Here I describe the change of the clock backup battery.

I had this problem, the clock always was 0:00 when the radio had been off

The original battery is a ML414H and has only 1mAh of capacity.

I had some somewhat larger cells in stock, a ML1220, these have a capacity of 17 mAh and are 12 x  2 mm in dimension.
So this is a rechargeable battery just like the one in the radio.
A lot of people describe to replace the battery with a CR2032 lithium cell.
I do not know, but can these cells withstand the charging current?

Her the schematic and board layout around the battery:
 
So there is a 3K3 resistor which charges the battery

I opened the bottom of the radio and found the battery here:

So out with it and in with the replacement:

OUUUUUPSS

So I had to improvise to mount the new cell:
   

So far it works very well, Maybe in some years I have to change again …