In this article David answers a reader’s question about getting an accurate reading on their caravan batteries.
Hi 12V Guru,
Hope you can help with this one — we’re trying to understand why it’s impossible to really understand what’s happening with the batteries in our van. We appear to be constantly getting different voltages randomly showing up on our monitor and it gives us no confidence in the battery or the system. Is there a better way to figure this out?
Frustrated,
Margie and George Elsworth
Travelling WA
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 and 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 battery 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).
For DIY instructions on adding a battery monitor and shunt to your caravan, check out this feature by John Hughes.
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 such as the health of battery or remaining capacity. One thing that is constant though is that monitoring the current flow 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 status and health and state of charge (SOC). This accuracy is especially important in off-grid applications such as 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 how much energy usage is passing through your battery and a battery monitor will not provide an accurate understanding of the actual 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 RVs have a complete power management system (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 energy consumption and combine this data with other system data to accurately manage the battery in totality.
Trust this clarifies your next steps to understanding your battery.
Safe travels,
12V Guru
THE NEXT STEP
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