Motorhome advice: How to fit a leisure battery current monitor

Motorhomes, unlike domestic homes, come with two power systems. One is 230V AC, which is available when we hook up to mains electric and the other is 12V DC, which is supplied internally. The latter is used as a supply for the lights, pumps, entertainment systems, etc. Mains items such as microwaves and electric hobs are only usable when hooked up (unless high-powered inverters are used, which is more the exception than the rule).

The 12V DC is supplied from the leisure battery (sometimes called the habitation battery). However, when hooked up to 230V, the mains is stepped down within the vehicle to 12V DC and takes over from the leisure battery. Solar/wind chargers can also assume the role when not hooked up.

However, the leisure battery’s situation needs monitoring. First you must know what is going into and out of the leisure battery. Some motorhomes provide a current monitor, others give just an indication of the leisure battery voltage, which doesn’t tell you by how much you may be eating into its ampere/hour capacity.

This project details the fitting of a digital ammeter that is positioned to measure directly the actual current going into (or out of) the leisure battery thereby giving a good indication of any impending over demand problems that might occur. It is based on a readily available digital 50A ammeter kit, which meets all of the criteria mentioned above.

The digital display is mounted on a plastic box, inside of which is a 9V PP3 alkaline battery. The battery is used to drive the display. A PP3 battery has been used as opposed to a 12V supply from the leisure battery, which would require extra wiring and an inline fuse. The display current is very small, on top of which it is switched off when not being read, hence the battery will last quite some time. It is easily replaced should the need arise.

As with most ammeters, an inline shunt is used. This is a very low-value resistor across which, courtesy of Ohm’s Law, a voltage is developed proportional to the current flowing through it. The digital display monitors this voltage and displays a value calibrated in amperes. The shunt is placed in series with the negative (-) supply wire connected to the leisure battery and not the positive (+). This ensures no accidental short circuits occur should something touch one of the contacts. Any connecting wire should be able to handle the maximum currents involved during charging, etc. I used wire with a 40 amp capacity.

Ebay was my main source of bits; however, other suppliers such as Maplin are equally suitable for most parts. The total cost was about £20 and the whole process takes about two hours.

To fit a leisure battery current monitor, you will need:

• Conventional toolbox (screwdrivers, pliers, etc)
• Pad saw
• Cordless drill and metal drill bits
• Tenon saw (to cut the plastic sheet)
• Soldering iron
• Heat shrink gun
• Crimp tool
• 50A digital ammeter kit (must be able to read current in both directions, positive and negative. Mine was from ebay for £12.99)
• An ABS box with external dimensions of 116mm x 78mm x 37mm þ Three M2 x 8 screws and nuts (to secure battery holder)
• Two M5 x 25 countersunk screws and nuts (to secure shunt to base)
• M5 washers (If spacers are not used on the shunt, I cut some 10mm aluminium tube for mine, then a washer stack can be used)
• Small toggle switch
• PP3 battery holder with flying leads
• Grommet (for shunt sensor wire exit hole in box)
• Double-sided sticky pads
• Heat shrink sleeving, wire and wire terminals to suit 
• 3mm thick plastic sheet for shunt base (metal or plywood can also be used).

The 50A ammeter kit comprising digital display unit, current shunt and interconnecting wiring loom.

After covering the box lid with masking tape, mark up the clearance hole for the digital display and drill a hole just inside of the four corners along with a clearance hole for the switch.

Use a pad saw to cut between the holes and remove the shaded area. Then file the edges until the digital meter is a snug fit.

Next fit the digital display and toggle switch to the box lid.

Create the drilling template for the battery holder by drawing around the perimeter and marking through the holes, then use it to mark the battery holder drilling points on the side of the box using a pointed tool.

Drill three 2.5mm holes for the battery holder screws.

Attach the digital display loom to the 9V PP3 battery holder. Note the extension to the yellow and white wires, which should be long enough to reach the shunt.

The finished inside of digital display box with 9V PP3 battery fitted.

The ready to install display unit with shunt sensor connecting wire emerging from the bottom of the box (you’ll need to drill a hole for this).

A base plate was made for the current shunt. Dimensions should be 10mm larger than the shunt all round. Holes were countersunk so that M5 screws sit flush with its bottom surface.

The shunt assembly shown with the connections underneath. This method ensures good ventilation and keeps the connections isolated.

The spacer and screw lengths were chosen to ensure the terminal connecting screws are clear of the base wires once they’re connected.

The digital display to shunt connection details – the colours should be as indicated to ensure correct current direction sensing.

Shunt located in negative supply line adjacent to the leisure battery. It is secured using double-sided sticky pads.

The digital display box is mounted in a convenient position using a double-sided sticky pad. The display shows nine amps is flowing into the battery. A negative sign in front would denote that the current is outgoing.

This is the wiring diagram to clarify the wiring of the shunt and digital display.

This feature was originally published in the January 2018 issue of MMM magazine. Want to read more like it? Subscribe to MMM magazine today!