Cranking Voltage Drop
The fall in battery terminal voltage for a few seconds caused by the very high current drawn by the starter motor. On a sound 12V battery with a warm engine this value typically does not fall below 10.5 V; the official pass limit of a battery load test is 9.6 V. Values dropping to the 8-9 V range indicate a battery, cable, ground or starter motor problem.
The cause of the drop is the battery's internal resistance: a cranking current of hundreds of amps creates a voltage loss across the internal resistance and the value seen at the terminal falls. As the battery ages the internal resistance rises, so the drop deepens.
- Measurement: with the voltmeter on the battery posts, the starter is engaged and the lowest value is read.
- Interpreting the result: above 10.5 V with a warm engine is normal; the official pass limit of a battery load test is 9.6 V at 21 °C, meaning that on a diesel tractor in cold weather a little below 10.5 V is not on its own a fault. 9 V and below indicates a battery or connection problem; getting a different result at every crank is a sign of a loose terminal clamp or a bad ground.
- Effect on electronics: systems that fall below their supply range reset during cranking, the LED goes out and the command is lost.
This is why the lower limit of the operating voltage range matters when choosing a product. The voltage stability of the HCT402 has been tested by the manufacturer at a point as low as 7 V; this is meaningful in terms of the system staying alive through the drop at cranking. The general installation rule is this: do not supply the kit from the same line as the starter circuit or the starter cable, take the supply directly from the battery post through a fuse.
The context where this term is used: Converting a tractor hydraulic lever to electronic control