Traditionally, lead acid batteries have been able to "self-balance" using a combination of appropriate absorption charge setpoints with periodic equalization maintenance charging. This characteristic of lead acid batteries is enabled by a secondary electrolysis (hydrogen producing) reaction within the electrolyte of the batteries. The produced hydrogen gas either vents (for flooded batteries) or is recombined into the electrolyte (for OPzV Gel and AGM batteries), expelling energy. This energy effectively allows all series-connected batteries to "balance", or come to the same state of charge (SOC) or "fullness".
This balancing is required due to small changes in the batteries due to manufacturing, the dynamic nature of lead-acid batteries, temperature or current gradient within packs, inconsistent wear, or numerous other reasons. Without appropriate absorption and equalize (equalization should not be used with LFP batteries) voltage settings, packs will become imbalanced. As cells "walk away" or begin to diverge in SOC, they risk being systemically undercharged, which may cause accelerated wear and failure. Lead acid batteries are relatively robust to this mistreatment, and the safety risks, such as rapid battery failure, internal short circuiting, etc. are less likely to occur than newer chemistries including lithium-ion chemistries.
With the development of various lithium-ion battery chemistries such as lithium iron phosphate (LFP), there is no longer available material in the batteries to be used up, replenished, recombined, etc. And secondary reactions within a lithium-ion battery, including LFP, use active material within the battery, which is unrecoverable and poses safety risks. Because lithium-ion batteries incorporate a BMS which protects the cells from unsafe voltage, current and temperature, the battery will not enter these conditions. Due to these hard stops in the BMS, performance will suffer and overall effective pack capacity will be reduced corresponding to the level of imbalance if nothing is done. If the imbalance continues to worsen, effective pack capacity will approach zero. This is recoverable, however, via balancing cells as they are cycled, similar to how lead-acid batteries are left on absorption, and periodically equalized (equalization should not be used with LFP batteries).
Rolls LFP Batteries use "bleed resistors" to dissipate small amounts of energy from the cells when they pass a certain threshold, when at high SOC and actively being charged, which keeps the cells in a given battery at the same SOC as they are cycled, accounting for small variances between them. This industry standard method is known as "top-balancing", where batteries have energy dissipated at the high point of their state of charge. Because lithium batteries are less dynamic than lead-acid batteries, with very tight manufacturing tolerances, only a small amount of heat must be dissipated to maintain cell balance compared to gassing reactions. Because balancing batteries at largely different SOC would generate substantial heat, requiring expensive dissipation considerations, the balancing in the internal BMS for Rolls LFP batteries is not sufficient to balance series-connected cells which are connected at very different SOC (>5%). A situation like this may occur when series-connecting batteries for the first time without a prior balancing cycle in parallel as detailed in Procedure to Initially Balance Batteries for Series-Connection.
More information on balancing is available on our YouTube video, here.