Battery power weak

have the ES 7036 for 3 years now. Use the device to 90 percent as pure Trockenreasierer. Initially, the small hairs on the neck were still well shaved. After 2 years was no longer that.
The biggest problem is the battery. The duration decreases constantly. It is also not possible to operate the device when the battery is charging. So only with electricity does not go.
Battery performance breaks a matter of seconds, so that shaving on a business trip or weekend break is not possible.