Technically yes, practically it is usually a false economy, and there is a real risk in doing it badly.
In a series string, every battery carries the same current. A new battery has lower internal resistance and higher capacity than its aged neighbours, so on discharge the older units reach their end voltage first and the new one is never fully exercised; on charge, the string voltage divides unevenly. The result is that the new battery is dragged toward the condition of the old ones, and the string’s capacity is still set by its weakest member. You have spent money without buying runtime.
A weak cell in a string can be over-charged by the float voltage intended for the string as a whole. Sustained over-charging is a recognised thermal runaway mechanism in VRLA batteries, and a failure mode observed across utility fleets managing critical infrastructure.
Sensible practice:
In order of impact: temperature (roughly 50% life reduction per 10 °C sustained above 20 °C), depth of discharge (the deeper the cycle, the shorter the life), sulphation from deep discharges and poor charging, dryout from high temperature and over-charging, charge regime including whether the charger is temperature-compensated, and self-discharge during inactive periods exceeding six months.
Lead sulphate crystals forming on the plates during discharge, which fail to convert back on charge if the battery is left discharged or is chronically under-charged. It is probably the single largest killer of lead-acid batteries in the field. Prevention: recharge promptly after discharge, avoid deep discharges where the design does not call for them, and ensure the charger delivers a correct and complete charge rather than a partial one.
Yes. Over-charging accelerates both sulphation and dryout, and at elevated temperature it is a recognised thermal runaway mechanism. Causes: an oversized or faulty charger, a charger without temperature compensation holding a 20 °C float voltage at a 45 °C cell, or one weak battery in a series string absorbing a disproportionate share of the string voltage.
Yes. For a 12 V VRLA, end-of-discharge voltage is typically around 9.5 to 10 V; taking it below that causes damage that is often permanent. The usual cause is not operator error but a system drawing more power than the battery was sized to deliver, a load that has grown since the bank was specified. If a bank is repeatedly going flat, the problem is the sizing, not the battery.
Inversely related to depth of discharge. A typical VRLA discharged to 100% of capacity each cycle manages around 300 cycles; the same battery at 30% depth of discharge manages around 800. LiFePOâ‚„ is a different order of magnitude, thousands of cycles at 80% depth of discharge. This relationship is why shallow cycling extends life, and why chemistry choice for a daily-cycling duty is usually decisive.
No. VRLA deep cycle batteries do not have a memory effect. Shallow discharges with regular recharging give the longest usable life, not a degraded one, the opposite of the folklore. The memory effect belongs to nickel-cadmium chemistry and has been incorrectly generalised to everything since.
End of life is conventionally 80% of rated capacity. Age-based, impedance-trend and timed-discharge approaches each have a role; only a discharge test measures capacity directly.
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