Battery fuel gauge discharges e-bike battery

Batteries, Chargers, and Battery Management Systems.
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avandalen   10 W

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Battery fuel gauge discharges e-bike battery

Post by avandalen » Dec 12 2012 10:48am

I have seen that e-bike batteries may drain by itself. Batteries charged to 40%, which is a good practice to extend lifetime, were completely discharged after one month. So I opened a battery and tried to investigate what happened. By doing measurements at the electronics, I found out that the cause of the discharge is the fuel gauge, which had a quiescent current of 6mA. This will take just one month to discharge a battery which is charged to 4Ah. See more here:

http://www.avdweb.nl/solar-bike/batteri ... gauge.html

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Who had the same experience?

circuitsmith   1 W

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Re: Battery fuel gauge discharges e-bike battery

Post by circuitsmith » Dec 12 2012 11:07am

6mA is a ridiculous amount of unswitched parasitic drain.
I might live with that much drain on a battery for an electric car, but nothing smaller.
I once worked at a company where a young engineer designed a battery operated circuit that used 100uA, then hung a 10mA LED on as a power indicator.

The BMS for my Currie lipo pack draws 100uA of the top of the stack and 4uA off each balance tap:

http://endless-sphere.com/forums/viewto ... 14&t=44100

The 100uA is primarily due to a schottky diode in the charger input.
A conventional diode or small relay could eliminate it.

999zip999   100 GW

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Re: Battery fuel gauge discharges e-bike battery

Post by 999zip999 » Dec 12 2012 11:11am

My A123 20ah 82v battery is plugged in for use or unplugged as 2-12s. All things are unplugged controller C.A. no bms ect. It's unplugged.Plug it in to charge and ride only.

SamTexas   100 MW

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Re: Battery fuel gauge discharges e-bike battery

Post by SamTexas » Dec 12 2012 12:25pm

circuitsmith wrote: The BMS for my Currie lipo pack draws 100uA of the top of the stack and 4uA off each balance tap:

http://endless-sphere.com/forums/viewto ... 14&t=44100

The 100uA is primarily due to a schottky diode in the charger input.
A conventional diode or small relay could eliminate it.
Do you provide a user switch to turn the BMS completely off?

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dogman dan   100 GW

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Re: Battery fuel gauge discharges e-bike battery

Post by dogman dan » Dec 13 2012 6:45am

This is why I don't recomend storing batteries with a bms, or any other device still attached at less than 100% charged. It's a tradeoff, better to lose some lifespan rather than blow it and end up with a ruined battery because you didn't check it this week.

The one time it got me, I arrived at home 90% discharged, and running a 103F fever. Sick, I forgot to turn off the controller, which I normally do by unplugging it. Nor did I put it on the charger. By the time I got well, the battery was a dead duck. About a 3 amp drain did the trick in no time.

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t3sla   10 kW

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Re: Battery fuel gauge discharges e-bike battery

Post by t3sla » Dec 13 2012 7:23am

avandalen wrote:I have seen that e-bike batteries may drain by itself. Batteries charged to 40%, which is a good practice to extend lifetime, were completely discharged after one month. So I opened a battery and tried to investigate what happened. By doing measurements at the electronics, I found out that the cause of the discharge is the fuel gauge, which had a quiescent current of 6mA. This will take just one month to discharge a battery which is charged to 4Ah. See more here:

http://www.avdweb.nl/solar-bike/batteri ... gauge.html

Image
Image

Who had the same experience?

I have a feeling this is done by design, what's the technical term...designed to fail masked under the guise of elegant degradation.

It's really a sign of the times we live in. :|
Endless Spirit.

circuitsmith   1 W

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Re: Battery fuel gauge discharges e-bike battery

Post by circuitsmith » Dec 13 2012 10:24am

SamTexas wrote:
circuitsmith wrote: The BMS for my Currie lipo pack draws 100uA of the top of the stack and 4uA off each balance tap:

http://endless-sphere.com/forums/viewto ... 14&t=44100

The 100uA is primarily due to a schottky diode in the charger input.
A conventional diode or small relay could eliminate it.
Do you provide a user switch to turn the BMS completely off?
No, since it takes about 400 days to bleed one amp-hour off the 15AH pack.

Worst case BMS failure (shorted FET) will cause a drain of <400mA.
If a cell gets drained below 3V it will refuse to charge.

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