There is a 3.6V rechargeable NiCad battery soldered to the motherboard, so it should be replaceable by any 3 NiCad cells in series: http://paulq.org/amiga/battery.html
An seemingly excellent procedure is described at this page:http://amiga.serveftp.net/battery.html
In my A3000 I replaced the soldered battery with a portable phone battery pack, three shrink-wrapped AA NiCads. After replacing, I had to use the SetBatt program to change the SCSI timeout back to LONG, having noticed that the computer no longer automatically mounted the Quantum hard drive, only the faster Maxtor that luckily had the system on it.
I also set the date and time, which is working fine. Maybe this SCIS wait time being forgotten by the real time clock battery going dead is the reason my A3000UX is not recognizing its hard drive. Next time I do this, if I don't want to worry about what happens to the NVRAM when I disconnect a battery, I should just clip a battery to the parallel capacitor while I do the swap.