A 1500mAh rechargeable battery, fully charged. If it can discharge at 1500mA for one hour, the capacity is sufficient. However, it's still discharging at 750mA for two hours for testing. But even at 0.9V (for a 1.2V rechargeable battery), it's still discharging the nominal current. How can this be explained?
The battery capacity we refer to is actually the battery's usable capacity. In other words, the amount of electricity a 1500mAh rechargeable battery discharges from 1.45V to 0.9V is the capacity we need. The capacity discharged below 0.9V is unusable and minimal (too low a discharge rate will also affect battery lifespan). Generally, good rechargeable batteries have a good discharge plateau; for example, a 1500mAh battery discharging from 1.45V to 0.9V...
If the current C discharges to 0.9V over 120 minutes, then we say the battery's usable capacity is 1500mAh. The relationship between current, voltage, and time during this period is shown in the figure:
A good battery's discharge plateau will show a rapid voltage drop after 0.9V, resulting in very little charge being discharged in a short period. Conversely, a poor-quality battery will show a rapid voltage drop between 1.45V and 0.9V, followed by a very slow voltage drop after 0.9V. Such batteries generally have poor performance and very low capacity.
Note: I: Ampere or milliampere;
f: [1/hour], when describing, it is often only said as "number" without mentioning the unit;
C: Battery capacity, unit: ampere-hour or milliampere-hour.
For example, if the battery capacity is 800mAh and it is charged at a current of 0.1C, it is equivalent to:
The charging current for the battery is I = 0.1 [1/h] x 800 [mAh] = 80 [mA]
f is also called the "charge (discharge) rate in n hours", f = 1/(n hours)
Measuring battery capacity generally involves measuring battery voltage. The charger detects the battery voltage during charging, and considers it fully charged when the voltage reaches a specified value. For example, a lithium battery is considered fully charged when its voltage reaches 4.2V. The voltmeter used to detect the voltage must be accurate to ±1%, as overcharging a lithium battery will damage it. If you want to monitor battery charging and discharging, there are well-established circuits available in textbooks. If you just want to measure it, it's simple: calculate the load resistance based on the capacity, discharge the battery using a standard discharge current, and then measure the voltage. As long as the discharge time reaches or approaches the nominal value, it's acceptable.
