Usually it is CR2032 type, or CR2450 - 3 Volts.
Some instruments have special batteries of different shape, not cell type. One of my instruments has special backup capacitors, not batteries (I think Technics WSA1R). And some instruments have battery holder installed from factory.
Problem with some instruments is there's not enough space for the holder on the main board, even when used vertically. Then it's necessary to improvise a little bit, find another placement for the holder and connect it with PCB with cables. In such case it's good to cover holder with plastic tubing or tape to avoid possible short circuit. Or place the holder to small PCB and screw it somewhere firmly.
Solder plates on PCB for battery are usually big so pin distance is not so critical - if necessary, new holes can be drilled.
In some cases it's also good to think twice about the placement of battery holder - for example right now I have opened DX7 II D (I will change OS Eprom), where main board is placed (and screwed) so that battery is not accessible. In such case it can be a good idea to solder battery holder on opposite side, not to the component side...
Daniel Forro
Also, I would think that having the correct voltage is very import as they arent all the same. Its worth noting installing a battery holder with different specified voltage can permanently damage your equipment I would venture to guess.
Maybe I will propose and post this at Gearslutz likewise or someone else can start it if they like. To my knowledge the DX7 series, Matrix 6/1000, QS, Z1, Alpha Juno/MKS-50 and more all have soldered batteries.
These are very popular instruments and would think there is interest in having this reference info.