Hydrometer vs. Voltage vs Load Testing Like many of you I have many vehicles, beyond my two hot rods. I keep a half dozen batteries on the bench with the charger, some that belong in a vehicle and some spares, and pick the worst one when it's time to trade in a core. Here's what I'm finding: Any battery that won't hold 12.5 volts is the first to go. But some that hold voltage, have high hydrometer readings fail a load test (common hand held load tester). Other batteries with low hydrometer readings still pass load testing. What gives? All are about the same size, group 24 etc..