Battery Trouble (again)?

[allotmenteer]

Hello all

I've noticed that my battery has been giving lower and lower readings when fully charged. This has happened over the last dozen or so charges. Battery is a 24V 6.8Ah li-ion. It used to read 29V straight of the charger. After a few charge cycles it read 28.7V and now is down to 28.5V. What is going on? Do I have a failing cell?

Regards

Paul

 

[flecc]

Bit early to say Paul, cell resistance increases during the battery life which can account for a degree of change, and another possibility is variation in the charger's cut-off point. I think watch and wait for a little while before taking any further action as that change is so small at present.
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[allotmenteer]

Thanks flecc, I'll give it some more time. I have checked the charger and it is putting out 29.1V or 29.2V (from memory). I think officially it should put out 29.4V. Perhaps my multimeter may be slightly inaccurate.

Have your (or anybody's) battery packs had the same issue?

Paul

 

[flecc]

I haven't actually measured the Li-ions for decline Paul, just mentally noting the performance decline, but with my ones that shows up more as an inclination to cutting out under extreme load with voltage drop. The incidence of that rose markedly in mid winter when the batteries were 6 months old, and the one I retained is still cutting out frequently now in warmer weather under heavy load at 10 months old. All these incidents are indicative of a voltage decline, my near 4 month old Li-ion not cutting yet.
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[allotmenteer]

I've been thinking about it and maybe I do have a cell that has a problem.
The pack is made up of 7 cells of 3.7V. Each cell when fully charged used to be 4.143V (29V / 7). When discharged I got a reading of 25.8V off the pack (3.69V each cell). Now what if one cell is discharged to that level and is now not accepting charge (perhaps a BMS issue) then the pack voltage would be (6*4.143) + 3.69 = 28.548V. This makes sense (to me).

Could I be right in this? Is there any way to tell? I'll open the pack and check for any issues. I seem to remember on my last pack which failed there was a green led glowing from under the shrink wrap - wonder if that was a cell failure indicator?

I have read that battery packs can have some sort of cut-off device that may need resetting. Does anyone know more about these things?

Paul

 

[flecc]

You can check the individual cell voltages if you open the pack, but be careful with the cell monitoring circuits. It's sometimes difficult to get access to the cell terminals directly since they can be capped by insulating plates and circuit boards. There's usually a mainboard with control circuits and regulation and there can be an integrated cutout. Here's some photos of the eZe Li-ion implementation.

 

[allotmenteer]

You can check the individual cell voltages if you open the pack, but be careful with the cell monitoring circuits. It's sometimes difficult to get access to the cell terminals directly since they can be capped by insulating plates and circuit boards. There's usually a mainboard with control circuits and regulation and there can be an integrated cutout. Here's some photos of the eZe Li-ion implementation.

Thanks for the info flecc. I've opened the pack but all the cells are wrapped in blue plastic and only the main board is accessible. Unfortunately this board seems to be potted (I believe that is the term for being sealed in some kind of plasticy stuff) so I don't think there is much I can do with trying to test the voltage of each cell. I think I will contact the shop (who shall remain nameless) that sold me the kit.

I don't know if the pack is faulty but I will have to take it out for a long run to see what the range is. If one cell is gone then I should lose a fair bit of range.

If it is faulty then I definitely don't want another of the same type.

Paul

 

[allotmenteer]

And falling...

A few charges later and the pack is now 28.1V. Looks like something is going wrong quite quickly.

Paul

 

[flecc]

Yes, looking more like a failed cell now Paul. My first Li-ion failed quite rapidly within months due to voltage drop.
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[allotmenteer]

A few charges later and the pack is now 28.1V. Looks like something is going wrong quite quickly.

Paul

Took a reading before going out to the allotment tonight - 28.4V. Seems after a rest (overnight) the reading went up. After the ride (4 miles) I charged the pack until the light on the charger was constantly green. Voltage reading is now 28.4V. Hmm...could the charger be at fault?
I've emailed the supplier to ask what they think.

Paul

 

[flecc]

It's possible Paul, but that very small variation between 28.1 and 28.4 is only 1% so could easily be within the normal tolerance of charge cut of variation. That 1% is considered a high degree of accuracy in consumer electronics generally, only test instruments and some Hi-Fi equipment often bettering it.
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[allotmenteer]

Took a reading before going out to the allotment tonight - 28.4V. Seems after a rest (overnight) the reading went up. After the ride (4 miles) I charged the pack until the light on the charger was constantly green. Voltage reading is now 28.4V. Hmm...could the charger be at fault?
I've emailed the supplier to ask what they think.

Paul

I've been taking regular readings after charging and before and after riding. I have found that after a ride to exhaustion (battery, not me ) the pack reads 25.2V but if you leave it 1/2 hour it recovers to 25.4 or 25.5V. Same thing happens with the pack immediately after charging 28.1V goes up to 28.4V. Sometimes after charging it reads 28.4V straight away. Perhaps what I am seeing with the small variations is down to temperature of both the battery pack and the ambient temperature. I know that lower temperatures can cause a drop in voltage but do higher temperatures cause a voltage drop with li-ion?

Paul

 

[flecc]

Only when under current drain conditions normally Paul. Voltage drop under load is very much a feature of Li-ion performance though, so perhaps these small variations are also to be expected.

The recovery from exhaustion that you mention is common to my Li-ion batteries as well. If I run one to empty and cutout, then rest it a few minutes, it will run again idling, but won't accept a load. If I cause a premature cutout a little before the battery is naturally empty, it will accept some load after a rest, but doesn't fully recover what was lost at first.
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