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: enter image description here

  • \$\begingroup\$ however as these fuses will have nominally 12V at both ends I figure this idea won't reliably work. That is wrong, an LED will have not problems with that whatsoever. You can use a standard LED with a resistor. The problem might be more that there is no voltage difference as the other batteries supply 12V. \$\endgroup\$
    – Oldfart
    Apr 14, 2018 at 10:36
  • \$\begingroup\$ Can you post a drawing or schematics of your setup? \$\endgroup\$
    – Lior Bilia
    Apr 14, 2018 at 16:31
  • \$\begingroup\$ It is not clear why the battery at fuse S7 is getting special treatment (or requires two fuses) unless it is the engine start battery. I would make an effort to have all the batteries fused to a single common accessory rail even at the cost and weight of longer additional cable runs, if the main rail is shorted to ground all the fuses should open, no obvious need to segment it. \$\endgroup\$
    – KalleMP
    Apr 15, 2018 at 13:52

2 Answers 2


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.

  • \$\begingroup\$ That is very good, thank you for sharing. I will have to look in to isolated DC-DC converters. I have understood everything there except why the LEDs wont work if all fuses are blown. Can you please explain? \$\endgroup\$
    – Sebastian
    Apr 14, 2018 at 21:07
  • \$\begingroup\$ Are the resistor values in your diagram tailored to a 12V output DC-DC converter? \$\endgroup\$
    – Sebastian
    Apr 14, 2018 at 21:19
  • \$\begingroup\$ The resistor value R1 is for a 5V DC-DC and a ~1.5mA LED. R3 is always the same. If all fuses are blown none of the DC-DC converters get any power at their input as they are driven from the common 12V line in my diagram. Unless you give them their own supply. \$\endgroup\$
    – Oldfart
    Apr 15, 2018 at 4:09
  • \$\begingroup\$ Thanks for answering. I realise now that was a silly question. \$\endgroup\$
    – Sebastian
    Apr 15, 2018 at 11:01
  • \$\begingroup\$ Robust solution. You can use a pair of diodes to supply +12V power from both sides of the fuse to the converter making it work if there is power in the common rail. \$\endgroup\$
    – KalleMP
    Apr 15, 2018 at 13:49


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.
  • \$\begingroup\$ Thank you Transistor, that's a idea I ought to have realised, thank you for sharing. I will look in to the cost implication. \$\endgroup\$
    – Sebastian
    Apr 14, 2018 at 21:11

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