How to measure battery capacity?

Re: How to measure battery capacity?

It's a difficult one to determine without running your batteries to the point where the inverter shuts down due to low voltage, however if you choose to go this route in determining your battery capacity it will give you the most accurate result at the expense of some battery life. Just make sure to recharge them immediately at the C/10 rate after your test. Note that at the point where inverters usually shut down there is still "20% or so of the Ah remaining in the batteries"

A less accurate but far less harmful test is the "rapid discharge test". Put the heaviest quantifiable load you can on the fully charged battery and let it run until it has removed about 25% of the rated Ah of the battery. After removing the load let the battery sit for a few hours and check the voltage. In the case of a 12v battery the voltage should still be around 12.5v if it's in good condition. Also during the test take note of the voltage sag.

As far as plotting a "death curve" is concerned I think it may be a hard trend to spot as capacity readings can vary due to charging conditions (high current rapid charging on sunny days vs slow trickle charging on cloudy days). Battery temperature will also mess with capacity readings. The onset of battery failure can sometimes be very sudden. In a controlled laboratory environment plotting a death curve may be possible, but due to the inconsistent charging and discharging regimes of off-grid systems it may be far more difficult to spot. Most off grid users develop a "feel" for their batteries and one glance at the dashboard can usually pick up a concerning change in performance...