Even if it’s about the rechargeable and not the disposable type, the measuring technique to gauge the remaining juice in rechargeable batteries remains the same, using voltage measurement. You can only measure voltage on a battery or current discharged by the battery. But usually for the current discharged through the battery, unless you can get fancy enough and keep track of the cumulative amount each time to get a whole picture from a fully charged state, you can’t really tell the battery percentage status if you’re only looking at a snapshot in time and not the entire discharged history that would need to be monitored and stored somewhere.
But the mfg can get more fancy in tracking the voltage measurements and maybe they can also monitor current discharge to get a more accurate picture of battery life. That’s probably why Oticon wants you to reset the batteries when you replace them with new ones, so it can keep track of the statistics to help determine freshness over time to factor that into the battery capacity measurement.
For laptops or smart phones, they also have the facilities to keep track of battery usage statistics in order to help them determine battery health (which is its max capacity when fully charged compared to when the battery was brand new). It’s possible that they also monitor current usage statistics in addition to voltage snapshot measurements in order to be able to give users a more accurate picture of the battery’s remaining juice, on top of the battery’s health.
You may notice that sometimes for Li-ion batteries based laptops, when you want to know what the battery health is, they may ask you to run the battery down to depletion and recharge it back fully before it can accurately give you an estimate of the battery health, because that’s the only way it can measure the amount of juice that can be put back into it, hence its health. Other smart phones may not require you to do this because they probably keep better statistics of your usage cycles through voltage and current measurements to be able to make a good guess of the battery health without needing to ask users to deplete and recharge fully the battery like some laptops do.