A multimeter gives you a voltage reading. An internal resistance tester gives you the true health of the battery. For electric and plug-in hybrid vehicle battery service, true health is what counts.The condition of a traction battery cell or module lives in its internal resistance. Aged electrode materials, corroded busbars, loose weld joints or moisture damage all drive resistance higher. The battery may still show a normal open-circuit voltage — until you apply load during acceleration or fast charging, and it drops out. A standard multimeter only reads static voltage. Only an internal resistance tester reveals the milliohm-level drift, the weak cell in a high-voltage pack, the hidden high-resistance connection that will trigger an under-voltage fault mid-drive.12V auxiliary batteries, 48V systems, and complete high-voltage battery packs — all require fast, non-disassembly health screening during routine maintenance, pre-delivery inspection and out-of-warranty assessment. A multimeter can tell you the battery is charged. An internal resistance tester tells you how much capacity remains, how quickly it will degrade, whether a full pack replacement is needed or just a single faulty cell.