Charge Level Accuracy after 12 v. Has Been Disconnected

I disconnected the 12 v. battery on my i-MiEV before storing it 6 months ago. The charge level when I stored it was ~40%. I was pleasantly surprised that the 12 v. battery's voltage was 12.3 after sitting for 6 months, so I have just reconnected it.

When I started the car, the battery pack's charge level gauge was initially at 0% but then increased to ~40% at the rate of ~1 bar/second. How accurate do you think this is? I expected the battery pack's charge level to drop over 6 months, but the gauge suggests that it hasn't. How does the battery management system determine the battery pack's charge level after the 12 v. power has been cut?

I still can't charge my car in its parking space, so I need to drive to a public charging station. I'm a little concerned that while driving, the battery management system might determine that the actual charge level is lower than indicated which would then cause a downward recalibration of the charge level gauge. I don't want to suddenly find that I don't have sufficient remaining power to reach a charging station.

Thanks for any insight that you have!

I've seen mine do that once. Lithium ion has a very slow rate of self-discharge, so your pack shouldn't have dropped more than 5-10%, if even that much.

PV1 said:

Lithium ion has a very slow rate of self-discharge, so your pack shouldn't have dropped more than 5-10%, if even that much.

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Correct. 1% or so per month is the generally accepted rate of self discharge

Don

The charge level should be close. However, long term disconnection of the 12 volt battery (and six months is about it) possibly will lead to some factory data about the pack that is stored in the BMS being lost. This information is used to control how the BMS determines the "health" of the pack over its lifetime. One of the parameters is the build date and there is some other charge/discharge data that could be lost. It won't be fatal, but it would be best to leave a trickle charger on the 12 volt battery when in storage if you have power for it available. If not, you can buy a small solar panel for a few bucks that will keep it charged outdoors away from a plug. I pulled a BMS out of a wrecked I-MiEV and after being disconnected from the 12 volt for almost 8 months the data was still there.

alohart said:

I disconnected the 12 v. battery on my i-MiEV before storing it 6 months ago. The charge level when I stored it was ~40%. I was pleasantly surprised that the 12 v. battery's voltage was 12.3 after sitting for 6 months, so I have just reconnected it.

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Well, apparently 12.3 v. isn't sufficient. My 12 v. battery warning light is illuminated. Hopefully, during a full battery pack charge tomorrow thanks to a nearby PlugShare member, the 12 v. battery will be charged sufficiently to turn off the warning light.

I noticed this same before when I was playing with the ethanol heater and relocating the battery to the trunk of my car.

If you know the chemistry and you assume that the cells have been resting, you can in fact determine state of charge from the cell voltages, if you can measure them very precisely. When you have 88 cells in series, you should be able to make a pretty good guess of SOC just based on total voltage. You have to keep in mind that the bars and the SOC percentage are not absolute measure by any means. More like an educated guess. I had a nasty reminder of this once when I quick charged to 80%, but once I had driven a hundred meters I was down to 71%.

Lithium cells have basically non-existent self discharge if completely disconnected. In practice, most BMS will draw some milli-amps from the cells, which accumulate over time, and this is why you shouldn't leave you pack unattended for long periods of time.

siai47 said:

The charge level should be close. However, long term disconnection of the 12 volt battery (and six months is about it) possibly will lead to some factory data about the pack that is stored in the BMS being lost. This information is used to control how the BMS determines the "health" of the pack over its lifetime. One of the parameters is the build date and there is some other charge/discharge data that could be lost. It won't be fatal, but it would be best to leave a trickle charger on the 12 volt battery when in storage if you have power for it available. If not, you can buy small solar power systems for a few bucks that will keep it charged outdoors away from a plug. I pulled a BMS out of a wrecked I-MiEV and after being disconnected from the 12 volt for almost 8 months the data was still there.

Click to expand...

Thanks for sharing out the charging solutions.. Even I would love to make use of solar power pretty soon..