If you want the remaining shaving time to be as accurate as possible, let the battery run completely empty* before you recharge it.
If your shaver is new or if you haven’t done this before, we advise you to continue shaving until the battery is completely empty before you recharge it. Ignore the battery low indication and keep using the shaver until the motor stops. Do this three times in a row. After that, it is enough to repeat this twice a year.
*Please note that leaving the shaver in the charger continually will not harm the battery. Using the shaver until the battery is completely empty is only necessary if you want an accurate remaining shaving minutes indication.
Why the shaving time is shorter in case of heavier beard growth can best be explained by comparing the fuel consumption of two identical cars. Car 1 has a lot of luggage on board. Car 2 has no luggage at all. Apart from that, they are the same. When both cars drive the same route, the heavily loaded car will use more fuel.