Question about Simplehuman Foam dispenser

The foam dispenser contains a cartridge that you can then refill. Instead of the - in my opinion - relatively expensive refill concentrates, I use "normal" liquid hand soap (without "peeling grains" or the like in it!) and mix it in a ratio of 1 part soap to 3 parts filtered (because of the limescale) water. Works perfectly! You can determine how thick the foam should be using the three different tubes provided.
I have read in many tests that the battery runs out relatively quickly. I have had the appliance for 2 years now and have had no problems so far. I leave the device on the charging cable for at least 24 hours every 5-6 months.