This is a fairly niche post, but it is information that I think needs to be out there.
Three years ago I bought a Taylor acoustic guitar with money I earned on St++m (cannot bear to use the name). This was a big upgrade from my old Dean guitar, but it was worth it as a much better quality instrument. It is actually a low-end Taylor that is made in Mexico. This is an electro-acoustic model that requires a 9V battery to power the pre-amplifier. Eventually that battery gave up and so I bought another. I bought a Duracell as they are usually good quality and that was what was in it before. This is when I discovered the problem.
On the left is the Duracell Duralock battery fitted in the guitar at the factory and next to it the Duracell Ultra I bought. You may be able to see there is a slight difference in height. The battery holder is only big enough to take the smaller one and trying to force anything else in is likely to break it. On ebay I found someone selling batteries that are guaranteed to fit a Taylor. It seems that particular Duracell is not available to buy, but the Energizer Max is also slightly smaller than others. I had assume that there was a standard size as lots of devices take 9V batteries. I actually tries snipping some of the plastic case off another battery I had, but it still would not fit.
The Energizer fits easily. On the right is the holder from my Dean that can take either size, so that is possible. They will generally have spring contacts inside the guitar to make a good connection with the terminals.
This seems like a mistake by Taylor who normally seem pretty thoughtful about their designs. If you are about to play a gig and find the battery is dead you will want to be able to just buy another, but it may be that these particular types are not available.
Taylor knew about this problem and their suggestion was to cut part of the battery holder off. I do not expect this with a £1000 guitar. Maybe they have fixed it now. It should be possible to allow for some size variation.
There may be other cases of devices where some batteries are a tight fit, so the Energizer may help there too.
I have actually just bought a load of different batteries. I needed some button cells for various devices, including my D'Addario guitar tuner. These take a CR2032 cell, which is a standard size as the number says it is 20mm diameter and 3.2mm thick. Again I went to ebay and found some packs of those, but I slipped up a bit. You may notice it says 'mixed pack'. The bottom two are CR2032, but the others are CR2025 and CR2016, i.e. thinner. They are all 3V, but the thinner ones will have lower capacity. I got one of those to work in my tuner by packing it out with some foil. It does not draw much power and so should still last a while. That will teach me to read the postings properly, but they were a good price anyway.
The other battery I have on order is a little 12V for my multimeter. I left it switched on and drained the old battery. I need this so I can check batteries in general to see if they are really dead. Something else I need to buy is a new battery charger as mine is not working. That was needed more when my kids were young for all their electrical toys.