Another general way to measure the health and charge capacity of a battery is to fully charge it from a discharged state and while doing so, measure the mAh required to fully recharge the battery (Volusiano mentions this w.r.t. built-in laptop circuitry). To do so, it’s handy to be able to insert a USB digital multimeter in the charging circuit pathway and set it to record mAh passing through. A variety of such devices are sold on Amazon.
Each Li-ion battery is rated in its capacity when discharging at an average voltage, usually 3.7 v, as specified on its label. Recharging a Li-ion battery is usually done at 5 v or higher (the USB multimeter will display the charging voltage in the circuit). So, to correct the 5 v mAh input into a battery to the 3.7 discharge capacity, one would have to multiply the input mAh by 5/3.7.
The other thing is that charging is not 100% efficient. Wired charging is about 95% efficient in charging a Li-ion battery, wireless charging only 60% to 70% efficient (the other 40% to 30% of input energy is wasted as heat, same for the loss in wired charging). So, you ain’t gonna get good figures for recharging your HA’s by wireless induction even if it’s possible to stick a digital USB multimeter in the circuit path.
You can also get battery health apps, at least for Android, that essentially do the same via phone circuitry. I’ve used one in the past, AccuBattery, for my Galaxy Note 8. AccuBattery recommends that one charge through at least a 20% state of charge difference to get enough significant figures in the measurement to accurately estimate remaining recharge capacity compared to the stated battery capacity.