Lithium Ion battery dead?

Batteries, Chargers, and Battery Management Systems.
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dolby   100 mW

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Lithium Ion battery dead?

Post by dolby » Mar 10 2018 5:49am

I have a 37v 10 Amp hr Lithium Ion Polymer battery that has been sitting around unused for 12 months. I am not sure if the charger is able to charge due to the battery being over discharged. I measure 44 volts from the battery before turning the throttle on the bike. If I sit on the bike the power dies almost immediately. I then measure the battery voltage at 2.5 volts. If I then put the battery back on charge the charge light does not come on instead the green led light is illuminated suggesting that the battery is fully charged. I can then measure 44 volts from the battery again.
Is my battery recoverable? I understand that some batteries have inbuilt protection to prevent you from charging batteries that have been overdischarged. I don't have a balance charger but would using one and charging in NiMH mode be able to increase the voltage high enough so that I can resume normal charging again. I have seen some youtube videos showing how overdischarged batteries are recovered by charging in NiMH mode.

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amberwolf   100 GW

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Re: Lithium Ion battery dead?

Post by amberwolf » Mar 10 2018 7:28am

Lithium batteries that have been overdischarged can be internally damaged in ways that make them a fire risk.

If you want to find out how bad the problem is, you'd need to open the pack up and measure each cell group, and post up those voltages in a list here on this thread.

If you're not sure where/how to do this, you can look in other battery and/or BMS troubleshooting threads around the forums (there's a LOT of them) and it's explained in quite a few. ;)

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dogman dan   100 GW

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Re: Lithium Ion battery dead?

Post by dogman dan » Mar 10 2018 9:58am

Sounds like an undercharged cell is unable to take the load, tripping the bms.

Try balancing the battery, by repeated charges. If the charger will not restart 30 min after it finishes, you will need to gently discharge the battery enough to get it to run again for a brief top off. Try running the bike with the wheel off the ground.

If it won't even do that after a full charge, then you likely have a dead battery now. For whatever reason, some cells self discharged over the 12 months, killing it.

dolby   100 mW

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Re: Lithium Ion battery dead?

Post by dolby » Mar 10 2018 6:08pm

So the idea is to increase the voltage of the undercharged cell by repeatedly discharging all cells by running the bike wheel of the ground. The recharging process will hopefully bring the undercharged cell back up closer to the other cells until eventually they are all the same again. Is this correct?
The bike does run with the wheel off the ground and if I let it run for a minute and then reconnect the battery charger I can see that the charging light does come back on despite the battery voltage not dropping much at all. The charging light seems to stay on for half an hour before saying fully charged again. I have repeated this process about 3 times now. Should I run the bike with the wheel off the ground for longer than 1 minute or until the pack voltage drops to a certain level before reconnecting the battery charger?
The battery pack does have a balance lead however I don't have a meter to be able to read these voltages from this plug. Measuring each cell from soldered connections looks a bit tricky as they are all connected to each other so I am not sure how to read each cell individually.

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Re: Lithium Ion battery dead?

Post by amberwolf » Mar 11 2018 2:16am

dolby wrote:
Mar 10 2018 6:08pm
So the idea is to increase the voltage of the undercharged cell by repeatedly discharging all cells by running the bike wheel of the ground. The recharging process will hopefully bring the undercharged cell back up closer to the other cells until eventually they are all the same again. Is this correct?
No. The undercharged cell is discharged at teh same rate all the others are, so all you're doing is discharging it further.

For the process to do what you want, you'd have to do something that discharges *only* the fuller cells. to bring them down to the same SoC as the emptier ones.

To do taht, you have to open up the pack and connect a load across just the high ones, either one at a time or a series of groups, to drain them down without further draining the already-low ones.

If you're opening up the pack anyway, it's simpler to use a single-cell charger to bring up the low ones to match the high ones.


Should I run the bike with the wheel off the ground for longer than 1 minute or until the pack voltage drops to a certain level before reconnecting the battery charger?
No, because it's draining all cells equally, which doesn't help you.

To fix the problem without manually draining just high cells or charging just low cells, you would need to just leave the pack on the charger for the hours, days, or weeks it will take for the BMS to automatically drain down high cells a bit, restart charge until HVC on those cells happens again, drain them down, and continue that cycle over and over until the low cells charge up to match the high ones. Because the little bitty balancing resistors in tehre can only drain a little bit of charge very slowly, it can literally take weeks for a badly-imbalanced battery to rebalance itself.

And this only works if the charger is the type that automatically comes back on after it's shut off--some of them don't, in which case you have to unplug it and replug it (sometimes from both the battery and the wall) each time the charger shuts off (usually waiting a while afterward first).



The battery pack does have a balance lead however I don't have a meter to be able to read these voltages from this plug.
Multimeters are really cheap; even if they're not great you can get them for a few dollars at Harbor Freight and the like here in the USA, and HF even sends out coupons for free ones with any purchase, now and then.



Measuring each cell from soldered connections looks a bit tricky as they are all connected to each other so I am not sure how to read each cell individually.
You only need to measure each whole parallel group. There's a lot of threads about testing batteries and fixing them, at least some of which show how to test this. Fechter recently posted one way in such a testing/repair thread; can't remember which one though, so you'd have to look thru his posts for that.

dolby   100 mW

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Re: Lithium Ion battery dead?

Post by dolby » Mar 11 2018 3:14am

I didn't realize that I could take readings of the individual cells across the balance lead with a multimeter. I have done this now and here are the readings.
C1: 4.14V C2: 4.16V C3:4.15V C4:4.14V C5:4.08V C6:4.03V C7:4.17V C8:4.15V C9:4.15V C10:4.16V

Cells C5 and C6 are a bit low. I guess they are the ones cutting out my BMS. Would investing in a better balance charger solve my problem or are the voltages on C5 and C6 too low to recover?
My existing charger does not have the facility to connect the balance lead. It only has a 3 pin plug to connect to my battery pack. Does this mean that is cannot properly do a balance charge?

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Re: Lithium Ion battery dead?

Post by amberwolf » Mar 11 2018 3:37am

dolby wrote:
Mar 11 2018 3:14am
C1: 4.14V
C2: 4.16V
C3:4.15V
C4:4.14V
C5:4.08V
C6:4.03V
C7:4.17V
C8:4.15V
C9:4.15V
C10:4.16V
Those are pretty close; there's very little capacity difference between cells at those voltages.

My existing charger does not have the facility to connect the balance lead. It only has a 3 pin plug to connect to my battery pack. Does this mean that is cannot properly do a balance charge?
Does your pack have a BMS? It seems to based on the beginning of this thread (the system behavior only makes sense if it does).

If so, the BMS does the balancing, but that balance connector has to be plugged into it. Then as I said you leave the charger on for hours, days, or weeks until it's rebalanced.

If it does not have a BMS then you have to manually charge up just the low ones or manually discharge just the high ones.

Or you have to get a BMS for it.

Or you have to get a unit like the Battery Medics that can drain down cells like a BMS does.



All that said: that little bit of imbalance is not likely to be the issue causing your system to cut out.

It's probably one of these:

--your controller is drawing more power than the BMS can handle, so it shuts down. To fix that you'd have to lower the current limit in the controller, or replace the battery with one that's built to handle that load.

--your controller is drawing enough power to cause enough voltage drop in one or more cells to make them go below LVC, causing the BMS to shutdown. You'd have to measure each cell group while testing it under load to see which cell groups are dropping below LVC when the system cuts out.

--your controller is drawing enough power to cause voltage drop in the pack wiring or interconnects to cause the BMS to shutdown. You'd ahve to measure across each interconnect point while under load to see which ones have voltage drops across them, and then improve that connection.

There's probably other causes it could be too, but those are ones seen a lot in problem packs.

dolby   100 mW

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Re: Lithium Ion battery dead?

Post by dolby » Mar 11 2018 11:14pm

amberwolf wrote:
Mar 11 2018 3:37am
--your controller is drawing enough power to cause enough voltage drop in one or more cells to make them go below LVC, causing the BMS to shutdown. You'd have to measure each cell group while testing it under load to see which cell groups are dropping below LVC when the system cuts out.
I have performed tests on all the cells while under load with the bike wheel running off the ground. The results are: C1: 4.10V C2; 4.10V C3:4.11V C4:4.11V C5:3.62V C6:3.44V C7: 4.12V C8:4.09V C9:4.15V C10:4.15V
So It looks like I have 2 cells (C5 and C6) that are not coping under load and causing my BMS to cut out. I assume that if I put more load on the wheel then C5 and C6 would drop even further eventually causing the BMS to cut out. I guess I can get a new battery or try to replace the faulty cells. I have opened up the pack and it doesn't look easy to replace the faulty cells. Where can I get replacement lithium ion cells from?

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