Remotely monitor condition of fuse

Question

I am installing five 12V batteries in a motorhome. The batteries will be wired in parallel for a 12V system and will be contained across three different enclosures. Each positive cable link between battery enclosures will be fused at either end, as such there will be seven fuses mounted in locations with minimal accessibility.

For this reason I would like there to be an indication light on the outside of each battery enclosure, or a remote monitoring station, to alert of a blown fuse.

I am aware that in many scenarios a 12V LED may be connected across the fuse to light up when the negative side of the fuse is no longer supplied with voltage, however as these fuses will have nominally 12V at both ends I figure this idea won't reliably work.

The solution needs to be economical and reasonably compact, while I am happy for it to be a custom made solution. I appreciate any input.

Edit to add circuit diagram of current layout:

Answer 1

The simplest solution I can think off would be to use a 5V isolated DC-DC converter.

R1 & the LED are the basic indicator. When the fuse blows the LED lights up. The LED current is ~1.7mA.

D2 and R3 are there to protect the DC-DC output if the fuse is blown and the output of the whole battery section for some reason goes to 0 volts. R3 also protects the LED if the fuse is blown, BAT1 is removed and the BAT1 wires are then shorted.

You need only a very small DC-DC converter. The smallest are less then two dollars. You must use an isolated type. You need one circuit per fuse.

The DC-Dc converter input can come from the common battery 12V section. You will not get an indication if all fuses are blown! The alternative is to power the DC-Dc converters from an independent source.

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. The use of a 2-pole DC MCB makes the solution very simple.

Two options are shown in Figure 1.

• The left circuit will light the LED when the circuit-breaker is closed and gives an OK signal.
• The centre circuit normally shorts out the LED but when the circuit breaker opens the LED will light indicating a fault.
• With a three-pin red/green LED (if you can find one with this pin-out) or two LEDs you have an OK/FAULT indication.