Cell Balance

cell balancing

Battery cell balance is extremely important to any battery chemistry, with no exceptions.  Have you heard of Lead Acid batteries being “Equalized”?  This is to accomplish cell balance with a caveman-like approach.  We will cover what proper cell balance is, the importance of it, and what equipment is required to accomplish this.

If cells are not properly balanced, you are leaving performance on the table.  Not only can an imbalanced battery pack not properly reach a full charge, it can’t fully discharge either.  Let’s learn how we can avoid poor performance by equipping a battery pack with devices to maximize performance.

How is a battery's cell balance measured?

The terminal voltage of a battery typically relates to its state of charge (SOC).  Measuring cell balance is as simple as using a volt-meter to check the terminal voltage of all cells in a battery pack, to see if all cells are at the same state of charge.  If the SOC of all cells in the pack are equal, then the battery pack is considered balanced.

Caution must be exhibited with LiFePO4 batteries, since they have an extremely flat voltage-SOC curve.  We recommend that LiFePO4 batteries be manually top balanced during initial installation.  During use, cell balance should be checked when nearly fully charged, as they can develop a personality with age.  When LFP batteries are nearly full, their voltages curve is no longer flat, so accurate balancing can be established.

Why do batteries become unbalanced?

No two battery cells are created perfectly equal, even when brand new.  Over time, factors such as C-rate, temperature, depth of discharge, and vibration can cause cells to form a personality.  It is also important to properly layout your cells & buss bars to ensure all cells have equal current sharing.  Regularly checking your cells are in balance could help identify a cell that is failing before the whole system experiences lost capacity.

Why do batteries need a BMS

Over time, the previously mentioned factors will contribute to a battery pack that is not in balance.  With a BMS, each cell is individually monitored and balanced to keep every cell in the battery pack working in unison. Cells in a series connected battery pack typically cannot balance each other during charging process. This is because the charge current stops flowing when the cell is full and is why most batteries need management boards.  If you excluded a BMS, you could easily over-charge or over-discharge individual cells in a pack.

Balancing cells while connected in series, can be accomplished by two methods: Active Cell Balancing and Passive Cell Balancing.

lead acid never needed a BMS. Why?

Contrary to most battery types, Lead-Acid will still allow some charge current to flow when the cell is full, thus allowing charge current to reach cells that are at a lower SOC.  Flooded Lead Acid batteries can be balanced during an “equalize” charge, where the battery pack is brought to a higher terminal voltage to “force” charge into cells requiring extra charge.  A danger with Lead Acid is that during equalization, electrolysis takes place within the cell, leaving explosive HHO gas and heat as a byproduct that must be vented from battery enclosures.  Electrolyte levels must be regularly checked and distilled water must be added to make up for losses due to electrolysis.

VRLA, AGM and Gel type lead-acid batteries cannot be equalized simply because there is no way to replace any electrolyte that is lost due to electrolysis.  Adding a BMS or Active cell balancing to a Lead Acid battery bank could greatly improve cycle life and performance.

How does a BMS balance Cells

passive Cell Balancing

BMS designs will vary between manufacturers, but most will utilize a resistor bank that can dissipate a few ma of current from cells that have higher voltage.  Passive balancing usually occurs during charging, when a cell has reached 3.5v in order to bring cells to an equal top-balance.

It is common to see passive balancers because they are compact, cheap, and can easily be scaled to large number of series connected battery banks.

Passive balancing is not good for second-hand or heavily aged cells because the differences between cells may require a much higher balance current than brand-new well-matched cells.  Another pitfall to passive balancing is that it only works when cells are almost completely full and can only maintain a top-balance.  Passive balancers are also wasteful because power is turned into heat instead of diverted into cells that could benefit from additonal power.

active Cell balancing

The most common Active Cell Balancer is the QNBBM-A module.  These modules are much more capable than passive balancers because they can easily move 3 amps of current from a cell with a higher voltage into cell(s) with lower voltages.  97% of the power being balanced is retained instead of dissipated as heat.

Active Balancers go great with second-hand or aged cells because they can give the weaker cells in a pack a “crutch” curtesy of the stronger cells in the battery pack, leading to better overall system performance.  

Active  Balancing is rarely seen in large battery packs because large packs are generally made with new, closely matched cells which don’t see a performance loss due to unbalanced cells.

Is active balancing better than passive?

In some applications, such as with brand new matched cells, the cost of active cell balancing equipment is not worth the expense.  The small amount of balancing needed with brand new cells means that most of the time an active cell balancer wouldn’t be doing much.  In this case, you would not be better off with active balancing.

If however, you are using used, or slightly mismatched cells, utilizing an active balancer will allow energy from healthier cells to be moved into weaker cells, therefore extending the runtime of the system.  In this case a significant increase in usable capacity and remaining lifespan is found.