Greetings travellers,

You have uncovered a now, well known problem in relying upon voltage readings for batteries – it is only an indication of what’s really going on.  Especially as a battery degrades over time the voltage will fluctuate more widely as the internal capacity of the battery diminishes.

To understand what is happening within the battery you will need to add a shunt to your battery with an appropriate monitor. An electrical shunt is a critical component in electrical circuits, primarily used for measuring the current flowing through a system as well as the voltage. It works by diverting a portion of the electrical current through a low-resistance path, enabling the accurate measurement of current via the voltage drop across the shunt (a low impedance resistor)

The shunt is typically installed in series with the battery’s negative terminal in systems.  Unfortunately, not every shunt is the same and different manufacturers can use different calculations in determining things like health of battery or remaining capacity – Their IP!  One thing that is constant though is that monitoring the flow of current is essential to understanding what is happening.

The shunt provides critical information of all energy that flows in and out of the battery.  This includes Amps, Volts, Time and Temperature – all critical in the calculations to determine better how to manage your energy requirements. By better understanding this information you can calculate power consumption, battery health and state of charge (SOC). This accuracy is especially important in off-grid applications like RVs and marine vessels, where knowing the exact state of battery charge can prevent over-discharge or undercharge, both of which can reduce battery life. Worse still you could be stranded somewhere without power or any way of knowing what is really happening with your power management system.

Shunts are important for two main reasons – firstly they provide accuracy in current measurement.  Shunt-based monitoring systems offer more precise data compared to simple voltage-based systems. Since voltage alone can fluctuate due to factors like temperature, using a shunt ensures that real-time current and energy usage are measured, which is critical for maintaining battery efficiency, especially in lithium batteries. Voltage based monitors provide a much lower level of accuracy. When you can better understand what is happening you can manage it accordingly.

Secondly, in any battery type, a shunt is essential for monitoring both charge and discharge. Without a shunt you will be blind to what energy usage is passing through your battery and a monitor will not provide an truly accurate understanding of the capacity of the battery, meaning that you could head off to bed with what you think is a full battery to awake to no power at all because there was something connected directly to battery and its energy usage was not being monitored.  This is critically important for systems where a CPAP or other medical device is connected directly to the battery to ensure a shunt is placed in the circuit to provide a highly accurate understanding of the actual battery capacity.

This accuracy is crucial for preserving battery longevity. Many RV’s have a complete Power Management Systems (PMS) which performs the output distribution function for the RV (i.e. they send the power out to all parts of the RV) and therefore if a load such as an inverter or a device is directly connected to the battery terminals the PMS won’t see this energy usage.  Therefore, a shunt integrated into the system can measure this and combine this data with other system data to accurate manage the battery in totality.

Trust this clarifies your next steps to understanding your battery.

Safe travels

12V Guru