Solar Battery Monitors Demystified: Battery Monitor For RV And Off-Grid Solar Power Systems

What is a battery monitor?

The battery monitor monitors the battery state of charge and gives information about current battery capacity in ampere-hours and the battery’s ampere-hours consumed.

It can measure the power stored in the battery or drawn from the battery, or both.

The monitor also gives information about the battery voltage.

What types of battery monitors do exist?

Depending on the detecting mechanism, battery monitors could be based on shunt or Hall Effect.

The battery monitors using shunt could be unidirectional or bidirectional.

Unidirectional ones measure battery characteristics in one direction only: either the current and power stored in the battery or ones drawn from the battery.  

When deployed, you can choose the corresponding direction that suits you the most.

These are the cheapest battery monitors.

Bidirectional battery monitors measure the battery characteristics in both directions: the energy coming into the battery or discharged from it.

They are more expensive ones.

The main drawback of the shunt based battery monitors is that they require the shunt to be deployed in the solar power system, which leads to undesired rewiring of the segment of the solar power system around the battery.

The second disadvantage is that the shunt electrical characteristics limit the maximum current and power measured.

If you want to monitor the battery of higher capacity, you might need a more expensive shunt capable of sustaining higher currents. 

On the other hand, battery monitors relying on the Hall effect do not require rewiring because they employ the through-hole design and need not a shunt.

What is a Hall sensor?

The Hall sensor is a circle bead through which the solar wire passes. The Hall effect battery monitors measure the battery characteristics in both directions, and they could monitor high capacity battery or battery bank since they don’t use a shunt.

The big advantage of this type of monitor is that the hall sensor is electrically isolated from the measured circuit.

 They are also capable of sustaining high overloads.

How do battery monitors work?

Let’s get started with the working principle of shunt based battery monitors. The primary sensor of these battery monitors is the shunt.

What is a shunt?

The shunt is a standardized resistive element with a very small calibrated resistance capable of sustaining high currents.

What does a battery shunt do?

The shunt is deployed in a series of the circuit to be measured as shown on the picture below.

A voltmeter, shunt and Load

Depicted in the picture above is the direction of the battery discharge current.  

The discharge current flowing through the circuit causes a minimal voltage drop, in the vicinity of millivolts,  over the shunt, thanks to the shunt’s very low resistance.

A precise voltmeter measures the voltage drop over the shunt.

On the one hand, the voltage drop over the shunt is calculated to be so small so as not to influence the circuit behavior. 

On the other hand, it is high enough to be detected and precisely measured by a micro voltmeter.

By using Ohm law and knowing the shunt calibrated resistance and voltage drop over it, we can calculate at any given moment the magnitude of the current flowing through the circuit:

I= dV/R

Say, we have a 12V battery bank and the voltage drop measured over the shunt, caused by the battery discharging current, is 50mV and the shunt resistance is 250 micro Ohm then the flowing current is:

Where dV is the voltage drop over the shunt, and R is its standardized resistance.

I=(50E-03)/(250 E-06) =200 Ampere.

If we know for how many hours the current is flowing, we can calculate the discharged battery capacity.

Say the current is measured for 3 hours so the discharged capacity is 3hx200A=600Ah.

Then the energy drawn from the 12V battery would be 12Vx600Ah=7200Wh=7.2KWh.

In practice, the battery monitor automatically makes the calculations mentioned above and displays the corresponding values.

How do you hook up a battery monitor?

The connection of the battery monitor starts with the shunt.

The shunt must be placed between the battery’s most negative potential, which is the battery negative terminal, and the DC negative bus terminal, sometimes incorrectly called “ground.”

The load, that current consumption is measured, must be connected to the other side of the shunt that is to the negative DC bus terminal /Ground Bus Frame/ as shown in the picture below.

RV battery monitors with a shunt

 The cables connecting the battery most negative terminal to the shunt and ground bus terminal to the shunt must be as short as possible.

Important! If you connect another load directly to the most negative battery terminal before the shunt, its power consumption would not be measured.

This is an incorrect way of using the shunt for measuring the load consumption.

So be careful!

For example, the picture below shows such an incorrectly placed Load N, which consumption would not be calculated.

RV's battery monitor with an incorrectly placed load

Incorrectly placed loads like Load N is a widespread mistake observed in RV and marine solar power systems that leads to inaccurate measurement of the total load consumption.

How do you connect a battery monitor based on Hall effect?

It is known that every DC current creates around the wire, through which is flowing, a static magnetic field with a direction corresponding to the current one.

The magnitude of the magnetic field density is proportional to the magnitude of the current.

What is more, If we change the current direction, the direction of the magnetic field changes.

 So If we can find a way to measure the magnitude of the magnetic field, we can exactly define the current magnitude in both directions.

Here the Hall sensor comes into play.

Hall effect sensor is a device which in the presence of a magnetic field  generates a voltage proportional to the magnitude of the magnetic density of this field.

The magnetic field could be created either by permanent magnets or the current-carrying conductor.

Therefore,  If we place a hall effect sensor around the DC current-carrying wire, we could measure the magnitude of the DC current flowing through it because the voltage generated by the Hall sensor is proportional to the magnitude of the magnetic field, which in turn is proportional to the magnitude of the current flowing through the wire.

Actually, the effect of creating а voltage in the presence of the magnetic field is called the Hall effect.

The Hall sensor is a semiconductor device through which the current is constantly flowing. 

 In the presence of an external magnetic field, the sensor generates a voltage proportional to the magnitude of that magnetic field.

How is the Hall Battery monitor connected?

The implementation of the Hall battery monitor is very straightforward.

Just place the Hall sensor around the wire.

And power up the monitor via the battery being measured or other external sources.

The picture below presents a typical connection of the Hall sensor RV battery monitor.

Hall effect battery monitor

The sensor is placed around the measured wire.

The solar battery powers the battery monitor.  

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Lacho Pop, MSE, holds a Master's Degree in Electronics and Automatics. He has more than 15 years of experience in the design and implementation of various sophisticated electronic, solar power, and telecommunication systems.  He authored and co-authored several practical solar books in the field of solar power and solar photovoltaics. All the books were well-received by the public. You can discover more about his bestselling solar books on Amazon on his profile page here: Lacho Pop, MSE Profile