Case for a Battery Monitor

Case for a Battery Monitor

Introduction to the Need for Battery Monitoring:

Would you ever drive a car that didn't have a fuel gauge? Most likely not, and if you did, you would probably feel a bit nervous about it. Why, then, would you go camping without knowing how much power remains in your batteries? While RV camping with full hookups has its advantages, there's something alluring about heading off the grid "the boonies". 

Is it the spirit of adventure and self-reliance that motivates us to go boondocking? Perhaps it's also the financial aspect. When you invested in an RV, you might have envisioned saving money, only to discover that some RV campgrounds with full hookups can be quite expensive. I've personally encountered rates reaching three digits per night, which can be quite steep. While you could find a cheaper hotel room, it might not offer the same experience. Now, let's delve into the one challenges of boondocking, which revolves around electricity.


Battery Monitoring Challenges:

At the core of your RV's electrical system lies a battery. It could be a flooded lead-acid model, a newer AGM variant, or even a high-tech lithium battery, but they all share a common trait: they function like black boxes, providing electricity until they don't. Unlike a water tank, there's no visual inspection to determine a battery's remaining energy, or the SOC (State-Of-Charge).

Complicating matters is the fact that the energy a battery can deliver depends on factors like the load, temperature, battery age, and even previous charge cycles. Unfortunately, many RV manufacturers have not devoted much effort to solving this issue. Most RVs come equipped with a simple push button that illuminates a few LEDs, offering a rough estimate of the battery's charge level. You've probably noticed that these indicators typically display values like 0%, 25%, 50%, 75%, and 100%, providing limited resolution. This not only represents a crude way to assess your battery but also proves almost entirely useless.

Here's a practical test you can conduct: discharge the battery by running various appliances, and then connect your RV to AC power. Immediately press the battery indicator button, and you'll likely notice that the displayed charge level jumps to 75% or even 100%. However, it's important to realize that the battery didn't actually charge this rapidly. This phenomenon occurs because when you pressed the battery indicator button, it activated some basic electronics that measure the voltage of the battery and compare it to predefined thresholds. This voltage-based system is what triggers the indicator LEDs to display a certain charge level. In reality, this method doesn't accurately reflect the true state of charge of the battery. The battery would need to stay at rest for a few hours before the voltage starts reflecting the real state of charge. For lithium batteries, the voltage method is completely useless.


How Battery Fuel Gauges Work:

Now, consider how a cellphone estimates its remaining battery life. Inside a cellphone, tablet, or laptop, there's a circuit known as a battery fuel gauge. In technical terms, a battery fuel gauge operates by integrating the current over time. To put it simply, if you have a battery with a capacity of 1 Ah (pronounced Ampere-hour or Amp-hour), it means it can provide a continuous current of 1 Ampere for one hour before becoming depleted, or 0.5A for 2 hours, or 0.1A for 10 hours. 

This principle, revolves around tracking the current entering or leaving a battery over a period of time. The concept is that the current remains relatively stable over short time intervals. By taking samples at specific, brief intervals and summing up these measurements, we can determine the total energy that has flowed in or out of the battery. This process is referred to as integration over time. This is how devices such as laptops, smartphones, tablets, and Thornwave Labs' PowerMon Battery Monitor work.


PowerMon Features:

PowerMon samples the current at almost 1000 times per second and aggregates this data to accurately estimate the charge entering or leaving the battery. With knowledge of the initial battery capacity, it can determine the battery's state of charge. Additionally, by considering the remaining charge and current power consumption, it can estimate how long the battery will provide power under the current load conditions.

Beyond battery monitoring, PowerMon offers real-time information such as voltage, current, power, temperature, energy consumption (Wh), charge (Ah), and various statistics. These features help you understand how much energy various appliances consume, akin to a trip odometer in a car.


Ghost Loads and Battery Management:

Ghost loads, often unnoticed, represent power consumed by devices that remain on even when not in use. TVs, phone chargers, laptop chargers, smoke detectors, and alarm clocks are typical examples. While individually they draw minimal power, their cumulative effect can be significant, especially in an RV where every bit of power matters.

In a house, the impact of ghost loads is negligible compared to the daily total energy consumption. However, in an RV, where the ghost load typically ranges from 0.1A to 0.3A, it can drain the battery significantly over time. To mitigate this, you should check your RV every two weeks or so and recharge the battery unless you have solar panels.

Some RV owners address this issue by installing a battery disconnect switch to fully disconnect the battery from the RV. PowerMon offers a similar solution; it can control an external relay, effectively providing you with a remote ON/OFF switch you can operate using your cellphone.


Low-Voltage Disconnect and Other Features:

A crucial feature is the low-voltage disconnect, which prevents battery damage by disconnecting loads when the voltage drops too low. Over-discharging lead-acid batteries, even deep cycle ones, can significantly reduce their lifespan.

PowerMon also offers high-voltage disconnect, over-current disconnect (acting like a circuit breaker), disconnect based on the fuel gauge (disconnects when the state of charge reaches a specific percentage), and timer-based control.


Solid-State Relays (SSRs):

PowerMon can drive a relay to implement the disconnect features. However, it's important to note that traditional relays, when kept engaged, consume additional power due to the relay coil's current.

A more efficient alternative is a solid-state relay (SSR), which operates electronically and offers advantages such as no wear and tear on contacts and low current requirements to stay engaged. While most SSRs on the market are designed for AC, Thornwave Labs has created the TL-SSR12100DCU, a 12V 100A DC unidirectional SSR designed to work seamlessly with PowerMon.


Data Logging and Storage:

A standout feature of PowerMon is its data logging capability, introduced in 2019. It records all the data it measures, allowing you to monitor power usage patterns, appliance runtimes, and overall energy consumption.

Importantly, data is logged in internal FLASH memory, regardless of smartphone connectivity. When your phone reconnects to the device, it downloads the data, presenting it in chart form.

Users can select the sample rate, ranging from once per second to once per minute, which affects how much data can be stored. Even at the highest resolution, you can review all electrical data for the past 18 days.


Wireless Convenience:

Notably, PowerMon is wireless (uses Bluetooth Low Energy), eliminating the need for drilling and installing new wires. Installation involves mounting the device near the battery, possibly inside the battery case, and connecting it in line with existing wires.



In conclusion, adding this high-tech device to your RV offers numerous benefits. Knowing your battery's charge level and being able to monitor it from your cellphone enhances predictability during camping trips and ensures the longevity of your RV battery.

Reading next

Leave a comment

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.