UPS Battery Internal Resistance: When Should You Replace It? Internal resistance is one of the most reliable early indicators of VRLA battery health, but there's no single universal number that tells you when to replace a battery — the reading only means something against the cell's own baseline. This article breaks down the 50%-above-baseline replacement rule, the 20–30% cell-to-cell deviation check for spotting a lagging unit, and why a sudden jump in resistance matters more than a high absolute value. It also covers why resistance alone isn't enough — pairing it with float voltage checks, capacity discharge testing, and visual inspection — and how DFUN's integrated battery monitoring and capacity testing platform automates threshold tracking across an entire string.