Unfortunately there is rather a lot wrong with this post. I hate picking holes in what others say, but with the greatest of respect, I'm not sure what you've proposed is valid.Here is a simple way to test the battery capacity with out any expensive tools or testing stations. As we all know a lithium battery cell is considered fully charged at 4.2V and fully depleted at 3.0V. In most cases the BMS will prevent the cells from over charge and stop the cycle at 4 volts, with 12 cells in series = 48V. The BMS will also most likely stop the cell from discharging past 3.2V or right around 38V on your pack.
So now that we know we can have a safe differential voltage of 10 How are we to find the true capacity of the battery? well that can be simple. what you need is a lamp, a 48v DC bulb, and an volt/amp meter. you could probably buy it all on amazon for 30 bux...
step 1, measure the battery voltage at full charge to verify 48 volts
step 2, connect the lamp leads to the battery + and - using some insulated stakon female flat connectors (make sure you use the insulated connectors to protect the battery from a possible cross connection)
step 3, Turn on the lamp and measure the amperage draw, since we already know the true voltage we can multiply the amperage for the real wattage consumption. A 9W bulb may in fact draw 10W due to power factor or other electrical losses.
step 4, Now you can do this step a couple ways but I would rather not actually measure how long it takes to consume 10 volts on a 9W bulb because that may take days. Instead, fully charge the pack again, verify 48volts, hook up the lamp, turn it on, start the timer, wait for the volt meter to show a drop by 1 volt and record the time.
Lets hypothetically say it took 240 minutes (4 hours) to discharge 1volt at 209 milliamps or 10W. We can take that time period of 240 minutes multiplied by 10(the amount of consumable volts between full charge 48v and empty at 38v) and we get 2400 minutes or run time / that by 60 minutes to get 40 hours. Now you take the available run time of 40 hours and multiply it by the wattage consumed (10watts) = 400Watts.
So now we know the battery can run 400 watts for 1 hour at 48V giving it an approximate 400Whr rating. I did this test on a few cheapo lithium batteries and found them also to be over rated on the label. Your real world experience CAN be different once the cells get out of balance. My girlfriends specialized levo would not charge all the way and would also discharge very fast when it was low. This was because the BMS of the pack is cheap as hell and the charger does not balance the cells the way it should. As soon as any of the battery banks reached 4.2 volts it stopped charging. I found 2 banks with cells that were only at 3.6v while the others were at 4.2 so it never got a 100 percent charge. Also when the cells began to drain those two banks that had a low voltage condition would drop out and it was the equivalent of losing 20 percent all at once.
For a start, not all batteries will alow you to just discharge them. Shimano as example has electronic switches inside it to prevent just this without being installed in the system.
You also assume a linear discharge over the voltage range, which you don't get with batteries, and as already mentioned, the power available will be dependent on the discharge rate, which at 9W, will not be representative of use and so will result in an over reading of capacity.
I won't go into the rest of it.