Originally Posted By: JHZR2
I agree that it is not a constant value. However one can see systemmatic trends and can also calculate voltage drop roughly at any point.
We always know the SOC vs voltage, within a tenth or so, temperature-dependent. And a multimeter is the easiest measurement out there... so one would instantly know if something within the battery is out of whack.
If the SOC is not at/near 100%, as it should reasonably be diagnosed that either something else is wrong (parasitic, short, bad charging system, bad alt diode, etc), and corrective action should be taken on the battery to (1) prevent further sulfation and (2) create a proper charging scenario.
And its not like a 400A load exercises the battry that much. Most SLI batteries are 500Wh-1kWh at the 10hr discharge rate (very low amps). At high rate, about 20-30% of the nameplate energy is accessible before they polarize and impedance causes an excessive voltage drop. But 400A for 15 s is a very small amount of actual total energy and thus SOC pulled out.
Re SOC vs Voltage, check this thread out for my weird OCV
http://www.bobistheoilguy.com/forums/ubb...726#Post2973726
The 400A test is only good for seeing if there is any life left in the battery at all i.e. the battery will be able to start the engine at least once if it is charged (given same temperature testing vs starting)
A toaster will have to be left on four times as long, 60 seconds and they are not designed for that and will burn up inside.
Conductance testers give you an accurate picture most of the time but not always. Measuring the SG of the acid is a pretty good test if you can open up the caps on the battery.