My Navigo 4.3" battery has never been much use even from new, losing charge within seconds lately when the cigarette lighter/usb cable would pop out mid journey.
Decided to solder (I have very poor soldering skills) in a Nokia BR-5C battery which went surprisingly easily,however managed to break the tiny speaker wire connections.
Had already decided that if I cocked up soldering them on again, I would just plug a speaker into the external phono socket.Needn't have worried as this went well too.
Before you attempt opening up the unit, I recommend you get some double sided tape as the battery bouncing about inside would not be too healthy.
Thanks to all those that provided the very useful info to enable this replacement