Battery Monitoring

Wattmon has built-in features for battery monitoring, and with a few configuration settings you can keep track of a battery bank with voltages ranging from 12V DC to over 300V DC.

How does it work?

There are various ways of determining the remaining capacity in a battery. One of these ways is called Coulomb Counting. Another uses voltage to determine the remaining energy. A third method will actually check the specific gravity of the battery liquid to determine the state of charge.

Wattmon monitors inbound and outbound current from the battery bank and uses the Coulomb Counting method. Current flowing through the sensors is read once a second and accumulated in registers.  By configuring the total capacity of the battery, an accurate view of remaining energy is then calculated by subtracting and adding from this value. 

For example: 

A 100 Ah (Ampere hour) battery will be (theoretically) able to supply 10 Amperes for 10 hours (10 A/h * 10h = 100 Ah).  If you take out 10 A for 1 hour, Wattmon will show your battery State of Charge (SoC) as 90%.  If you then charge the battery at 10 Amps for 30 minutes, the battery will show 95% full (90 Ah + 5 Ah).

The Algorithm

Wattmon lets you set the various parameters in the Control Panel. Wattmon will reset the battery status to full when the battery voltage is higher than the 'full voltage' threshold (adjustable) and the current drops below a specific value (adjustable) for a set period of minutes (adjustable). This flexible set of options ensures that most requirements can be easily met.

Different battery chemistries have different efficiencies. You can enter the charge efficiency if you desire to get even more accurate results. If you are using a lead acid chemistry, you can also enter the Peukert constant of your battery bank to get an accurate discharge profile for higher currents.
This method has in our experimental setups proven to be quite accurate. It is definitely more than sufficient to get a good overview of remaining energy.
The device will, based on the remaining energy and the average load over the last few minutes calculate the run time through a web interface from your PC or mobile.


There are many devices which claim to have an accuracy of below 1%. We claim no such thing! It is accurate when it comes to estimating the remaining capacity, however there are cases when it can be off by up to 5 or 10%. Lead acid chemistry is in general difficult to measure accurately. There are temperature differences and many other factors that will affect the state of charge of a battery.

Required Hardware

In order to monitor a battery bank, you will require a Wattmon and a current sensor module.  There are multiple DC energy meters with Modbus RTU output that could be interfaced to get the values required for SoC calculations.