Understanding your BMS overhead result
Why a BMS draws power even when nothing else is happening
The battery management system continuously monitors cell voltages, temperatures, and current, and keeps communication and protection circuits active, all of which draws a small but constant amount of power. That standby draw is a fixed cost that exists whether the battery cycles heavily or barely gets used at all, one worth factoring in alongside the battery kWh to runtime converter if you're estimating how long a given capacity will actually last you.
Why this overhead hits backup-only systems harder
A battery cycled daily processes a large amount of energy, so a fixed standby draw is a rounding error against the total. A battery kept mostly idle for occasional backup processes far less energy, so the same fixed draw eats up a much larger share, sometimes 20% or more of total throughput, a usage-dependent overhead effect NREL's energy storage research accounts for when modeling real-world storage efficiency across different duty cycles. This is one more factor worth weighing in the best chemistry for backup only use comparison.
Where cell balancing losses come from
Even cells manufactured to tight tolerances drift apart slightly over time, and the BMS periodically bleeds excess energy from higher-voltage cells to keep the pack balanced. This same BMS behavior is part of what the UL 9540A test method factors into its safety evaluation, since a properly functioning BMS is central to preventing the overcharge conditions that can lead to thermal runaway. The cell balancing loss calculator looks at this specific loss in more depth, including how it changes as a pack ages.
What to check next
Once you know your BMS overhead, the round trip efficiency comparison puts it in context against a chemistry's overall efficiency figure, which includes this overhead along with charge and discharge losses. If you're comparing how different systems report this kind of data, the inverter monitoring data comparison tool covers the equivalent question on the inverter side.