Why a battery tests good cold and fails hot

A battery that spins the engine instantly in the morning and leaves it dead in the afternoon, engine hot, is not necessarily discharged. It can read 12.6 V at rest and have nothing left the moment you ask it for current. The resting figure says almost nothing.

Surface charge misleads

A battery that has just been driven or charged carries a surface charge: a few tenths of a volt sitting on the surface of the plates. Measured straight away, it reads high. You either let it rest, or load it, to read the true voltage. That is why a voltmeter check right after a drive reassures you wrongly.

What resting voltage says

Engine off long enough, a charged lead-acid battery reads around 12.6 V. Near 12.4 V it is partly discharged, near 12.0 V it is weak, and below that it will soon fail to crank. These are orders of magnitude: voltage tells you the state of charge, not the health of the plates.

Health is the load test

To measure real capacity you must draw a heavy current for a few seconds and watch the drop. That is what a battery tester does: it measures a cold-cranking current and compares it with the original figure. A battery can read 12.6 V and fail that test — sulphated plates, a tired internal cell. That one starts in mild weather and gives up at the first cold snap or with the engine hot.

Why "hot" changes everything

Internal resistance rises with temperature, and a hot starter draws more current. Add an oxidised ground or a thinned cable, and voltage drop grows exactly when demand grows. A weak battery and a tired cable produce the same symptom: the load test plus voltage-drop measurements are what tell them apart.

Check the charging, not just the battery

With the engine running, voltage across the terminals should sit in the range of an alternator under load — typically a little above 13 V, and never collapsed. A low figure when hot points to the alternator or its circuit. Measuring the drop between the alternator terminal and the battery terminal, then between the negative post and the ground, separates the battery from the wiring.