State of health

State of health (SOH) describes how much of its original capability a battery still has, comparing its present condition with its condition when new. It is usually given as a percentage: the battery's current usable capacity divided by its nominal capacity. Battery management uses SOH to report how much life the storage device has left and to give early warning of failures. On a Sunny Island, SOH starts at 100% after commissioning, based on the nominal capacity that was set. The value is then refined during operation, but only with cycled lead-acid operation and regular idle periods. A loss of a few percent per year or per 100 cycles is normal aging. A drop of several tens of percent in the first months points to a possible fault, and an SOH that stays at 100% means the system cannot learn it. The inverter's SOH estimate is only accurate to about plus or minus 15 percent, so an accurate value needs a proper battery capacity test with special measuring equipment. Some SMA devices report a battery error when SOH falls below 70%. SOH is normally shown next to state of charge (SOC) in battery menus.

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• sentence 7: support check: partial - The 70% battery error is stated and SOC/SOH are paired in a section heading, but 'normally shown next to SOC in battery menus' and 'some SMA devices' are only loosely supported.
• sentence 7: review: partial - The battery error below 70% SOH is stated, but the passages only pair SOC and SOH in a section heading and do not say SOH is normally shown next to SOC in battery menus. -->