How do you fix water damage in a nokia bl 4c phone

As we all want longer stand-by and talking times with our mobile phones and sometimes get problems to get an original battery for our old Nokias, here is a tip for this matter:

Most of the former Nokia mobile phones use the battery with the model type number "BL-4C". There are some models using type "BL-5C". This "5C" model has more capacity and could also be used instead of the "4C" model. The 5C model is slightly fatter than the 4C one.
Using BL-5C needs with some Nokia mobile phone models a little modification (like 6100) while others do not (like 6260) and require a bit harder close of the battery lid. Cutting both edges on the batteries sides to fitting in the battery compartment is easy done. Charging process is not influenced and works accurate.