State of Health (SOH)

The difference from state of charge

The two are routinely confused, and the distinction is worth being precise about.

State of charge is how full the tank is today. State of health is how big the tank has become. A three-year-old pack can be at 100% SOC and 78% SOH: completely full, and holding 78% of what it held when new.

Conventionally a pack is considered end-of-life for its original application at 80% SOH, which is the same threshold used to define cycle life.

Why it matters commercially

SOH is where warranty disputes are won and lost. A customer reporting “my battery only lasts half as long” is describing SOH, and without data the conversation is one word against another.

Packs that log cycle count, cumulative amp-hours, and capacity measurements give you something to reason with. For a fleet or rental operator, the same data is an asset-management input: it tells you which packs to retire before they strand a rider, rather than replacing on a fixed calendar.

It also matters for resale and second-life decisions. A pack at 75% SOH is finished as a high-duty fleet battery and perfectly serviceable in a lower-demand application.

Ask what the BMS actually records

Ask whether the BMS records anything, and whether that record can be read out. Many low-cost BMS designs track nothing at all, which means every warranty claim becomes a judgement call.

Ask also how SOH is estimated. Cycle-count proxies are crude — a count says nothing about the depth or temperature of those cycles. Capacity-based estimation, updated on full charge-discharge events, is meaningfully better.

For programs where field data matters, raise it at specification stage. Logging and read-out capability is a BMS firmware decision, and retrofitting it to packs already in the field is not possible.