Yes, the battery is for memory retention, the standard memory for that processor is SRAM so it needed a battery. Depending on the vintage, you may have been able to add an additional long term battery to the rack, but if you didn't mention it, it's not likely there. Also depending on which version you have, there is/was a non-volatile memory module that could have been added. There are 7 versions of the 5555, 4 of them (M12, 13, 14, 16) could NOT accept the non-volatile memory card, 3 of them (M22, 23, 24) could.
If you have powered it up and the Batt light is red, then it's likely toast. Maximum life AFTER the warning light comes on is 26 weeks (1/2 year) but can be as short as 8 weeks depending on the size of the program and ambient temperature it was kept at. If the Batt light is not on, you may be OK, life expectancy is up to 3 years (again de-pending on program size and ambient temperature).
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