Used LiFePO4 behavior

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energyi   10 W

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Used LiFePO4 behavior

Post by energyi » Mar 05 2013 1:42pm

Have a question for ES on a used LiFePO4 battery (48V, 20 AH Golden Oasis). It seems to self discharge. Is this consistent for LiFePO4 technology?

In the first hour after charging with no load (20 ma voltmeter) it drops from 57.97 V to 57.00 V. It further dropped to 56.74 V in the next two hours. The next morning it had dropped to 55.64 in the first hour of monitoring.

It seems like it might be the BMS that is using the power, or redistributing it. When the volt meter is attached, it shows the previous voltage, but quickly reduces to the numbers I posted to you above. I was testing every ten minutes, about a volt every hour reduction. And again, there was a 20ma draw on the volt meter, but I would assume that doesn't account for the large voltage drop.

I ran my hub motor and the battery went down to 52.6 V, and it hovered around that voltage, back and forth between 52.6 and 52.5. Then I came back maybe 10 minutes later and it had gone up to 53.1V.

Is this behavior consistent with a one year old LifePO4 battery/BMS with approximately 50 charge cycles? Thanks in advance for any insights.

Best,

Joe Mc
HS3540 20" on Terratrike Tour Recumbent, LifePO4 48V 20 amp (was 12S2P Lipo), Greentime controller w/regen. Denver Area.

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Re: Used LiFePO4 behavior

Post by dnmun » Mar 05 2013 1:58pm

yes. you can measure the BMS circuit current it you wanna.

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Ypedal   100 GW

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Re: Used LiFePO4 behavior

Post by Ypedal » Mar 05 2013 2:06pm

The charger should bring all cells to 3.6v~3.7v before it stops charging,

Assuming this is a 48v , 16 cell pack.

3.6v x 16 = 57.6v

Some lifepo4 cells retain this voltage, while others will drop to 3.4v all on their own, and this is not a problem.. in a situation like this .. 3.4 x 16 = 54.4v

When you put the pack to use and draw power, they drop to 3.2~3.0v depending on load and C rate of the cells.. all normal....

below 2.7v per cell the pack is near empty.. and 2.0v is absolute completely drained to 100%
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Re: Used LiFePO4 behavior

Post by icecube57 » Mar 05 2013 5:26pm

This is somewhat normal behavior. Its the voltage rebound of the cells. I saw the same thing with the 48v 10AH lifepo4 pack i just sold. Most of the voltage rebound instantly but afteral several minutes the resting voltage came back up. Remember a .025v rise in voltage across 16 cells is alot.

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Re: Used LiFePO4 behavior

Post by amberwolf » Mar 05 2013 5:51pm


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dogman dan   100 GW

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Re: Used LiFePO4 behavior

Post by dogman dan » Mar 05 2013 5:52pm

I'd call it normal. New, it should hold it's full charge as long as overnight, even with a bms slowly draining it. Dropping to 56v, all you've lost is surface charge. That's a slight overcharge that is done to ensure that the cells are truly fully charged, rather than stopped too soon.

But by cycle 50, most will drop to 56v overnight. Then in use, dropping again to about 52-53v is pretty normal. And from there, dropping 2-3v more under load is not unsual, but the voltage will come back some when you stop, or ride under much less load.

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geeeyejo1   1 kW

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Re: Used LiFePO4 behavior

Post by geeeyejo1 » Mar 05 2013 6:02pm

dogman wrote:I'd call it normal. New, it should hold it's full charge as long as overnight, even with a bms slowly draining it. Dropping to 56v, all you've lost is surface charge. That's a slight overcharge that is done to ensure that the cells are truly fully charged, rather than stopped too soon.

But by cycle 50, most will drop to 56v overnight. Then in use, dropping again to about 52-53v is pretty normal. And from there, dropping 2-3v more under load is not unsual, but the voltage will come back some when you stop, or ride under much less load.
My Sunthing Pack does the same - charged to 60.4V Sunday afternoon now down to 56.0V on Tuesday two + days later. I have a LCD voltmeter connected in line with my pack that was bleeding away the surface charge and then some - leaving the pack at about 53-54V after maybe two days. I now disconnect my in-line fuse right after charging (which cuts current to my packs Voltmeter) and now, except for the BMS I draw very little current.
1997 Gary Fisher HooKooEKoo Hard Tail MTB, 48V 1000W Yescom Rear Kit, 48V 15 AH Lifepo4 Sunthing28 Pack

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circuit   100 kW

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Re: Used LiFePO4 behavior

Post by circuit » Mar 06 2013 12:29am

Voltage does not directly reflect capacity. LiFePO4 cells usually drop to 3.2-3.4V in few hours. This is absolutely normal.
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fechter   100 GW

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Re: Used LiFePO4 behavior

Post by fechter » Mar 06 2013 1:14am

It might be a good idea to try measuring the individual cell voltages after it has dropped to make sure you don't have a bad cell. I've seen abused cells that will drop fast and keep dropping to below 2v. If this was the case, your pack would have significantly less capacity. If they are relatively even, I wouldn't worry about it.
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energyi   10 W

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Re: Used LiFePO4 behavior

Post by energyi » Mar 09 2013 3:20pm

Thanks to all for the ES knowledge base. Most appreciated.
HS3540 20" on Terratrike Tour Recumbent, LifePO4 48V 20 amp (was 12S2P Lipo), Greentime controller w/regen. Denver Area.

energyi   10 W

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Re: Used LiFePO4 behavior

Post by energyi » May 14 2015 9:30pm

Just a positive update two years later, that pack is still going strong. Very happy with LiFePO4 decision.
HS3540 20" on Terratrike Tour Recumbent, LifePO4 48V 20 amp (was 12S2P Lipo), Greentime controller w/regen. Denver Area.

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fechter   100 GW

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Re: Used LiFePO4 behavior

Post by fechter » May 14 2015 10:48pm

Thanks for the report. Nice to get some long-term results.
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