How to size a fuse for lithium batteries

Without the correct fuse, your system could draw too many amps, heating your cables and system, to the point that they melt, fail, and / or cause a fire.

• To size a fuse for lithium batteries, choose a fuse rating that is at least 25% larger than the maximum continuous current you plan to draw from the battery, ensuring it’s always lower than the current carrying capacity of the battery cables. Essentially, the fuse should be the weakest point in the circuit to protect the wires from overheating in case of a fault.
• It’s just as important that your select a fuse type, capable of interrupting very high amps provided by the battery in the event of a dead short. Some fuses may simply melt together rather than breaking. For typical Lithium battery installations on boats, RV motorhomes, 4×4, and tiny houses, we recommend MRBF fuses mounted directly to the battery terminal, as these can interrupt 10,000A @ 14V DC!

• Formula to calculate fuse size: 

  Fuse size (Amps) = Maximum continuous current draw (Amps) x 1.25 (safety factor)

• Example: If your system has a maximum continuous current draw of 100 Amps, and your lithium batteries can provide this or more, then you should choose a fuse with a rating of at least 125 Amps. However your battery cables also need to be able to handle 125 amps. If your invterter and cables can’t or won’t draw this much, then down size the fuse to suit your system draw and battery cable size. It is OK if surge current (for a couple seconds to start a motor) is higher, up to twice fuse rating.