Dear YM – In Nigel Calder’s Boatowner’s Mechanical and Electrical Manual he states that batteries are rated according to the amount of time they can deliver a certain amperage (C/20) before their voltage drops to below 10.5V. He also uses the battery’s stated Ah capacity to calculate the total required battery capacity on board (depending on average discharge and charging intervals, among other things). So far all is clear.

Good question! The short answer is ‘Yes’. From a practical point of view, 10.5V on a C20 test pretty much equates with 11.8V on an open circuit test, but only if the battery has been rested long enough for its open circuit voltage to stabilize and it really is 11.8V.

Obviously, in order to ensure consistency when testing batteries you need to test to the same parameters every time. The industry has settled on 10.5V as the test termination voltage for 12V battery capacity tests. I don’t know where this number came from – it’s been around a long time.

The C20 test you mention is a common capacity test. The battery is discharged at 5% of its rated capacity (e.g. a 100Ah battery is discharged at a rate of 5A) until its voltage falls to 10.5V. The capacity is then defined as the 5% rate of discharge multiplied by the time, in hours, it took to get to 10.5V (if it took 17 hours, the capacity in this example is 5 x 17 = 85Ah, or 85% of rated capacity).

When you discharge a battery, the voltage measured across its terminals is always a little lower than the ‘rested’ voltage. This is because the discharge takes the charge preferentially off the surface of the battery plates, and it is this surface voltage the voltmeter is reading. When the discharge ceases, the inner plate areas, which have not been so deeply discharged, equalize with the surface areas, restoring some of the surface charge. Over time, this causes the measured voltage to bounce back (generally to somewhere in the 11.0V-12.0V range). The higher the preceding rate of discharge, the more the voltage will bounce back.

If the same battery tested above at the C20 rate is discharged at the C5 rate (20% of capacity – e.g. 20A for a 100Ah battery), also down to 10.5V, when the load is taken off there will be a much greater bounce back (to something over 12.0V). If the battery is discharged under a really heavy load (e.g. an inverter powering a microwave) down to 10.5V, it may bounce back to well above 12.0V (how far it bounces back will be a function of the size of the battery bank relative to the load, the temperature, and other variables – this stuff all gets rather complicated).

Even though the C20 discharge rate (5% of rated battery capacity) is relatively low, there will still be a significant bounce back when the test is terminated at 10.5V and the battery is allowed to rest. This will take the battery back up to something over 11V. This is unlikely to be exactly 11.8V, which, as an open circuit voltage (i.e. rested voltage) measurement is another widely used indicator of a discharged battery, but in any event you can be pretty sure that any battery discharged to 10.5V in a C20 test, or with a voltage of 11.8V on an open circuit (rested) test, has very little life left in it! Note that the 11.8V referred to here is a RESTED voltage, which is to say the battery has been left unused long enough for its internal voltage to stabilize. This may well take several hours.