How do I know when to replace a front terminal battery?

Battery end-of-life and testing approaches.

The industry convention is end of life at 80% of rated capacity—the point at which the battery can no longer be relied on to deliver its specified runtime. This is applied explicitly across critical infrastructure.

Three approaches to determining end of life, in increasing order of reliability:

Age-based. Simple but crude. Most fleets use it. Typical replacement ages for VRLA are 12 years for 2 V cells and 8 years for 12 V monoblocs—note the significant difference in lifespan between cell types.

Impedance or conductance trend. Fast, non-invasive, performed on a live bank. Reliably catches open cells, dry-out and connection degradation. Rising impedance correlates with capacity fade. Requires a baseline reading from when the bank was new; without one, you are comparing to a manufacturer’s typical value rather than to this bank’s actual history.

Timed discharge test. The only direct measurement of capacity. Everything else is inference.

One critical point: impedance is a trend indicator, not a capacity measurement. Discharge testing on single-bank substations is often impractical, which is why effective asset management pursues continuous cell monitoring and capacity test methods in combination, not one instead of the other.