Do I need a fuse between charge controller and battery?
The answer is yes, you do need a fuse between your charge controller and battery bank. Should an overcurrent occur, you would want to have an overcurrent protection device (OCPD) placed between your charge controller and battery.
An overcurrent can occur due to short circuits, incorrect wiring, excessive load, etc… which in turn could cause overheating, fire, or even explosions. A fuse placed between your charge controller and battery would protect these components as well as your wires against these risks.
What size fuse between battery and charge controller?
The fuse that you need between your battery and charge controller should have the same Amp rating as the output current rating of your charge controller. For example, if you’re using a 30A solar charge controller, the fuse should be rated at 30 Amps.
The 30 Amp charge controller should not be able to output more than 30 amps of current, if it does, the 30A fuse will blow immediately and disconnect the circuit.
Another important thing to note is that you’ll also have to make sure the amp rating of the fuse does not exceed the Ampacity of the wire. If your wire is correctly sized, this should not be an issue.
If you haven’t yet sized your wire, please refer to this page: Charge controller to battery wire size calculator