The fuse should be in series with one of the wires to the battery. Preferably, use a covered fuse holder, with a very short wire between the fuse and the battery.
With lead acid batteries, the potential fault current may be several hundred Amps. So you should select a fuse with an interrupt rating of at least 1000 Amps. If the wire has to run a long way, you may need to use heavier wire to avoid excessive voltage drop under maximum load. That is fine. You can still use the 100 A fuse with larger wire.
Do I need a fuse to fuse a battery?
In between batteries (unless regulations require) it is not necessary. Fusing is for wiring not so much equipment protection. The bms takes care of the battery as you said. There is a decent product that acts a a rapid disconnect and fusing, look up the mersen fuse disconnec t. There is quite a range of them for different amperages needed.
Internal battery cabinet output fuse is 80 A for battery protection. In-line battery fuse requirement: To protect the output conductors, install a line fuse rated properly to the load current in the battery plus side of each output pair. Wire per applicable local code.
Is fusing a battery a good idea?
Fusing is for wiring not so much equipment protection. The bms takes care of the battery as you said. There is a decent product that acts a a rapid disconnect and fusing, look up the mersen fuse disconnec t. There is quite a range of them for different amperages needed. What is the bank powering? What size wiring as well?
There is even a mod for the lynx power in to take the fuses if you dont want to use the electronic mod to power up the lynx distributor without the lynx bms. That is the ultimate best way. As for it being a requirement, usually fusing is for wiring, not equipment protection.