Restoring an unbalanced ebike battery (smart BMS/Neptune15)?

[GaryTheGolfer]

Q: will any BMS or smart BMS actively rebalance a substantially out of balance ebike battery?

Background:
I have a cheap Chinese ebike. It is at an overseas holiday home and is only used for 4-8 weeks per year. It is hard to imagine a less favourable ebike battery environment! The LiPo battery (10s/2p; 36v; 10Ah) had been stored long term before I got it. Taking the pack apart was a nightmare. It has a typical (cheap) BMS. The cells do not rebalance using existing BMS and bulk charge. Low cell: 3.2v; high cell 4.3v (!!). Total voltage 41.1v.

I realise that I MAY be able to patiently recharge each cell (pair) to, say, 3.8v and then let the existing BMS/bulk charger take care of full charging. But the internal resistance mismatches (and long periods of non-use) may well cause the battery to become unbalanced again quite quickly. I don't mind a one-time fix to rebalance the battery (if necessary) but I really want a solution that will actively rebalance the battery even if it becomes badly unbalanced during extended periods of (non-charging) storage.

I believe that a cheap BMS will rebalance a battery that is mildly out of balance (eg cells between, say, 3.7v and 4.1v).....but can't deal with high levels of imbalance.

I wonder if a smart BMS such as Neptune15 might be able (and needed) to actively rebalance such a battery? I realise the Neptune15 is very expensive but I have a battery that is just about impossible to replace. No air courier company will export my battery (for repair). And I can't take a replacement battery or enough cells on an aircraft as hold or carry-on luggage. And battery suppliers won't ship to that country. Only batteries installed in a new bike seem to be allowed!

I do have access to a balance charger but connecting this to the battery is mechanically difficult except perhaps as a one-off rebalance since it requires access to disconnect the existing BMS each time.

Use of BMS balancing (for a frequently unbalanced battery) would be the most elegant solution I can think of for my situation.

I just can't find any information on just how unbalanced a battery can be rebalanced by any particular BMS.

I realise that a smart BMS will tell me the individual cell voltages but knowing these is not enough for my purpose.

Does anyone have any advice or experience of dealing with such frequently unbalanced batteries and or smart BMS?

Thanks for reading this far!

 

[wheeliepete]

In theory any BMS can rebalance a battery, but in practice this could take weeks or months depending on the voltage difference of the groups. Obviously leaving the pack on charge for this long is not recommended. It sounds like some of the cells in your pack are self-discharging, so I don't think you are going to find a solution to your issue. To test this, manually charge/discharge all cell groups to 3.8v, disconnect the BMS, and recheck after a week or two. How many cell groups are showing lower voltage?

 

[wheeliepete]

Q: will any BMS or smart BMS actively rebalance a substantially out of balance ebike battery?

 

[GaryTheGolfer]

In theory any BMS can rebalance a battery, but in practice this could take weeks or months depending on the voltage difference of the groups. Obviously leaving the pack on charge for this long is not recommended. It sounds like some of the cells in your pack are self-discharging, so I don't think you are going to find a solution to your issue. To test this, manually charge/discharge all cell groups to 3.8v, disconnect the BMS, and recheck after a week or two. How many cell groups are showing lower voltage?

Hi and thanks for your answer. Sadly the battery is 3500 miles away so I can't do any testing. I was hoping to find out if smart BMS might do the job before I leave. If so, I would have to buy the BMS here before I leave UK (obviously no time soon!). A Neptune15, for example, would be quite an investment relative to a $10 BMS so I would like to know how good it might be at substantial rebalancing. And, perhaps after an eventual manual rebalance, could it keep the battery effectively balanced.

If some cells are beyond saving, that will be an entirely different problem.

I did manage to get the battery to work (5 mile journey) but at the time didn't know the state of individual cells. I have since found out how poorly balanced the cells are after a bulk charge.

Thanks again

 

[wheeliepete]

A true smart BMS with would allow you to set the balance voltage to around 3.8-3.9v, which is good for long term storage, but this will only help if your cells are all good. If the bike was cheap, then there's a fair chance the cells in the battery are likely to be cheap also, so even less chance of keeping the pack in good order, esp. if it has been stored badly. Looks like the Neptune has this function.

 

[GaryTheGolfer]

A true smart BMS with would allow you to set the balance voltage to around 3.8-3.9v, which is good for long term storage, but this will only help if your cells are all good. If the bike was cheap, then there's a fair chance the cells in the battery are likely to be cheap also, so even less chance of keeping the pack in good order, esp. if it has been stored badly. Looks like the Neptune has this function.

Thanks again and I understand (and agree) your thoughts on the battery quality. A balanced 3.8v would be fine. If I used a smart BMS and got to that stage....and then left the battery for 6 months and the voltage spread was then, say 3.2 to 3.7v, should I expect a Neptune15 to rebalance (in one cycle) to its target voltage (say 4.05v)? Or would I need to manually rebalance before letting the smart BMS finish the balancing?

Does anyone know what range a smart BMS would balance voltage?

I plan to take about 4 replacement pouch cells out with me. This would allow me to replace some cells if they are defective.

In theory all the BMS boards SHOULD balance a battery but in practice, with the cheap BMS at least, their capability is limited. Are there any comparable/better smart BMS (other than Neptune15) that I should be looking at?

Thanks for any advice.

 

[wheeliepete]

Others may know better, but I'm not aware of any BMS that will balance a battery with wide voltage variables. If you have damaged sells that are self discharging they will also sag badly with use, so the pack will be impossible to keep balanced. Make sure any cells you buy are the same capacity, or this will cause further balance issues. Can you not source a replacement in your destination country?, it would prob. be the best solution.

 

[Nealh]

BMS whether generic or smart only balance at approx 60 -100ma, the smart versions allow you to set balance, charge and discharge V's as well as being able to remotely see cell/battery data on a phone or pc. Good cells even with a generic BMS should keep good balance what ever voltage you choose to charge to for some time.
I charge my batteries with generic BMS to 4.1v and every 2 or 3 months carry out a full balance charge just to ensure no straying.
That said some generic BMS are better then others , they aren't all tarred with the same brush.
Moe then 0.2v is quite badly out of balance, even 0.1v isn't good.

 

[GaryTheGolfer]

BMS whether generic or smart only balance at approx 60 -100ma, the smart versions allow you to set balance, charge and discharge V's as well as being able to remotely see cell/battery data on a phone or pc. Good cells even with a generic BMS should keep good balance what ever voltage you choose to charge to for some time.
I charge my batteries with generic BMS to 4.1v and every 2 or 3 months carry out a full balance charge just to ensure no straying.
That said some generic BMS are better then others , they aren't all tarred with the same brush.
Moe then 0.2v is quite badly out of balance, even 0.1v isn't good.

Many thanks for your comments.
My issues with an embedded standard/generic BMS are the lack of control: I can't set the cell battery voltages. All that happens is a gross 42v (36v 10s/2p LiPo) is the point that the PSU brick cuts off. And in my situation any unbalance has 10 months between use to fall into more significant imbalance. Further, I can't set the cells to storage voltages.

The use of an external balance charger seems to have been rejected by manufacturers simply on basis of cost. A decent 10s balance charger is quite a rare animal and costs well north of £100....and often more. Of course once an owner sees the imbalance problem, they regret not having an external balance charger (or an effective internal BMS).

So for a similar cost, I might consider a 10s balance charger or a smart BMS. The balance charger affords/requires access to cell junctions (and temporary disconnection of the internal BMS?). So also perhaps requires battery dismantling each time? If a cell or two is badly unbalanced, they could also be corrected INDIVIDUALLY.

Alternatively, the smart BMS is more elegant. And, as you say, displays cell level voltages and gives more control. I just don't know if it gives enough control!

But IF the smart BMS can't deal with a voltage imbalance of the order of 0.5v, then it doesn't meet my need and I should spend my beer tokens on a decent balance charger and accept the inconveniences.

I think you have helped me accept that the smart BMS probably won't do it for me.....

Whilst pottering, I have already built a little external interface box with cabling that will connect to the 11 battery/BMS cell terminals; connects to a 10s balance charger and will use 2 of those nice little 6 way voltage monitors (about $2 from China). I hope to be able to leave this permanently connected and use the balance charger when needed but mostly use the bulk charger when the cells are back in balance.

I could post my diagram if anyone is interested.

Thanks again

 

[Nealh]

If the lipo is seriously going out of balance then the cells doing this will or usually have higher IR, all you can do is solder on a set of balance leads to monitor the cells.

I charge my 5 1/2 y/o 29E's now to 4.1v and they are behaving.

BMS deal with top balancing if the cells are within reason but not 0.5v.

 

[GaryTheGolfer]

BMS deal with top balancing if the cells are within reason but not 0.5v.

Thanks. That's exactly confirming my fear. So it's down to an external balance charger then.....