Weird State of Charge (SOC) Reading Errors

shrink · Aug 26, 2015

So I really weird issue with the battery SOC readings today.

Because of the desert heat in Phoenix, I usually keep the charge around 55-60% and run COOLDOWN cycles at night. My RT commute to work is only 7 miles so I don't need much range. I know balancing can be an issue, so I try to do a full standard charge about once a month and my last one was about 15 days ago.

Anyway, this morning, I drove to work as usual. I think the SOC was around 60% when I left. I only worked 1/2 day, so after work, or about 3-1/2 hours later, when I get back in the car the battery level was in red and the SOC reading around 15-20%. I got home and charged at the full 70A. I was, however, only charging between 43 and 53 amps, probably because it was 105 F outside. I let the charge go for about 2-1/2 hours (with a COOLDOWN cycle) the the SOC got back up to 50%. I did see a DMC WARNING Charge Thermal Limit error - probably due to the heat.

I ran an errand and drove 2.2 miles. I was in the store for about 15 minutes and when I get back to the car, the SOC was at 85%.

I called the service center and my car is there now.

Has anyone ever seen an issue like this?

My car was in for annual service 2 weeks ago and got a clean bill of health.

djp · Aug 26, 2015

You probably had a faulty voltage reading on a single brick. The displayed range is based on the SOC of the lowest brick, so one bad reading can bring the estimated range down for the entire pack. This would also explain why it bounced back so quickly.

shrink · Aug 27, 2015

Hmm - that would make sense because it confused the hell out of me. Any idea what the fix is?

wiztecy · Aug 27, 2015

shrink said:
Hmm - that would make sense because it confused the hell out of me. Any idea what the fix is?

Shrink, did you dump your logs before handing your ride over to Tesla? The ahr.log file would show you the V reading on each brick and if that indeed was the real issue with the SOC being off as explained above.

I wouldn't speculate at this point for that if the SOC / brick voltage was way down for one brick significantly, its "typically" not a good thing and could point to a hardware/BMB/ESS issue. But the fact that it recovered back again is a positive sign. I've had my CAC go down over time from 154 down to 146 and then its now back up to 152. But never experienced a drastic SOC change. In my case I have overall a very very healthy pack and lately (8 months time) have been observing brick #75 and #90 fighting for the lowest brick which brings down the overall CAC. Interesting enough I see my CAC reported a little higher than their own, as if all the calculated Ah capacity of all the bricks get averaged into one final CAC. Never went back and did the math but the CAC reported in the VDS appears to reflect that when the value goes up or down. I've moved to downtown San Jose for work and have the luxury of having 5 chargepoint 30amp chargers that I use for the cooldown. When I arrive at work, 35 miles drive the battery temp starts off at 24/25C. At work its 31-33C. I then run a 16Amp cooldown @220v in Range mode for 1hr. Outside temps are anywhere from 85-90F. I drive the temps down to 24-5C where they equalize to 26C and that's my target temp. When I get home in the evening I do the same thing. Don't know if its correlated, but when my CAC was dropping I wasn't using any cool-down at work. No chargers. But right after we moved and after a week or so the CAC started jumping and climbing back up. Possibly it woke up the weaker bricks (#75 & #90) by low amps and / or the cooldown to 26C at work is helping it. Either way its always a plus whenever you see the ESS climb but hurts to look at when you see it drop.

Again with your logfile you most likely can see how each voltage, Ah, and overall CAC health was for that day.

Sending good vibes your way that its nothing that a quick reset of the computer won't fix as a long-term solution.

hcsharp · Aug 27, 2015

shrink said:
So I really weird issue with the battery SOC readings today.

Because of the desert heat in Phoenix, I usually keep the charge around 55-60% and run COOLDOWN cycles at night. My RT commute to work is only 7 miles so I don't need much range. I know balancing can be an issue, so I try to do a full standard charge about once a month and my last one was about 15 days ago.
...

I know you're running cool-down cycles but keep in mind that your battery runs cooler at higher SOCs. It's also better for your pack to keep it balanced because the cells are stressed more evenly. For those two reasons I would do a full std charge more like every other day rather than once a month. I would also give it an hour after it's done before driving to let it balance. A lot of people are trying to program their charging to finish right before they drive. Depending on your driving and charging habits, that may not be such a great idea.

wiztecy · Aug 27, 2015

hcsharp said:
I know you're running cool-down cycles but keep in mind that your battery runs cooler at higher SOCs. It's also better for your pack to keep it balanced because the cells are stressed more evenly. For those two reasons I would do a full std charge more like every other day rather than once a month. I would also give it an hour after it's done before driving to let it balance. A lot of people are trying to program their charging to finish right before they drive. Depending on your driving and charging habits, that may not be such a great idea.

Also keep in mind that your battery changes going up and down in its mood(CAC/balancing), and if you have access, to verify if balancing has been fully completed also keep a record of how much time it took to balance after the charge completes. Holding things constant, I've seen where my ESS would fully balance in 15 mins, 30 mins, and at times 6-12hours. Again holding my driving/charge behavior as constant as I can.

Lastly if for some reason I didn't get a jump on the charging and running late. I at least try to get the SOC / std. ideal miles to be in the 174 mile range if I can't wait to do a full 181 ideal mile charge and don't have time to wait for the pack to balance out. At 174 miles the pack will settle down to 172, which is the cutoff point where the balancing begins for my ESS. So that time where the Roadster would have been charging, it is using the time to balance.

shrink · Aug 28, 2015

No, I didn't pull the logs. I wanted to, but I noticed the jump from 50% to about 80% around 4:30 pm that afternoon and I had to get my car up to the SC that night before they closed at 6 pm. I also had a flight to catch the next morning at 8 am, so things were a bit hectic.

In consideration of Henry's charging advice above, what would you guys recommend in my situation:

- Phoenix summer with highs between 105-115 F
- I don't drive the Roadster everyday. It depends on my wife's work schedule. So, sometimes I'll drive it 7-10 miles to/from work 2-3 days/week; sometimes farther if I have errands to run. Sometimes I drive it once every 10 days or so.
- Should I: a) Do full standard charge nightly or every other night? b) Keep the SOC lower (to protect against the desert heat) and run COOLDOWN cycles?

The SC knows I'm traveling and still has my car. They unplugged the OVMS so I have no idea what's going on.

shrink · Sep 3, 2015

Well, Tesla installed a refurbished battery pack in my Roadster. I'm still under CPO warranty so this worked out well. They didn't tell me what was wrong with the old one. They just put a refurb in.

skadax · Sep 3, 2015

shrink said:
Well, Tesla installed a refurbished battery pack in my Roadster. I'm still under CPO warranty so this worked out well. They didn't tell me what was wrong with the old one. They just put a refurb in.

wow. I thought I got a heck of a deal with a new PEM under warranty. whole new (refurb) battery? have you noted any change in CAC or miles on a charge?

wiztecy · Sep 4, 2015

skadax said:
...have you noted any change in CAC or miles on a charge?

It'll take one to three months for shrink to balance and condition the battery to get any meaningful numbers for the above.

shrink · Sep 4, 2015

I actually don't have the car back yet and I'm still out of town. After installing the refurb battery, Tesla said they were still troubleshooting an ABS error message.

I probably won't get the car back until after Labor Day.

ML Auto · Sep 4, 2015

shrink said:
Well, Tesla installed a refurbished battery pack in my Roadster. I'm still under CPO warranty so this worked out well. They didn't tell me what was wrong with the old one. They just put a refurb in.

It was probably a loose voltage sense wire on one of the bricks. They are just connected to the sheet side plates with aluminum pop rivets. I personally believe this is going to more of a problem as these cars age. Its a shame it takes 6 hours to R&R and open the pack to replace a 2 cent rivet. And Tesla charges $5K to do it.

shrink · Sep 11, 2015

Well, I finally got my car back a couple of days ago.

These were the comments on the service records:

Performed battery analysis and ESS bleed test and found sheets 4, 5 and 7 with rivet failures. Recommend battery assembly.

They also fixed some ABS issues. I had never seen an ABS warning previously and the car was in for annual service about 2 weeks prior to my battery issues:

Replaced abs modulator assembly. Performed firmware update and brake system bleed.Verified proper operation after repair.

Performed diagnosis of ABS system and found current alert for left rear wheel speedsensor failure

Confirmed left rear wheel speed sensor failure. Performed electrical test and found leftrear sensor reading incorrect. Replaced left rear HUB assembly.

CAC on the reburb is: 149.72 and it charged to 180 miles in standard mode. Before going it, the CAC was reading around 150-something and charging to about 177 ideal miles in standard mode. My ideal miles were always a little lower than expected for the CAC reading. We'll see if it increases as I balance it over the next few months.

I was taking pictures of the new labels and it looks like my PEM is a refurb, too. I never noticed that previously. That must have been done before I bought the car.

wiztecy · Sep 11, 2015

shrink said:
Well, I finally got my car back a couple of days ago.

Congrats on getting your baby back!

shrink said:
CAC on the reburb is: 149.72 and it charged to 180 miles in standard mode. Before going it, the CAC was reading around 150-something and charging to about 177 ideal miles in standard mode. My ideal miles were always a little lower than expected for the CAC reading. We'll see if it increases as I balance it over the next few months.

The ideal mile / CAC relationship puzzles me. My original pack, the Roadster charged up to 188 ideal miles when I first picked it up and the CAC was around 154/155. It started dropping after 4k miles down to 181 and the CAC I believe was down to where your current refurb pack is, around 149. Tesla replaced my pack @20k miles, they had it out due to a 12v APS issue and replaced the pack as well. I happily was able to get this refurb pack to hit 160 CAC, but never hit the ideal miles any further than 184. @ 160 it was fully balanced.... did all the calibration tricks using range mode and long one key drives. I have about 30k on this refub pack, and now in the 50k clubs for the chassis. The pack had dropped down to as low as 146 CAC where my ideal miles were hitting 174. I've been doing aggressive cool-downs at every opportunity @ 16 amps and have seen the CAC rise back up which its still climbing where its now at 153.43 and the ideal miles are hitting 181.

Can't wait to see how high yours will go!

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