I read in this forum about someone that has turned on a unit like this and to their surprise the constants where lost before they could back it up. So finally decided to change batteries.
Hp was nice enough to provide the battery installation date on a sticker on back of the units. Both units have a Battery install date of 1991. I did check the battery dimensions inside the unit and they are 50mm long and 15mm diameter with Axial tabs. Original battery was made by Tadiran and the only marking I could see was AA.
I did a little research for the right battery choice. First we need to know the current requirements. The HP 8591A uses Static RAM TC55257 which only provides max standby current range between 2uA to 30uA per chip. This is useless information. Next step would be to ignore the current usage since after all the battery lasted almost 30 years and still works.
These are ~2Ah batteries (assuming all the capacity is used up by now ) so we can estimate that the average current to be no more than 7.8uA . In reality this is probably a lot less.
So you want a battery with low current discharge and long life. The TL-4903 is rated for 40Years on shelf and is rated for ~55.000 hours (6.3Years) at 38uA discharge while producing 2.1Ah. This should be a very good battery for this application.
Based on the data sheets I ordered TADIRAN TL-4903/P batteries..
To get ready to replace batteries on 2 of mine HP 8591A spectrum analyzers we need to backup Correction constants that are stored in RAM.
First step is to get the manual and print out the section 9-2..9.6 "Backing up Analyzer Correction Constants"
In worst case these could be lost if something goes wrong so worth having a hard copy (or Soft copy) to avoid expensive calibration.
Here are the calibration constants from the 2 units I have. Maybe someone can use this info.
With this data I feel good about waiting for the battery. No more stress over lost Calibration .