Battery Monitor


Full Member
Bit of info and a bit of a plug ...
It is important to have some kind of way to keep an eye on the Batteries.
Many Motorhomes come with just a set of LEDs which are not that useful.
A Voltmeter can be a very useful bit of kit, especially if you either fit a pair - one for Starter and one for Leisure,, or have a switch to use one meter to show both batteries
The system that gives the best level of information is the SOC - State of Charge - Monitor. This gets round the problem of the voltmeter potentially giving misleading information if the electrical system is either under charge or being used. These SOC Monitors have the monitor itself, plus a 'shunt' that is fitted onto the negative terminal of the Leisure Battery and is a device that allows the current to be calculated. Basically if you know the Battery Bank size, the Battery Voltage and the Current Flow, the amount of charge left in the battery can be calculated and that information is extremely useful.

This is the CBE Version of the WBM-350 Battery Monitor kit, designed to match in with the other CBE Fittings that you may have or plan to get in your Campervan.

WBM-350 - SOC
by David, on Flickr

The Battery Monitor is the centre unit. Note that the devices to the left and right can be various types. In these photos I have a 4-way CBE Fuse Box and a 3-way switch, but they can be anything. The image above shows the Monitor showing the State of Charge (%) . This is the amount of charge left in the battery (bearing in mind 50% is typically the lowest you would want to go with Lead Acid Batteries)

This photo shows the AmpereHours (Ah).

WBM-350 - Amphours
by David, on Flickr
Batteries are measured in Ah rather than watts. Not always that useful a measure TBH, but that's how it is. WIth a Battery Monitor, you first set it up by telling it how many Ah you have when the batteries are full. Then with some Monitors, such as the Victron BMV, it shows a negative number telling you how much has been taken out. Others, such as this WBM-350, tell you how much battery power is left. Just different ways to tell you the same thing really.

This photo is the Battery Voltage (V)

WBM-350 - Voltage
by David, on Flickr
Basicsally this is just a voltmeter view showing the voltage of the Leisure Battery

And finally the Current in amps (A)

WBM-350 - Amps
by David, on Flickr
This shows the Current that is either being pulled from the battery or put into the battery. How you know if it is coming out or going in is the symbol on the left hand side. the "-" in the downpointing arrow means it is a draw and the battery power is reducing; the "+" in the upward pointing arrow (like in the photos) means the current is a charge current. Also as a quick check, when the battery is being charged, the display has like a heartbeat pulse with the backlight so you can just glance at the display and tell if you are in net draw or net charge.

Now the plug. I will be putting on my on-line shop a complete add-on Battery Monitor kit and as a introduction Motorhome Builder Members can get this a a special price.

The Kit comprises:
The WBM-350 Monitor, complete with 350A Shunt
25mm Cable to connect Battery -ve to Shunt - 200mm long
You move the existing Battery cable to the other side of the Shunt, so additional ring terminals are supplied if you need to change the existing battery terminal to suit the shunt Block (it uses M10 Bolts)
+12V sense cable to go ftom Battery +ve to Shunt connector
Triple Gang CBE Inner Frame for Monitor + Triple Gang CBE Outer Cover for Monitor
CBE 4-Way Fuse Box OR Dual USB Sockets in CBE Mount with Green Illumination
Rocker Switch (3-way or 2-Way with Green Light) OR 12V Accessory Socket
Selection of 1mm/16A Cable for wiring up Switches/Sockets
Selection of Crimp Spade Terminals

The Complete Kit as listed about will be £83 including postage. It will be listed in my store for £99.99 inc Postage. The NASA Marine BM2 (200A) Monitor is around £125 for just the monitor and you have to add the shunt cable and you don't get the CBE Extras; The Victron BMVs (500A) start from around £140, again with no Shunt cable and no CBE bits and pieces.

I had this test system setup while I was doing my B2B testing and it was very good. I had it running in parallel with the BMV -712 and the results were very comparable. In terms of using the WBM, it was actually more user-friendly then the BMV with an easier to see display and buttons to take you direct to what you want to see instead of having to scroll through numerous buttons.
If I didn't use the connectivity features of the BMV-712 for my advanced data logging, I would actually prefer to have the WBM despite it being a much cheaper product.