HP3478A Replacement Battery

[Miti]

After getting my two HP3478A meters, the first thing I did, even though I regret it now, was to replace the batteries. I saved the calibration constants soon after that and it was proven to be a very smart move because both replacement batteries died, within one year the first one and 2.5 years the second one. The batteries were bought from a local store since DK doesn’t carry the lithium batteries in Canada. After the first replacement died, I went to the store and asked them to let me select a good battery and I got the one with the highest voltage from the shelf. That still measures around 3.7V after more than a year now. The second one just died this weekend. It is interesting that the voltage on the dead battery was around 10mV so those memories retain the data all the way to hundreds or even tenths of mV.
I went back to the store to select another battery but unfortunately they don’t carry them anymore.
So I improvised. See attached. I hope it will last at least for few years. It is sure easy to change.

[EHT]

There are special types of long life batteries to use such as Lithium Thionyl Chloride and Panasonic's BR2/3 series. These are all designed to have a constant voltage until EOL so I think measuring the voltage won't give much of a warning. They can last a very long time, e.g. 10Y design life.

I noticed that in the circuit of some of this old HP kit (3457A, 8904A), the batteries are hooked up via diodes so I have attached new batteries in series with new diodes and left the old batteries in place. I suppose I'll need to remove these at some point in case they leak. The diodes are ideally meant to be a special low reverse-current type.

[Miti]

There are special types of long life batteries to use such as Lithium Thionyl Chloride and Panasonic's BR2/3 series. These are all designed to have a constant voltage until EOL so I think measuring the voltage won't give much of a warning. They can last a very long time, e.g. 10Y design life.

The batteries that failed are Lithium Thionyl Chloride, see here : https://www.eemb.com/model/er17335.html
Unfortunately they are made in China. It is also possible that someone dropped them in the store and that would increase the self discharge current many times.
I'm not sure about that myth about constant voltage. In a bunch of maybe 10 batteries, I found everything from completely dead to 3.7V, which I bought. They may stay at about 3V for a long time with a tiny load, but if I find a new battery at 3V, that's a clear indication that something is wrong, there's a big internal discharge current.
The proof is that the one I bought still reads around 3.7V after more than a year.
Since I can read/write the calibration constants through the GPIB, all I care is that I don't need to do it every time I power it up. If I need to replace the CR2032 in two - three years, so be it.

[wiesl]

Which batteries do you recommend?

[Miti]

Which batteries do you recommend?

If you can find the genuine Panasonic BR2 that is recommended by HP, that’s what I would use. If not, see my modification above, but don’t forget to save the calibration constants.