The Nimbus range of computers with a real time clock RTC require a battery to power them when the mains is switched off.
A big problem with older PC batteries is that they leak and damage the motherboard, fortunately RM fitted external lithium batteries in a small box.
You will mostly find the battery is fine, but dead...

	This one from 1991 has long since run out of power, but condition wise looks brand new, which for being 25 years old is impressive.
They are deigned to be placed in a small plastic box that velcros onto equipment.
	The modern day replacement can be purchased from RS Components in the UK: Part Number: 594-274 http://uk.rs-online.com/web/
Here is a new battery fitted to an RM Nimbus VX/2