BMS questions!

Im having a hard time getting home built ebike batteries to work due to conflicting info about BMS's. Maybe we can clear up some confusion here.

1. How do people get away with "bulk charging" large batteries? Wouldnt this lead to the battery becoming unbalanced?

2. Are the balance wires accurately colored? Some have all red (positive) with one black (negative), some are reversed.

3. Can you measure the charging voltage on the balance wires? Will they increase by 3.6-4.2V as you move from one end of the battery to another?

4. What are the benefits of "bypassing" the BMS? Will it still balance charge?

5. Why are unbalanced groups a bad thing? Isnt the whole point of the BMS to preferentially charge groups until they reach cutoff voltage? Seems like as long as the batteries in the parallel groups are close.

6. How does "one bad cell" screw up a whole pack? They're all connected, seems like it would just decrease power by 1 cell.

A simple reply.

1. Bulk charging requires closer monitoring, if cells are all good then they should charge close to within reason.
For bulk charging use a lower terminal top voltage so not to over charge and monitor cell charging.
2. Dosen't matter their colour, most bms board jst are marked B-/B0 for v- connection and last cell marked with b**+.
3. Yes.
4. Probably ok for charging but leaves battery open to failure if you don't monitor individual cell discharge by loosing lvc protection.
5.Bms will only balance if cells are close to start with, if cells are far out balance will cease when one hits the desired terminal voltage. Any cell group/string that is low will hit lvc sooner reducing range and battery performance. 0.1v would be too far out for balance to work, cells need to be much closer like < 0.03v.
6. A bad cell in a P group/string will make the other cells in that P group/string work harder to make up for it's deficiency, also that P group/string will sag more then the rest of the battery groups/strings. Ultimately battery performance will suffer greatly under higher load demand and typically range will be affected, lvc will come in to play a lot earlier.

SwampDonkey said:

Im having a hard time getting home built ebike batteries to work due to conflicting info about BMS's. Maybe we can clear up some confusion here.

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There are many many variables, not least battery quality and the use case, so never a simple one-answer-fits-all.

> How do people get away with "bulk charging" large batteries? Wouldn't this lead to the battery becoming unbalanced?

With quality brand-new batteries, gentle c-rates and avoiding the voltage shoulders, batteries can go long past the vendor-rated cycle lifetime never needing any balancing ever, only pack-level LVD / HVD and temp protection needed.

But few here can do that, every bit of range is squeezed, c-rates are high, people cheap out on their cells even use second-hand units long past EoL. In which case yes, heroic per-cell balancing and ideally in-use protective measures are required.

> What are the benefits of "bypassing" the BMS? Will it still balance charge?

A BMS that just takes raw PSU input and handled all the charge regulation sounds like a great idea, but personally, I think chargers should have all the required controls built in.

To me redundancy is required, BMS is last-ditch protection, meaning only needs to be engaged after the upper layer control hardware has failed.

> Isn't the whole point of the BMS to preferentially charge groups until they reach cutoff voltage?

As above, to me, the **charger's** whole point is to get the whole bank to (your definition of) 100% Full, ideally in a balanced state, to achieve the desired compromise between longevity vs range.

The whole point of a BMS (IMO) is to prevent user error or hardware failure from damaging the very expensive bank.

> Why are unbalanced groups a bad thing?

No idea what that even's asking. Any significant imbalance reduces cycling performance and threatens longevity.

> How does "one bad cell" screw up a whole pack? They're all connected, seems like it would just decrease power by 1 cell.

No, unevenly worn cells cause potentially dangerous conditions from uneven current flows, varying resistance. Thermal runaway is a real danger with lipo.

And even with the most sophisticated engineered thermal management systems, such imbalances drastically reduce both range / capacity and the bank's cycling lifetime.

"> Why are unbalanced groups a bad thing?

No idea what that even's asking. Any significant imbalance reduces cycling performance and threatens longevity."

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Because wouldnt the voltage normalize since their all connected? If one battery is at 3V and the rest are at 4, wouldnt the 4V cells donate electrons and charge the 3V cell? Seems like the it would just bring the average voltage down slightly, which would then be fully charged by the BMS.

3. Can you measure the charging voltage on the balance wires? Will they increase by 3.6-4.2V as you move from one end of the battery to another?

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Im not seeing this happen. Ive been troubleshooting, trying to figure out how to wire this waterproof Ebay BMS and its just not working. There is no voltage coming from the balancing wires. Also, why would you get LESS voltage out of the BMS mains than the pack itself? I have a 48V pack at 50V, but when I check the voltage from the BMS leads its at 46V. WTF?

This is the THIRD BMS in a row that just didnt work once wired up. To confuse matters more, I just took apart an old, working pack, and it had 2 charging wires soldered to the same group. How would that work?

you got some very good replies, very hard for me to do better.

But we can't reply better till you provide a lot more information. My best guess why your bms's don't work is you made some mistake connecting them. For us to see if that is what happened, we need really good clear pictures of both the bms, and its connecting plug, plus a very clear picture of each wire leading to the bms plug.

Clarifying some other questions you already asked. People who bulk charge without a bms are the bms. They are a human bms. A slight undercharge prevents any cell from a drastic overcharge, in a reasonably balanced pack. After the charge is done, each group of parallel cells is checked. Example, I usually bulk charge to 4.15v. If when full none of my cells is over 4.25v, I'm happy. But I will do a discharge of that 4.25v cell to 4.15v once it gets that far out of balance.


WTH is out of balance? In my world. .1v off is out of balance. 4.25v if its supposed to be 4.15v. Any less, I call it balanced enough, for the way I run my packs. I don't run them deep enough to drive them way out of whack each cycle, so I end up actually doing a manual balancing about once every 3 months. at most.


Others shit bricks if they are .01 out. I'm certain sure the accuracy of my cellog 8's is not that good. So I take notice by .03v or so, and ignore anything less. Again, I can since I don't run till the pack is 100% empty, ever.


You asked about the cell next door charging a weak or damaged cell. This is what happens when they are connected in parallel. If you pack is 4p, and 13 in series, then a weak cell in one group will be supported by the ones in its parallel group. But think about what happens then to the whole pack. lets say each cell is 2.5 ah, so 4p is a 10 ah pack. one cell in the whole pack is shitty. It only holds 1.5 ah. What does this mean? It means the entire packs has only 9 ah in it. Because one of the 13 groups is not 10 ah, its 9 ah. Cells will not support the one next to it, in a series connection. So if you pull 9.1 ah from this pack, that 9 ah cell group will hit zero volts and get ruined. A good bms will prevent that, shutting down the whole pack if one cell is too low. If you are doing the human bms thing, then you need to stop way before that can happen. You need to know all about it, if you have a weak cell group. you can then watch the voltage of that known weak group, and stop when you should.


This situation would be considered an extremely unbalanced pack. Permanently unbalanced. On recharge, the weak cell group will overcharge, unless something stops it.


In other cases, there are many ways one cell group can get discharged more than the others, and unbalance the pack. This often happens if the pack is discharged at high rates that heat the cells, or if the pack is deeply discharged 100% empty, or both. In this case, balancing the pack at full charge is the fix, and easy. If you heat up your pack, some cells heat more, and end up deeper discharged than others. unbalancing the pack. Discharging deep, some cells will have slightly more capacity than the others, unbalancing the pack when you recharge.

Lastly, with a bms, long term storage can unbalance the pack. Often a bms runs on just the first two cell groups. So by spring, those two groups are discharged a lot more than the others. Disconnect bms for long term storage, or, charge and let the bms balance the pack monthly all winter.

SwampDonkey said:

"> Why are unbalanced groups a bad thing?

No idea what that even's asking. Any significant imbalance reduces cycling performance and threatens longevity."

Click to expand...

Because wouldnt the voltage normalize since their all connected? If one battery is at 3V and the rest are at 4, wouldnt the 4V cells donate electrons and charge the 3V cell?

Click to expand...

No, it does not work that way, each cell must have the same capacity, resistance, relationship between SoC and voltage level, within very close tolerances.

Otherwise can be a safety issue, certainly reduces lifetime drastically, and cripples performance from the outset.

> Seems like the it would just bring the average voltage down slightly, which would then be fully charged by the BMS.

What does that "which" refer to there? There is nothing collective for a source to charge, only individual cells.

You do know cells/blocks in series interact completely differently from strings in parallel right?

Do you know how a BMS works in trying to keep cells balanced?