The name "VARTA" strikes fear into the hearts of Amiga 500 plus, and A501 memory expansion owners: VARTA rechargeable batteries have been built into these devices, and with growing age are prone to leak green acid, damaging the printed circuit boards and other components.

Original rechargeable battery from an A501 memory expansion

So it's an absolute requirement to remove that rechargeable battery, and clean the PCB from all acid that might have spilled out, if you want to keep your device intact. Most Amigans seem to use vinegar and alcohol for cleaning, sometimes using a tooth-brush to rub off any damaged material. If you don't catch the "infection" in time, traces or components might be damaged, requiring repairs.

But still it would be nice to have a working backup battery installed to your machine. Coin-cells, type CR2032, are a good replacement, they're widely available, and do not leak. But keep in mind that these aren't rechargeable - the Amiga provides recharge-current to it's batteries, so when replacing the original rechargeable one, we need to take care of that to avoid (additional) damage.

Removing the rechargeable VARTA battery shouldn't be a problem, you just need a soldering iron. Next is the cleaning - and if you're unlucky - repair job.

When you're done so far you can install you coin-cell. The original VARTA batteries are 3.6V. CR2032 coin-cells are 3V, that's within range for the clock chips used in Amiga 500 pluses and A501 memory expansions (mostly OKI M6242 chips).

With some googling I found Daniel Schneller's nice article on how to do the replacement. He's done what's required, and added a few details, mainly putting the new battery on a separate board to avoid stress to the original PCB when exchanging the coin-cell in the future - an idea I like very much. I basically did the same, but wanted to keep my A501 in "one-piece", so I decided to put the extra board onto the A501 itself. According to one CR2032 manufacturer's docs, a diode and a 200 ohms (minimum) resistor are required to keep the Amiga's recharge-current away from the new, non-rechargeable battery. I'm using a 220 ohms resistor, just for a little bit of extra-safety.

Here's a schematic of the new battery board:

Battery board schematic (click to enlarge)

Nothing spectacular - have a piece of drilled board ready, the components as shown in the schematic, add a coin-cell holder, wires, and pin headers, and basically all that's left is the soldering.

New coin-cell battery on a separate drilled board

To fix the new board to my A501 I added several layers of duct-tape to the bottom side for padding, and a small piece of extra-strong double sided sticky tape as a (hopefully) removable glue.

The extra board is held in place by double-sided sticky tape

I'm quite pleased with the result! It doesn't look totally amateurish, there's close to zero modification to the original PCB, the sticky tape should keep the new board properly in place - and if I should ever want to do so, I could probably revert the modification completely.

And best of all: it works. :-)

Thank you very much, Daniel Schneller, for sharing your know-how and experience!

Sources:
http://www.danielschneller.com/2018/02/amiga-500-restoration-a501-memory... 
http://www.farnell.com/datasheets/1496885.pdf
http://thomas-rapp.homepage.t-online.de/index.html
http://thomas-rapp.homepage.t-online.de/download/RestartClock.lha