Battery pack cell balancing

[Fordulike]

I have read in many posts on this site and others, that if you suspect some cells to be out of balance with the others, then it's best leave the pack on charge for a long time. Some say as long as a few days.

I was just wondering why a BMS takes so long to balance individual cells? A layman's explanation or a link would be fine.

Also, it it a good idea to periodically give the pack a nice long charge, even if you don't suspect any cells to be out of balance?

 

[danielrlee]

The main reason for this is because the bleed-down resistors on the BMS used to discharge the high cells are pretty weedy and can only do so at a trickle.

 

[Fordulike]

The main reason for this is because the bleed-down resistors on the BMS used to discharge the high cells are pretty weedy and can only do so at a trickle.

Ahem, layman's

 

[Alan Quay]

Ahem, layman's

If it went to fast all the magic smoke would escape.

In my experience, the balance function of a BMS can only do so much. If a cell (s) are a long way out it probably needs a manual balance. I've done this on a few batteries with success.

Sent from my HTC One_M8 using Tapatalk

 

[Fordulike]

If it went to fast all the magic smoke would escape.

In my experience, the balance function of a BMS can only do so much. If a cell (s) are a long way out it probably needs a manual balance. I've done this on a few batteries with success.

Sent from my HTC One_M8 using Tapatalk

So the charger will charge the fuller cells until a certain level, then the BMS discharges them slowly until the less full cells catch up. Have I got that right?

 

[Alan Quay]

So the charger will charge the fuller cells until a certain level, then the BMS discharges them slowly until the less full cells catch up. Have I got that right?

That's it.

Sent from my HTC One_M8 using Tapatalk

 

[danielrlee]

Ahem, layman's

Okay.... A pack becomes unbalanced when the voltage of individual cells differ - The higher the difference, the greater the imbalance.

BMS's use a series of resistors to bleed (lower) the voltage of the higher voltage cells in order to equalize the voltage of each cell in the pack.

So, charging a pack with a BMS goes as follows.... The charger sinks current into the pack causing the voltage of each cell to rise. Once the BMS detects that an individual cell has reached the upper voltage for the cell type, it halts the flow of current into the pack and starts to drain the individual high cell down to a lower level. It is this bleeding process that takes so long. Once the BMS has lowered the high cell enough, it will allow current from the charger to flow into the pack again. When the BMS detects another high cell, the charging is halted again and the bleeding process is repeated for that cell. This process is repeated until the voltage of each cell is full and equal, or 'balanced' as it is otherwise known.

 

[Fordulike]

Thanks for your replies Daniel and Alan.

Just one more question. Would a pack, even with all good cells, eventually go out of balance through normal discharge and charge use, and would it be a good idea to charge it for a couple of days, say every six months?

 

[danielrlee]

Thanks for your replies Daniel and Alan.

Just one more question. Would a pack, even with all good cells, eventually go out of balance through normal discharge and charge use, and would it be a good idea to charge it for a couple of days, say every six months?

Yes, absolutely. As a pack ages, it will require more frequent balancing. A couple of days is likely somewhat overkill, but a long balance is never a bad thing for a pack.

 

[anotherkiwi]

I think that if you leave the charger on overnight it should do the trick. If you still have unbalance after 12 hours then you may need to take more drastic measures.

My bottle battery has gone out of balance once in a year. I knew because instead of getting 60-70 km range I only got 40 in identical usage. I left it on the charger for 24 hours and it has never done that since. I regularly charge overnight: 11 p.m. - 8 a.m. which is 9 hours instead of the 6 it requires to fill.