Unfortunately not all A1181s use the replaceable battery - some use a Super-Cap. Certainly mine did, after spending 1/2 hour opening it up and flipper the case over I find no replaceable battery, rather a small device labeled “V C2800” almost under J4501 (HDD). Mine charges up to 2.8V with the AC power adapter hooked up, but quickly discharges to 0.9VDC once power is removed. I shall see if the SuperCap at Digi-Key works better…https://www.digikey.ca/product-detail/en...
Here is discussion on the SuperCap version: Is there no CMOS RAM battery on this model?