I did check the backup battery and it reads 3.3V. Seems pretty good. I have found the same exact replacement available - should I replace it? All opinions are read and considered! 
Don't go by the voltage. Go by the date code on the battery. THAT is your only real yardstick. If you're lucky, it's been replaced recently.
mnem
*More than the sum of my differential pairs*
I did an in-depth study on backup batteries for a major aerospace project. The requirement was to predict battery status, i.e. remaining charge, independent of temperature and without charge counting. What we found is that the most significant measurement is the recovery voltage after a load pulse.
So we implemented a measure-load-measure cycle as BIT.
The problem with that is not the state of charge; it is the expected life cycle of the product. Most of these backup batteries have an expected life cycle of 10 years or less. Anything beyond that and you have no idea if the battery is just about to develop a faulty plate or leak and take all your critical CMOS data with it. There is no form of electrical testing that can accurately predict this; the best you can do is perform reasonable preventive maintenance and be thankful that your "10 year or less" battery actually lasted 150-300% of expected life and you still have valid data preserved.
I did an in-depth study on backup batteries for a major aerospace project. The requirement was to predict battery status, i.e. remaining charge, independent of temperature and without charge counting. What we found is that the most significant measurement is the recovery voltage after a load pulse.
So we implemented a measure-load-measure cycle as BIT.
The only number I see that might be a date code is - 5024 - but I can't find the magic decoder for it. Anyway, I did find the exact battery on Ebay so might as well replace it. I'm getting ready to go in and add the Option 001 which adds modulation capability, I really can't see using any of the other options. Hope it works ...
Note: I usually write the date I change the battery ... on the battery itself. What a concept. 
Be sure to seek out any relevant eevblog threads on this unit... there may be some "magic process" that folks have devised to prevent data corruption during the battery swap,
as with the cal data from certain 24xx scopes, HP voltmeters, etc. Some devices actually have a process to make a backup to file which the device can later be restored from.
I make it a habit to do the exact same thing... and I've made a habit of assuming that if there is no such date code on a device, then the date of the battery is the manufacture date of the device. I also routinely measure the IR of my Lixx batteries over time; this figure I write on the battery along with a date.
Mr Sharpie is any technician's best friend.
Got me another HP triple supply (6236B)
Double bonus - got to drive out to Gloucestershire to get it which means I’ll take day out. This is the main excuse
Road trip! Road trip! Road Trip!!!mnem
glerp?
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