I'm looking for advice to how to replace a Duracell TR163 mercury battery in an old synthesiser I have (Oberheim OB-1 if anyone is interested). The battery is no longer available, at least not in the UK, so I'm looking to see if I can replace it with a circuit or some other type of battery. The battery itself is rated as 4.05V and 500mAh so I'm thinking that using 3xAA batteries and reducing the voltage from 4.5V to 4.05V would work. I'm very much a beginner with electronics and there seems to be several options (voltage divider, diodes etc) however I've not been able to find this specific situation to use as a reference.
My concern is that I don't know whether the load is fixed or variable so I'm not sure a voltage divider is sufficiently safe. I believe the battery powers the patch memory when the device is powered off but I don't have any more details at the moment.
Ideally I'd like some sort of rechargable battery system that is charged from the main power board when the synth is running, but that's probably reaching too far!
Any advice would be most gratefully received.