Lithium and LiPoly Voltages;
Cells / Discharged Voltage / Normal Voltage / Max Voltage
1 / 2.9v / 3.7v / 4.3v
10 (36V pack) / 29v / 37v / 43v
I have to say I'm disappointed if not surprised that bikes, particularly the more expensive ones, are being sold without balance chargers which read the voltages of the individual cells in the pack and ensure that when the charge sequence is terminated all of the cells within the pack are at or very near the same voltage.
Anyone involved with the use of lithium cells should be aware of the benefits of doing this - most radio control (RC) modellers who use these cells increasingly for motive power amongst other things are aware of the dangers of not so doing and I would be surprised if many, if any, do not use the balance feature of their chargers (which do not have to be expensive) every time they charge their battery packs.
If a balance charger is not used the simple charger in use measures only the total pack voltage. The ones supplied with bikes seems to be set at fairly low charge rates (modern lithium ells are capable of being charged in an hour or considerably shorter periods in some cases under controlled and observed conditions) which may allow some slower cells to catch up with the rest but the danger is when one or more cells have chemistry mismatched to the others or have even failed. Imagine two cells have failed and are actually below their safe discharged voltage of 2.9 volts each and remain there throughout the charge.....if the charger is looking to achieve 43 volts from the pack before terminating the charge (hopefully, in the circumstances, these things are set to look for a lower safer voltage) then it will have charged 8 of the cells to 4.65 volts to achieve that and if and when more of the pack cells fail the terminating charge for the pack will result in higher and higher voltages for the remaining cells.
Overheating and combustion has to be considered possible. Be aware that when lithium cells combust they tend to do so fiercely and their fires are self sustaining as oxygen is one product of the combustion process.
The other side of the problem with non balanced charging comes during the discharge process ie when the bike is being ridden. The speed controller (as RC modellers call them) (BMS?) is designed to terminate the discharge at a voltage to which the cells can safely be discharged and in RC terms this will be 2.9 volts per cell minimum, usually a bit higher in order to preserve to longevity of each cell. If a lithium cell is discharged below that minimum safe voltage that cell is considered permanently damaged and may not be capable of safe recovery.
Attempts to charge such cells may result in overheating or combustion.
If the BMS does not read individual cell voltages the pack will cut off at 29 volts or a bit higher but there is likely to be a variation in individual cell voltages and it is possible for some to remain a fair bit higher than the 'discharged minimum' and of course others to be at or below that.
Then the next non balanced charge phase begins with the charger again looking for an average voltage across all of the cells. Any that were at a higher voltage when the previous charge terminated will possibly reach 4.3 volts a while before the pack reaches 43 volts but they will continue to be charged beyond that voltage. Similarly, any that started off low may still be lower when the charge terminates and are likely to reach their safe minimum voltage before the others during the discharge phase.
And so on.....
The point of this ramble was to try to help anyone who is unaware of the benefits of maintaining cell balance within a lithium pack achieve some appreciation of what goes on in there and is not intended to be alarmist.
If the BMS reads individual cell voltages during the discharge phase then they should be quite capable of protecting against cells going 'under voltage' and that combined with the pretty low charge rates which appear to be common should keep most people out of trouble and preserve cells that are out of balance so that they can at least be re 'conditioned'.
Never, however, attempt to charge even a single lithium cell with any charger that is not a lithium charger and be aware that many devices that use lithium cells have the circuitry for the charge algorithm (which is complex) built into the device itself rather than that plug with the wire on the end that you plug into your phone or whatever....that may only be a 5 volt power source and contain no charger technology, in which case the charge will not terminate when the max voltage is reached.
The fairly benign (if technically inadequate) chargers that appear to be supplied with bikes will, at worse it seems, leave you with a hard pedal home. Connect the wrong thing to a lithium cell, or short it out, and you are likely to have to call the fire brigade and I apologise for that deliberately alarmist statement.
If you google 'lithium fires' I'm sure you'll get plenty of examples probably including the smoking remains of several modellers cars and houses.
Be safe!
JimB
Cells / Discharged Voltage / Normal Voltage / Max Voltage
1 / 2.9v / 3.7v / 4.3v
10 (36V pack) / 29v / 37v / 43v
I have to say I'm disappointed if not surprised that bikes, particularly the more expensive ones, are being sold without balance chargers which read the voltages of the individual cells in the pack and ensure that when the charge sequence is terminated all of the cells within the pack are at or very near the same voltage.
Anyone involved with the use of lithium cells should be aware of the benefits of doing this - most radio control (RC) modellers who use these cells increasingly for motive power amongst other things are aware of the dangers of not so doing and I would be surprised if many, if any, do not use the balance feature of their chargers (which do not have to be expensive) every time they charge their battery packs.
If a balance charger is not used the simple charger in use measures only the total pack voltage. The ones supplied with bikes seems to be set at fairly low charge rates (modern lithium ells are capable of being charged in an hour or considerably shorter periods in some cases under controlled and observed conditions) which may allow some slower cells to catch up with the rest but the danger is when one or more cells have chemistry mismatched to the others or have even failed. Imagine two cells have failed and are actually below their safe discharged voltage of 2.9 volts each and remain there throughout the charge.....if the charger is looking to achieve 43 volts from the pack before terminating the charge (hopefully, in the circumstances, these things are set to look for a lower safer voltage) then it will have charged 8 of the cells to 4.65 volts to achieve that and if and when more of the pack cells fail the terminating charge for the pack will result in higher and higher voltages for the remaining cells.
Overheating and combustion has to be considered possible. Be aware that when lithium cells combust they tend to do so fiercely and their fires are self sustaining as oxygen is one product of the combustion process.
The other side of the problem with non balanced charging comes during the discharge process ie when the bike is being ridden. The speed controller (as RC modellers call them) (BMS?) is designed to terminate the discharge at a voltage to which the cells can safely be discharged and in RC terms this will be 2.9 volts per cell minimum, usually a bit higher in order to preserve to longevity of each cell. If a lithium cell is discharged below that minimum safe voltage that cell is considered permanently damaged and may not be capable of safe recovery.
Attempts to charge such cells may result in overheating or combustion.
If the BMS does not read individual cell voltages the pack will cut off at 29 volts or a bit higher but there is likely to be a variation in individual cell voltages and it is possible for some to remain a fair bit higher than the 'discharged minimum' and of course others to be at or below that.
Then the next non balanced charge phase begins with the charger again looking for an average voltage across all of the cells. Any that were at a higher voltage when the previous charge terminated will possibly reach 4.3 volts a while before the pack reaches 43 volts but they will continue to be charged beyond that voltage. Similarly, any that started off low may still be lower when the charge terminates and are likely to reach their safe minimum voltage before the others during the discharge phase.
And so on.....
The point of this ramble was to try to help anyone who is unaware of the benefits of maintaining cell balance within a lithium pack achieve some appreciation of what goes on in there and is not intended to be alarmist.
If the BMS reads individual cell voltages during the discharge phase then they should be quite capable of protecting against cells going 'under voltage' and that combined with the pretty low charge rates which appear to be common should keep most people out of trouble and preserve cells that are out of balance so that they can at least be re 'conditioned'.
Never, however, attempt to charge even a single lithium cell with any charger that is not a lithium charger and be aware that many devices that use lithium cells have the circuitry for the charge algorithm (which is complex) built into the device itself rather than that plug with the wire on the end that you plug into your phone or whatever....that may only be a 5 volt power source and contain no charger technology, in which case the charge will not terminate when the max voltage is reached.
The fairly benign (if technically inadequate) chargers that appear to be supplied with bikes will, at worse it seems, leave you with a hard pedal home. Connect the wrong thing to a lithium cell, or short it out, and you are likely to have to call the fire brigade and I apologise for that deliberately alarmist statement.
If you google 'lithium fires' I'm sure you'll get plenty of examples probably including the smoking remains of several modellers cars and houses.
Be safe!
JimB
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