Measuring the capacity of a 40v lithium ion, lawnmower battery
Get four car headlamp bulbs in series. Charge the battery then connect the headlamps and use a multimeter to measure the current. Measure the time the lamps are on. This will give you current and time - ie Ah.
Note that car headlamps get very hot! Be sure to mount them in a way that they won't cause a fire.