low voltage cut off when riding a long ride

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Glens carzy ebike   1 mW

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low voltage cut off when riding a long ride

Post by Glens carzy ebike » Nov 09 2019 3:43am

Hi i had a voltage cut of today on my e bike for the first time i was full throttle for about 65 km in distance doing a speed of 50km would i have damaged the battery or does the cut off stop it from being damaged ? :warn:

AngryBob   100 W

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Re: low voltage cut off when riding a long ride

Post by AngryBob » Nov 09 2019 6:22am

In theory, yes. In actual practice, it would depend if the LVC on either the controller, or the BMS, is 1.) set correctly for your battery, and 2.) what exactly constitutes "damage". This second parameter is a variable, and becomes a matter of just exactly how much degradation to your battery that you personally consider unreasonably harmful. It varies broadly with chemistry and somewhat more narrowly with particular cell types, formulations, and discharge curves. Also specific usage pattern.

That is a general answer intended to illustrate that with the extremely limited information given no more precise answer is possible.

A WAG based on minimal data would be "probably".

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dogman dan   100 GW

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Re: low voltage cut off when riding a long ride

Post by dogman dan » Nov 09 2019 8:27am

Better of course, to stop sooner. But if your battery is not a total train wreck, no harm.

What I mean is good cells can do this many times. A bad cell was a train wreck from the beginning, and discharging that low could be the last straw for it. So unlikely you damaged anything, and no you tested it. Next ride, you will know if a cell died pretty quick. If that does happen to be the case, nothing you could have done would have saved that bad cell.

What is very likely though, is discharging the battery to cutoff drives the cells out of balance some. I found it good practice to do a bit of extra charging before the next ride, or at least before the next long ride.

Charge till its full and unplug from the charger. wait half an hour, and plug it back into the charger. If it does not start charging, ride around the block and try again. This will give your bms an extra chance to balance the battery, getting all the cells equally full. Each time this is repeated, the bms gets another opportunity to balance it better.

Winter is coming,, over the winter, recharge the battery monthly or so. This gives it a better chance of remaining in balance till spring. Too long storage will result in the bms drawing enough power to unbalance the pack. Typically, bms runs on just one or two of the cells, so it can discharge those two cells in a half a year.

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E-HP   10 kW

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Re: low voltage cut off when riding a long ride

Post by E-HP » Nov 09 2019 8:51am

Glens carzy ebike wrote:
Nov 09 2019 3:43am
Hi i had a voltage cut of today on my e bike for the first time i was full throttle for about 65 km in distance doing a speed of 50km would i have damaged the battery or does the cut off stop it from being damaged ? :warn:
It's relative. What voltage level are you normally comfortable with the battery dropping to when you're paying attention? If the BMS is tripping below that voltage, then yes, you incrementally damaged the battery more than you're comfortable with.

docw009   100 kW

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Re: low voltage cut off when riding a long ride

Post by docw009 » Nov 09 2019 9:54am

There are two low voltage cutoffs. First is the one on your bike controller. Most of my 48V controllers are set for 41 volts. That's 3.1 volt per cell. Relatively safe. Second is the battery BMS LVC. Many go down to 2.7V per cell, which is lower than I'd want to go.

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fechter   100 GW

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Re: low voltage cut off when riding a long ride

Post by fechter » Nov 09 2019 4:22pm

Since he was running at full throttle when the pack cut off there would be quite a bit of voltage sag at the time. The cell voltage would immediately rebound once the discharge stopped so unlikely they got low enough to cause damage.
"One test is worth a thousand opinions"

Glens carzy ebike   1 mW

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Re: low voltage cut off when riding a long ride

Post by Glens carzy ebike » Nov 09 2019 5:29pm

dogman dan wrote:
Nov 09 2019 8:27am
Better of course, to stop sooner. But if your battery is not a total train wreck, no harm.

What I mean is good cells can do this many times. A bad cell was a train wreck from the beginning, and discharging that low could be the last straw for it. So unlikely you damaged anything, and no you tested it. Next ride, you will know if a cell died pretty quick. If that does happen to be the case, nothing you could have done would have saved that bad cell.

What is very likely though, is discharging the battery to cutoff drives the cells out of balance some. I found it good practice to do a bit of extra charging before the next ride, or at least before the next long ride.

Charge till its full and unplug from the charger. wait half an hour, and plug it back into the charger. If it does not start charging, ride around the block and try again. This will give your bms an extra chance to balance the battery, getting all the cells equally full. Each time this is repeated, the bms gets another opportunity to balance it better.

Winter is coming,, over the winter, recharge the battery monthly or so. This gives it a better chance of remaining in balance till spring. Too long storage will result in the bms drawing enough power to unbalance the pack. Typically, bms runs on just one or two of the cells, so it can discharge those two cells in a half a year.
Thanks it charged up to full my controller cut it off when i got home to look the battery was still at 70 volts but when i charged it it still went fine up to 83.9 volts

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Re: low voltage cut off when riding a long ride

Post by MikeSSS » Nov 13 2019 12:41am

This is an interesting and valuable thread.

Thanks to those who posted.

donn   1 kW

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Re: low voltage cut off when riding a long ride

Post by donn » Nov 13 2019 12:04pm

Bearing in mind that all of the above apparently assumes Lithium Cobalt battery chemistry, with 3.6V cells where that matters to the discussion.

Tommm   10 kW

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Re: low voltage cut off when riding a long ride

Post by Tommm » Nov 14 2019 11:21am

Glens carzy ebike wrote:
Nov 09 2019 5:29pm
Thanks it charged up to full my controller cut it off when i got home to look the battery was still at 70 volts but when i charged it it still went fine up to 83.9 volts
Your controller cut it off at about 62-65v under load. That is 3.1 to 3.25v under load. It bounced back to 70v or 3.5v resting.
You were at about 10-15% charge. Damage would occur below 2.5v under load or under 3.0v resting.

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steelmesh   100 W

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Re: low voltage cut off when riding a long ride

Post by steelmesh » Nov 14 2019 3:00pm

You may need to monitor cell voltages to see if you're losing a cell. Have you noticed a reduction in range or pack capacity?

If you hit the pack voltage LVC then yes you're risking overdischarging a cell if you don't monitor individual cells.
If you hit the cell voltage LVC looking at individual cells, then the BMS will have prevented overdischarging any cell.

Example where the controller LVC for pack voltage is 12.00 volts:
Cell 1 = 3.14 V
Cell 2 = 3.12 V
Cell 3 = 2.61 V
Cell 4 = 3.14 V
Sum(1-4) = 12.01 V

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