Lithium RXV - Talk me out of it

[KLA]

Originally Posted by Grinds6

And a BMS monitors the batteries and keeps them all kind of at the same charge/discharge correct? Having one helps the batteries life last longer than not having one correct?

Depends.

The simplest passive BMS systems monitor the cell voltages during charging and switch the charger off when any of the cells reaches its maximum BMS programmed voltage. After the charger is off, it discharges those cells (slowly) and switches the charger back on. This repeats until all cells are are at about the same voltage. This is top balancing. During discharge those simple BMS systems monitor each cell and provide a warning when one of the cells is discharged to its min voltage. This prolongs battery life because it keeps the cells within the recommended SOC range.

Active BMS typically don't just discharge the highest voltage cells, but feed their charge (minus some taken up by the efficiency of the DC-DC converters) to the other cells that have not reached the high voltage limit. This speeds up the charge and balance process. During discharge an active BMS also monitors the cell voltages, and if one cell drops below the min voltage, may charge it by taking energy from the cells that have not reached the min voltage limit. This way the total available pack capacity is close to the average capacity of all cells instead of being determined by the lowest capacity cell as with the simple passive BMS above.

Even more sophisticated BMS also track the internal resistance and temperature of each cell and from that, with a complicated battery technology dependent algorithm, predict the remaining capacity of each cell and distribute the cell charge as evenly as possible to maximize range before the min voltage limit is reached by a cell.

[interstate]

The battery management system will perform several tasks, but each BMS may have slight differences. There are active BMS units and passive BMS units. I choose to use an active unit... to...

Monitor total pack voltages
Monitor individual cell voltages

Keep individual cell voltages within a given tolerance of each other

Prevent the total pack voltage from over charging
Prevent individual cell voltages from over charging

Prevent the total pack voltage from over dis-charging
Prevent the total pack voltage from over dis-charging

Check cell/pack voltages to initiate a charge when plugged in
Turn the charger back on if the voltage drops during storage/balancing
Turn the charger off at a set voltage per cell AND per total pack

Calculate the capacity going into the battery
Calculate the capacity pulled from the battery
Calculate Actual real time current draw
Calculate actual real time charing current

Calculate present battery capacity still available

Reveal any defective or failing cells within the battery pack (depends on the series/parallel configuration)

Sound alarms as needed
Reduce power to motor/throttle as needed

All of these parameters are user programmable and adjustable in most of today's modern active BMS units that have computer interfaces.

[KLA]

Originally Posted by interstate

The battery management system will perform several tasks, but each BMS may have slight differences. There are active BMS units and passive BMS units. I choose to use an active unit... to...

....

Interstate, just to pick a nit:

There is an accepted definition in the EV car industry of what constitutes an active and what is a passive BMS.
All the features you mentioned are possible and available for both active and passive BMS depending on their sophistication. And most BMS today, even very sophisticated ones in electric cars, are still passive.

However, the difference in the definition of active vs. passive BMS is how they handle the case if one or more cells are in danger of being overcharged or overdischarged.
In the passive case the highest charged cells are discharged through a load resistor to even the charge (resulting in all cells at the same voltage at the end of the charge/balance process).
In the active BMS case those cells charge is distributed to the other cells to get them to the same state as fast as possible to reduce charge and balance time. This requires expensive DC-DC converters or charge shuffling electronics.

During discharge both passive and active BMS monitor the cells, however, only active BMS then take energy from the higher capacity cells to distribute the total charge evenly between all cells. Again, this takes expensive DC-DC converters to for example charge the most discharged cell from the total pack, thereby evening out the charge between cells.

[interstate]

Thanks for the educated correction.