worried about the battery at all. it's guaranteed for 8-years and I have never in my life owned a car that long anyway. I only full charge mine once a week the other 6 days are 90%. (I use almost the full battery everyday) I had a Tesla for 3 years before this and did the same thing and it only had about 14% degradation at the end.

 

[Triggerhappy007]

This will probably come off negatively, but I I'm really not worried about the battery at all. it's guaranteed for 8-years and I have never in my life owned a car that long anyway. I only full charge mine once a week the other 6 days are 90%. (I use almost the full battery everyday) I had a Tesla for 3 years before this and did the same thing and it only had about 14% degradation at the end.

How many miles a year do you drive? How many miles did your Tesla have?

 

[Crazypostman]

How many miles a year do you drive? How many miles did your Tesla have?

I only had 33,000

 

[Triggerhappy007]

I only had 33,000

You said you almost use the full battery (90%) everyday. How many miles do you drive a year?

 

[Crazypostman]

You said you almost use the full battery (90%) everyday. How many miles do you drive a year?

Yeah that was worded poorly above I don't drive as many miles as that made it seem. My mileage didn't increase until about the last few months I owned the Tesla through currently with the Mach-E this is supposed to been a temporary thing and it's just been dragged out. Currently it's about a 110 mi round trip 5 days and 1 day is 150 MI.

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