I have here some old golf trolley batteries which are no longer fit for golfing use. These were on their way to being scrapped before I 'intercepted' them, with a view to seeing whether they would be any good for storing excess PV power.
Attached pic is a test report from a test performed presumably by the golf pro at whatever club this battery was used.
The attached graph shows the voltage remaining steady for 20 mins & then dropping off significantly just after 22 minutes. As it doesn't say on the test report, I am assuming at this stage that the test was performed with a 200w load (I've heard that a golf trolley motor is typically 200w, though some are bigger). On this basis therefore, I would say this battery holds about 70wh (wh = watt hours) of energy (200w X 22/60 hour = 70wh ish), whereas, being a 20ah battery, it is supposed to deliver 12v X 20amp = 240wh of energy per discharge cycle. I.e., from the golf pro's test report it looks like this battery is only delivering 30% or its rated capacity.
(formula i'm using here is the schoolboy P=IV, power - current X voltage. if you are going to learn one formula, this is probably it)
in the next post i will describe how i perform my own test using a 45w load and a Turnigy power meter. The turnigy power meter is commonly used by model airplane enthusiasts to check the power/performance of their lithium batteries, but can equally be used on golf and other lead acid batteries. The Turning keeps a running total of watt hours.