How to calculate mAh capacity of a single module

Well... I always say the rebuilders that spend tens of thousands of dollars on lab grade electronic systems to measure cell capacity do it much more accurately and efficiently, and that hobby chargers and dischargers aren't very accurate because they can't be calibrated / fine tuned.

But then @TMR-JWAP will come along and explain that he finds hobby chargers accurate enough and his methods seem fairly straightforward.

If you're just trying to buy high quality used modules that aren't junk than buy from professor Joe. He not only has spent a fortune on battery rebuilding gear over the decades, but he teaches hyrbid and EV cars at the local college: Hybrid Car Battery Distributor | Contact Us | 2nd Life Battery

 

Battery capacity is the ability to give off an amount of energy per unit of time.
It is accepted that the battery has upper and lower voltage limits.
For a nickel-metal hydride battery module, energy is given off within the voltage range from 8.4 V to 7.2 V.
It is dangerous to charge above 8.4 V due to the boiling of the electrolyte and the destruction of the shell due to pressure.
The lower voltage limit of 7.2 V sets the limit after which the battery quickly loses its quality characteristics and becomes waste.

You can measure it using a car lamp by connecting it to the positive and negative terminals of the module.
You will need a voltmeter to monitor the voltage, an ammeter to measure the current and a stopwatch to measure the time.
If your module is discharged from 8.4 V to 7.2 V within one hour, and the current is equal to 5 A, then the capacity of the module will be 5 A * hour.
If the module is discharged in 45 minutes, the capacity will be 5A*3/4 hour=3.75 A*hour

Since the lamps can be of different power, the current must be measured in each case. In addition, the current may drop during discharge.
Then it is more accurate to measure the current over short periods of time (5-10 minutes) and sum up the values.
For example: 6A * 1/6 hour + 5.8A * 1/6 hour + 5.4A * 1/6 hour + 5A * 1/6 hour

 

The one variable that's hard to account for is heat throughout the entire circuit, especially when you have fans keeping things cool. Expensive equipment is calibrated to have accurate readings based on heat issues, cheap equipment is not...

Especially the temperature of the filament in the bulbs change the resistance and a dim bulb is a different resistance than a bright bulb, so lab grade gear uses resistors that don't vary in resistance. Of course I like the way the lights light up a wall and let me know with a click glance how things are going with each module with a volatge meter giving more clear details for each module.

Also NiMH is extraordinarily durable without damage even if you discharge down to 0 volts. I used to worry so much about that until I fell asleep once and 7 modules sat at 0-volts and was continually discharged by bulbs for 8 hours with no ill effects afterwards.

 

Whether using a bulb or a resistor, the discharge current will change over the duration of discharge, as the voltage drops. That isn't a problem if you have a good ammeter datalogging the actual current the whole time. Then you just numerically integrate to find the area under that time-current curve from the fully-charged to the fully-discharged condition, and that's the amp-hour capacity. The bulb and the resistor will give that curve different shapes, but the usual techniques for finding the area beneath work either way.

Or you could build a constant-current circuit to discharge with, and then finding the "area under that curve" is trivial because it's rectangular and you just multiply the constant current by the hours it took from the fully-charged to the fully-discharged points.

When comparing results reported by different people, it's important to find out what criteria they used for the fully-charged start point and fully-discharged end point. For example, 7.2 volts suggested in #3 is the nominal voltage for a module (1.2 V per NiMH cell). Some people will use 1.0 V per cell as a discharge cutoff. Different choices there will lead to different capacity figures.