Attaching a fuse to a new set of batteries?

Almot wrote:

DrewE wrote:
{alarm} will work fine because the resistance of the alarm is much much higher than that of the fuse, and so the high current will not flow through the alarm--it will be limited to whatever the alarm usually uses. It's no different than connecting the alarm directly to the battery.

If alarm is in parallel to the big fuse that is sitting on the Pos terminal - as it was suggested - then whatever high current flows through the big fuse and blows it, will flow through this alarm a moment later.

It won't be a high current that flows because the impedance of the alarm is much greater than the impedance of the fuse previously was. The alarm will limit the current. Electricity doesn't have "inertia" as such; it just follows ohm's law. (If the connected circuit has significant inductance, the sudden change in current will cause a voltage spike, which may cause its own problems. I would not expect a particularly large overall inductance in an RV's 12V system.)

This same basic setup is exactly how the little fuse blown lights on fancy 12V fuse panels work. The light (an LED and a current limiting resistor) is simply wired in parallel with the fuse, and when the fuse opens, the light illuminates due to the current path through the rest of the circuit. The suggestion here is exactly the same, except with an audible alarm rather than a visual one.