# Should I look at nominal or max battery voltage for choosing resetable fuse?

I am building a LED/Camera control circuit that is going to be using a 14.8V Li-Ion battery as a supply. I want to put a resettable fuse to prevent short circuits from blowing up too much since this is going in water.
At first I chose a 16 V fuse thinking it would be enough, but looking at the battery voltage at max charge, it reaches up around 16-16.4 V.

When I spec my fuse, should I look at the nominal battery voltage or the Max charge battery voltage?

Maximum since that is the voltage the fuse might have to interrupt. Voltage ratings don't play any role while the fuse is conducting. In your case though, you probably won't notice a difference in practice due to tolerances and battery voltage dropping as it is discharged. It's not exactly difficult to interrupt 16V or even 24V. Now, 100V or 200V or 1000V...that's different.

• What do you mean it's not difficult to interrupt the 16V fuse? I don't want any interruptions while the circuit is running. Commented Oct 29, 2019 at 19:50
• @Francoislandry Imagine what happens in a 1 million volt circuit if the fuse needs to blow but it only produces a 1mm air gap when it does. Commented Oct 29, 2019 at 19:52
• At that voltage a 1mm gap is probably as good as connected, but I don't understand the relation to my question, sorry. Commented Oct 29, 2019 at 19:54
• @Francoislandry Exactly. Now how big gap do you think 16V can actually arc across? Now look at the size of your fuses and think about what a reasonably manufacturable air gap is for those fuses. Commented Oct 29, 2019 at 19:55
• A 50v or 120v fuse will only work better than a 16v one. Commented Oct 29, 2019 at 19:57

So my question to you: when I spec my fuse, should I look at the nominal battery voltage or the Max charge battery voltage?

When specifying voltage on components, they need to be able to handle the max voltage for the design that they are used in. Another consideration is also spec for the max worst case voltage. There could be situations in a design (like when hot plugging cables with an inductive load) that the voltage could rise above the rail momentarily. For this reason, its better to even specify a max voltage above the normal max voltage just in case.

I would consider 18V (or more) to be a good starting point for a max voltage spec. Sometimes parts can't be located so that might not work.