ASK THE EXPERTS: Battery Management System (BMS)

Intermediate

I have an off-grid system with a Sunny Island 5048 inverter and a 475 Ah, 48 V FLA battery bank. The system is now about 10 years old. The batteries are down to 80% state of health (SOH), so nearing their end of life.

I’m looking at options for my next set of batteries. I am considering LiFePO4, 300 to 400 Ah at 48 V. It looks like I could bottom-balance the batteries and use a simple constant-current charge to around 90% without the complexities of a battery management system (BMS). Does this all sound reasonable, or am I missing something obvious?

Alistair Ward • via homepower.com

A BMS’s primary function is to prevent cells from overcharging by controlling a physical disconnect (a contactor) located on the battery. This monitoring and disconnect system can also help protect the battery from overdischarge, helping to extend its life, and prevents damage from charging when the battery temperature is 0°C or less.

When lithium batteries are charged, it’s common that one cell in the pack reaches full charge before the others. Without the BMS and under a heavy charge, this one full cell will jump from 3.45 V to 4 V almost immediately. Above 4.2 V, there is irreparable damage to the cell. This process can happen on the very first charge cycle of a new lithium battery’s life no matter how well the cells are pre-balanced.

Nominal cell voltage for lithium batteries is about 3.2 V per cell, and 100% state of charge happens at about 3.45 V per cell. If you set a charge controller or inverter’s charger to 3.45 V per cell times 16 cells (55.2 V for bulk and absorb), we would expect that all the cells would come up together to 55.2 V. The charger would hold 55.2 V during the absorb cycle for the predetermined time before transitioning to float. However, with the one errant cell, this is what happens:

Cycle 1

3.35 V × 15 cells = 50.25 V

3.45 V × 1 cell = 3.45 V

Pack voltage = 53.7 V

Target voltage = 55.2 V

At 53.7 V, that one cell at 3.45 V is about to run away. The charge controller is still in bulk, delivering maximum current to the battery. The higher impedance results in a cell’s voltage rapidly increasing, and it will accept even more energy from the charging source. This is where things get bad. The PV array will keep charging the battery until the pack reaches 55.2 V, the bulk/absorb transition point. That means that the battery will have to come up an additional 1.5 V before it begins the absorb countdown timer. During the absorb stage, the charging source will continue to push a good amount of power into the battery for at least an hour before it’s time for float.

Without a BMS, that one lithium cell previously at 3.45 V will climb to 4.9 V and hold that voltage for the entire absorb cycle. After that one charge cycle, the overcharged lithium cell will be damaged beyond repair. Its voltage will drop quickly as the battery begins to discharge. The next cycle will be worse, because this damaged cell will charge slower than the rest of the pack. A different cell will run away on this second charge cycle, and its overcharge will be even worse because of the damaged cell from the first cycle that may be holding 2.8 to 3.0 V.

Cycle 2

3.0 V × 1 damaged cell = 3.0 V

3.35 × 14 cells normally charged = 46.9 V

Subtotal = 49.9 V for 15 cells

Target voltage for solar charging = 55.2 V

That leaves room for the one overcharged cell to achieve 5.3 V. This is the point at which a lithium cobalt or lithium manganese battery will catch fire.

The BMS has the ability to control this overcharged cell though passive or active cell balancing. Resistors are used for passive cell balancing. Under charge, when one cell rises to about 3.3 V, the balancers activate and look for a voltage differential greater than 50 mV or 0.05 V per cell. If one cell starts climbing faster, the balance resistor begins to bleed a small amount of energy from that cell. On a high current charge like we see at 48 V, the balance current should equal 1% of the maximum charge rate. High-amperage resistor packs, sometimes 6 amps per cell, are used to keep big batteries from overcharging.

Beyond the BMS, consider that lithium iron phosphate chemistry is intrinsically much safer than other lithium chemistries. Lithium cobalt and lithium manganese are both volatile and prone to thermal runaway.

Brandon Williams • Iron Edison Battery Co.

Comments (1)

Fred Starkey_2's picture

Don't forget the discharge side. Upon full discharge, LiFePO4 cells immediately begin to self destruct. The electrolyte dissolves the foil "plates," destroying the cell. You must strictly adhere to the manufacturer's minimum cell voltage and break the circuit when that value is reached. The pack capacity is determined by the "smallest" cell. Continue to discharge the pack when the little one is empty and the empty cell will be destroyed within minutes.

Also consider the time between end of discharge and beginning of charge. Plan NOT to leave them setting at the bottom for an excessive time. The further away from that published minimum, the safer they will be, awaiting sunrise.

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