Many electric appliances, and in particular those used in daily life like a shaver or toothbrush, are battery driven to allow the user a mobile use and an easy handling of the appliance without the need of an power cable being inserted into the electrical wall socket during use. For ecological reasons and to avoid a frequent change of discharged batteries in the appliances, very often rechargeable batteries are used to power up the appliances. This technique is in particular useful for appliances that are used for a comparatively short time with regard to the idle time of the day.
One key asset for these appliances is a battery level indicator that indicates the need for charging the rechargeable battery (in the following also referred to “battery” only) to the user in order to avoid a breakdown of the appliance due to a discharged battery. This indication of the need to charge the battery shall be in time so that the user is able to finish the actual (normal) use of the appliance and after that, to recharge the battery e.g. by inserting the appliance in docking station having a charge. On the other hand it is to be avoided to indicate the need for charging to early as the user might regard this as a quality problem of the appliance itself.
Battery indicators are currently often based on a measurement of the battery voltage. If the measured battery voltage falls below a predetermined fixed threshold, the low battery level indicator is activated to indicate the need for charging the battery. Such a battery level indicator is disclosed in WO 95/34002 A1 or in WO 01/78211 A1. Further, the WO 2008/075246 A2 discloses a battery charge indicator detecting whether a battery voltage across the battery drops below a threshold value. A controller is determining whether a load of the apparatus is connected to the battery to activate the detector for comparing the battery voltage with the threshold value after the load has been disconnected from the battery.
However, it is difficult to use a fixed predetermined threshold as the battery voltage depends on many different environmental parameters and is not in all cases directly correlated to the remaining capacity of the battery defining the remaining battery power. For NiMH rechargeable batteries, for example, there is only low correlation between battery voltage and capacity. Therefore a discrepancy between a displayed battery level indicator and the real battery status is very likely.
Another disadvantage of detecting low battery level by a predetermined fixed voltage threshold measurement is that due to a wide variety of battery behavior (by distribution, temperature, current consumption, aging or the like) it is impossible, to find a universally valid voltage limit, that matches a low battery level under all conditions. So, the user will either get a low battery indication too early or too late to react and charge the battery before a breakdown. In both cases, the user or consumer will be unsatisfied with the battery level indicator and/or the runtime of the battery.
In the JPH 07294610 A, a possibility is disclosed to detect the remaining capacity of the battery by operating a self-discharging capacity based on self-discharging characteristics stored in a memory.
A further different approach for a battery level indicator, also applicable e.g. for NiMH batteries, is to count charge currents and discharge currents over time and to calculate the remaining capacity. But when not knowing exactly the charge and discharge currents, this also will result in a mismatch between indicated battery level on a display and the real remaining capacity. An exact measurement of the charge and discharge currents is extensive and power-consuming itself.