If you're having this problem then you need to at least, remove the (probably leaking!) original battery from the motherboard. That would be the 'low cost' option as you can do it for free.
The best option is to buy one of those lithium battery upgrades from eBay and solder or have it soldered in. Corrosion from leaking batteries is a well known villain around here :-/ And it is the silent killer of many an old Amiga.
Just a word to the wise.
