Voltage and Amp Meter
The house batteries in my Roadtrek are a vital component, so it's important that I know how they are performing and what's happening to them at any point in time. A simple light bar with four lights is insufficient. As a bare minimum, I'd like to know my actual battery voltage.
I came across an inexpensive multimeter on Amazon that looked like it might work for me (DC 6.5-100V 0-100A LCD Display Digital Current Voltage Power Energy Meter Multimeter Ammeter Voltmeter with 100A Current Shunt, about $19) and decided to try it. In addition to showing the voltage in real-time, this meter comes with an external shunt that enables you to measure the current entering or leaving your batteries. If it knows the voltage and current, the meter can also show the power in watts and energy used in watt-hours.
This meter can be hooked up to either show the amount of current flowing out of your batteries, or the amount of current flowing into your batteries. It can't do both. You could hook up a toggle switch to alternate between a current-in view or current-out view. I wanted to be able to see both without having to use a switch so I purchased two meters. Since the meters also keep a running tally of watt-hours, by having two meters I can see how many watt-hours total went in and went out of the batteries (this can be reset to zero at any time).
So how do these amp meters work? Basically, they use Ohm's law: Voltage = Current x Resistance (E = I x R). At your battery bank's negative terminal, you place a shunt so that any electricity coming or going into your battery bank must pass through this shunt. This shunt is calibrated to have a very small resistance, on the order of 0.00075 ohms. This tiny resistance will have no appreciable effect on the functioning of the battery, however it will create a tiny voltage drop across the shunt. If you measure that voltage drop, you can calculate how much current is flowing: Current = Voltage / Resistance or in our case Current = Voltage Drop / .00075. So if you measure a voltage drop of 7.5 millivolts, for example, that would indicate that 10 amps of current are flowing through the shunt.
Here's a picture of what the shunt looks like. I attached one side to the negative terminal of my battery bank and the other side goes to the negative battery cable (see my 12V diagram). Remember, any current flowing into or out of your batteries must pass through this shunt. You then need to run a small gauge wire from each of the small screw heads to the shunt connection terminals on your meter, enabling the meter to measure the voltage difference. In addition, you need to run 12v power to the meter by connecting additional wires to the positive and negative battery terminals. A standard 4-wire telephone cable works great for this. You can use the same shunt for both meters. Just run leads from the first meter to the second and reverse the leads going to the second meter shunt terminals.
I mounted my battery meters on the wall above the passenger-side second row seat, near the existing control panel. I used a double light switch cover plate as a backing for the meters. I just cut out the proper size openings for each meter using my scroll saw and they push right in.
I wired up one meter to measure all the current flowing out of the batteries (the top meter) and the second meter to measure all the current flowing into the batteries (the bottom meter). In this picture you can see that the battery voltage is about 12.79 volts and I'm using 30 milliamps of current out of my batteries - practically nothing. The meter shows that I've used a total of 2,835 watt-hours of energy since I last zeroed the top meter by holding down the small button on the right side of the meter.
The meters have a blue electroluminescent backlight that can be turned off. I like to leave mine on all the time. It uses very little power and makes a great night light.
Here's a picture of the meters a few minutes after I turned my battery charger on. You now see the voltage has gone up to 14.28 volts. Instead of current leaving the batteries, there is now 0.58 amps of current flowing into the batteries - equivalent to 8.2 watts of power ( 14.28V x 0.58A = 8.28 Watts ). You can also see that the watt-hour meter on the input meter has gone up by 8 watt-hrs since the above picture, showing that the battery is charging.
Now it is very easy to see what is happening with my batteries and charging system. I check these meters often and I would feel lost without them. This is definitely one of my better upgrades and I highly recommend it.
This is what the back side of the battery monitors look like. I split the four wire cable (telephone extension cabling) coming from the batteries and current shunt into two sub-cables that I ran to each of the meters. I just reversed the wires from the shunt to the second meter. This allows one meter to read current flowing into the batteries and the other meter to read current flowing out of the batteries.