Depleted battery Showing 18 kWh - Should I Worry?

[unplugged]

After reading about using the extended trip information screen to show battery usage, I finally ran it down to empty. I had about three miles remaining, and I used the suggestion to use the heater to deplete the battery to zero. The final battery usage was 18 kWh exactly. This causes me some concern because I have read that the battery should have around 19.4 kWh in it. Does this mean that in one year of usage, the battery has lost 7% of capacity?

This weekend, I am going to be doing some more long distance drives, and I can confirm my numbers. But now I am worried.

 

[v_traveller]

A couple of questions:

1) When you depleted the battery to zero, was the car actually unable to move? In my experience, there was maybe 0.7kWh-1kWh left after getting the "charge depleted" message, judging by how far I went when I screwed up and ran out of charge once.

2) When you charged up prior, did you unplug immediately after the FFE informed you it was fully charged, or was it plugged in for at least a half hour after that? From what I've seen while charing L2 on chargepoint, the FFE informs me that it's fully charged before chargepoint says that the FFE is drawing little or no current. I've seen the FFE draw as much as 0.5 kWh in that period of about 20 minutes after being told by the FFE that it was done... but that's on the chargepoint side, so one has to factor in inefficiencies in trying to figure how much actually got to the battery.

With all that, it still doesn't add up to the 1.8kWh discrepancy... but it could perhaps justify most of it...

 

[michael]

In my opinion, this is within the normal range and you should not worry.

First of all, I don't agree with the 19.4 target value. This number was seen often here but was based on assumptions about the usable percentage. Now that we have OBD scanners, it appears that the usable range is fromapproximately 90% to approximately 8%. 82% of 23 kWh is 18.9 kWh. I would consider this my current estimate of the nominal capacity when measured in a specific manner.

Then, your specific result is influenced by the details of your test. Key factors include:

1. Battery temperature during the test...lower temps give lower available energy

2. Power levels during the test...higher power levels turn more stored energy into heat, resulting in less delivered energy

3. Amount of regeneration during the test. Only some portion of the regenerated energy actually gets stored back in the battery, and some portion gets lost in the charging process. The trip meter sees regeneration flowing into the battery terminals, but it doesn't know how much of this energy is converted into chemical energy and how much is lost to heat.

The only way to get a solid number is to use a battery tester and perform the test under specified conditions. Using the trip energy meter is a good approximation but is not exact. Another approximation is the energy to empty value shown in the OBD scanner.


If you really want to know your battery's capacity, buy one of these:

http://www.avinc.com/testsystems/product_matrix/abc-150/#product

Otherwise, don't worry about small differences between your trip meter and what we believe to be the expected capacity. If it makes you feel better, do the test:

1. With the battery at 98 F
2. On a level course with no regeneration, driving maybe 30 mph

I think this will give you a slightly higher number.

 

[davideos]

Should you worry? No. I think you should just keep your eye on it. When my car is at 90% and I reset the trip meter just after charging, my car shows 1.6KW consumed for that first 10%. Each additional 10% is another 1.6KWs. After 50%, I start to get more charge for each 10% consumed. If I go to depletion...0mi remaining but not officially dead, I will have just about 18KWs consumed.

When I first got the car, the software didn't show the state of charge, so I couldn't do good comparisons; however, I have monitored what it takes to get to work and back and that hasn't changed over the last year and a half.

I think there are discrepancies in the way either of these are measured, and not necessarily the true capacity of the battery. If you've purchased your car, just keep track of your experiments and when your 8 year warranty is about up, check it again. Anyway, that's my approach.

Regards

 

[WattsUp]

Should you worry? No. I think you should just keep your eye on it. When my car is at 90% and I reset the trip meter just after charging, my car shows 1.6KW consumed for that first 10%. Each additional 10% is another 1.6KWs. After 50%, I start to get more charge for each 10% consumed. If I go to depletion...0mi remaining but not officially dead, I will have just about 18KWs consumed.

...also, people have bandied about the idea that the trip meter isn't completely accurate with regards to computing kWh consumed, so that's something to keep in mind.

But, I agree, don't worry. I think your FFE is saying normal things.

 

[unplugged]

Thanks to everyone for their intelligent and well worded responses. I will keep an eye on my range, but I do feel much better about thinking my battery was going bad.

 

[studio460]

I assume you deliberately drained your battery to 0% in an attempt to gauge its capacity. But, isn't that one of the life-shortening factors for Li-ions? As I understood it, you should never allow a Li-ion battery go to 0%.

 

[davideos]

The FFE will not let you actually drain to 0% or charge to 100%. Roughly, there is a 10%buffer at both ends. The usable capacity is around 80%... More or less.

 

[WattsUp]

The FFE will not let you actually drain to 0% or charge to 100%. Roughly, there is a 10%buffer at both ends. The usable capacity is around 80%... More or less.

Correct. The car will not "let" you ruin the battery.