Cell Balance and BMS Question/

Get all your technical information about electric bikes here.
Post Reply
User avatar
bobbill   100 W

100 W
Posts: 226
Joined: Apr 09 2018 10:15am
Location: SE MN Winona

Cell Balance and BMS Question/

Post by bobbill » Sep 14 2021 10:00am

I am a NooB which may for may not be obvious. I know nada.

My questions: If one places multiple (Say 7 or more cells) in a pack and all are within .5 or half a volt difference,

Will the cells balance themselves -sans BMS?

Does or will a BMS balance cells in a pack (these same cells) on its own, with no further ado?

I know this must be basic stuff, but I know little of 18650 actions.

Thanks.

User avatar
ZeroEm   1 MW

1 MW
Posts: 1799
Joined: May 03 2019 11:53am
Location: San Antonio, TX

Re: Cell Balance and BMS Question/

Post by ZeroEm » Sep 14 2021 10:23am

by bobbill » Sep 14 2021 10:00am

I am a NooB which may for may not be obvious. I know nada.

My questions: If one places multiple (Say 7 or more cells) in a pack and all are within .5 or half a volt difference,

Will the cells balance themselves -sans BMS?

Does or will a BMS balance cells in a pack (these same cells) on its own, with no further ado?

I know this must be basic stuff, but I know little of 18650 actions.

Thanks.
I would not worry about being obvious, want to know something ask. I have a bad habit of skipping the basics until they come back and bite me.

BMS are supposed to keep the battery in balance. Most (not all) only do this when the battery is fully charged. (which I don't do often enough) Think i'm going to follow the wise men here and buy me active balance boards for my bigger batteries.

Should try to get them as close as possible or .01, .5 is not close. They will balance if put in parallel but the current will rush into the lowest one at very high amp rates. This is bad.

So balance them before connecting and allow the BMS to keep them there.
2019 Performer E-Trike
2013 Nissan Leaf S 7 bars 331.5w/KM

john61ct   100 GW

100 GW
Posts: 7692
Joined: Dec 18 2018 2:06pm

Re: Cell Balance and BMS Question/

Post by john61ct » Sep 14 2021 10:44am


bobbill wrote:If one places multiple (Say 7 or more cells) in a pack and all are within .5 or half a volt difference,

Will the cells balance themselves -sans BMS?

Does or will a BMS balance cells in a pack (these same cells) on its own, with no further ado?
Cells that are connected directly to each other in a group - meaning in parallel, increasing Ah capacity but keeping Voltage the same at the 1S level - will when at rest and isolated, automatically without assistance, self-level to the same voltage.

They should be at close to the same voltage, say within 0.1V before being so connected.

This group will then act as a single larger unit, nothing can be detected or measured wrt the individual cells without atomizing the group, breaking the parallel connections.


john61ct   100 GW

100 GW
Posts: 7692
Joined: Dec 18 2018 2:06pm

Re: Cell Balance and BMS Question/

Post by john61ct » Sep 14 2021 10:47am

Cells or groups (as above) connected in series increase voltage but not Ah capacity.

No current can flow between each cell / group until a balancing circuit is activated between them. One such type of circuit is that incorporated into many protective BMS, whose primary function is usually HVC & LVC backup failsafes, but may also include temperature and over-current protections.

Best if each cell / group are exactly the same voltage before assembling the pack, or within a delta of at most 0.05V or 50mV

When the pack is being cycled (charged and discharged) the entire Amps current rate is "flowing through" each S-level equally, by definition.

While within the parallel intra-group connections, only a small amount of current load needs to flow, re-balancing the energy differences caused by inevitable differences in resistance between the cells.


User avatar
bobbill   100 W

100 W
Posts: 226
Joined: Apr 09 2018 10:15am
Location: SE MN Winona

Re: Cell Balance and BMS Question/

Post by bobbill » Sep 15 2021 10:56am

As I surmised!
Thank-you both...
Reality matches logic, 'cept maybe in Quantum Mechanics and MAGA minds.

And, seems to me, if so and one or so cells do exceed .5V variance, the over limit cells will (at rest) eventually balance to other cells.

Why I did not set series connections right off.

Please do not get me wrong, There is to me little logic unless one takes time to imagine end results before doing,,,

Again, thanks

User avatar
ZeroEm   1 MW

1 MW
Posts: 1799
Joined: May 03 2019 11:53am
Location: San Antonio, TX

Re: Cell Balance and BMS Question/

Post by ZeroEm » Sep 15 2021 11:17am

One thing not to over look on balancing is the cells in a pack will balance with each other but not in the next cell/pack in series. That is where the bms should balance each pack with each other.
by bobbill » Sep 15 2021 10:56am

As I surmised!
Thank-you both...
Reality matches logic, 'cept maybe in Quantum Mechanics and MAGA minds.

And, seems to me, if so and one or so cells do exceed .5V variance, the over limit cells will (at rest) eventually balance to other cells.

Why I did not set series connections right off.

Please do not get me wrong, There is to me little logic unless one takes time to imagine end results before doing,,,

Again, thanks
2019 Performer E-Trike
2013 Nissan Leaf S 7 bars 331.5w/KM

john61ct   100 GW

100 GW
Posts: 7692
Joined: Dec 18 2018 2:06pm

Re: Cell Balance and BMS Question/

Post by john61ct » Sep 15 2021 12:35pm


bobbill wrote:And, seems to me, if so and one or so cells do exceed .5V variance, the over limit cells will (at rest) eventually balance to other cells.
Sorry if I wasn't more clear. "Natural flow balancing" only happens over parallel connections.

And nothing to do with the delta / variance, even a 0.001V difference will cause some energy to transfer, albeit at a very slow rate.

Between serially connected units, the assistance of balancing circuitry is required, and that is where the three factors / setpoints come into play, at least the way most BMS work:

1. max transfer rate allowed

2. delta trigger Start

3. delta target Stop




User avatar
bobbill   100 W

100 W
Posts: 226
Joined: Apr 09 2018 10:15am
Location: SE MN Winona

Re: Cell Balance and BMS Question/

Post by bobbill » Sep 16 2021 5:39am

Thanks, John, makes sense to this dodger!

Post Reply