My original plan, as I have said, was to use an Arduino and a small display to handle these functions and display the discharge time, but I realised that it is possible to buy an analog quartz clock for 99¢ in a Dollar Shop which is designed to work with a 1.5V battery but will happily run from a 10μF capacitor across a working red LED (red LEDs light at ~1.7-1.9V and the clock movement tolerates this easily but other colours of LED have higher working voltages and should not be used - the 10μF capacitor is necessary to provide the pulse current when the quartz movement steps). If the clock is reset to 12:00 before each test it will run while the battery is discharging and stop when the "fully discharged" voltage is detected, displaying the discharge time.
The product of the discharge current and the discharge time is the battery capacity (strictly the capacity at that particular discharge current).
Since building this gadget I have tested every secondary battery in the house, and am appalled at the difference between the claimed performance printed on the battery and the actual measured capacity. This applies to batteries from reputable retailers, not just market stalls and dubious dealers on eBay. The majority of 18650 Li-Ion cells that I have measured had less than 40% of their advertised capacity, and some NiMH AAA cells, like the ones my sister bought for her cordless phone, had capacity of under 150mAH against an advertised value of 800mAH - less than 20%! Only batteries from electronic distributors have (generally, but not invariably) the capacity they purport to have.
Caveat emptor! And build this gadget and test 'em
Solar Photovoltaic Charger for a Lead-acid Battery
At the end of my garden is my woodshed. It would be difficult to connect it to