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Case for a Battery Monitor

by Razvan Turiac April 11, 2019 10 min read

Case for a Battery Monitor

Would you ever drive a car that didn’t have a fuel gauge? Most likely not, and if you did anyway you would probably be a bit nervous about it. Why would you ever go camping without knowing how much power you have left in your batteries? Sure, RV camping with full hookups has its advantages. You can treat the electricity and water at such a campground as infinite resources for any camping purposes but a lot of times the most fun places to go are off the grid. There is something attractive about the boonies. Could it be the spirit of adventure and self-reliance that drives us boondocking? Maybe because it makes more financial sense? You bought yourself an RV thinking about how much money you are going to save just to find out that some RV campgrounds that have full hookups can get pricey. I personally have seen rates in the three digits, per night. That is pretty steep. You could find a hotel room for cheaper than that but granted, that wouldn’t be a nice one. Back to the RV though! Boondocking is attractive but it comes with three problems: electricity, water, and sewer.

    At the center of your RV’s electrical system is a battery. It can be a flooded lead-acid one, a newer AGM model, or maybe even a high-tech lithium battery but they all have something in common. They are like black boxes. They provide electricity until they don’t anymore. Unlike a water tank, there is no way to visually inspect the amount of power a battery can still deliver. Couple this with the fact that the energy one can extract from a battery depends on the load, temperature, age of the battery and even the previous charge cycle makes it a pretty tough problem. Most RV manufacturers didn’t put much effort into solving this issue either. Most RVs offer a simple push button that will light a few LEDs giving an approximation of how much charge is left in the battery. You probably noticed that the possible values are 0%, 25%, 50%, 75%, and 100% so not much resolution at all. Not only this is a crude way to evaluate your battery but is also almost completely useless.

    Try this: discharge the battery with some loads and then plug your RV into AC power. Immediately press the battery indicator button and you will see that the indicated charge level will be 75% or even 100%, if not immediately, within a few minutes for sure. Did the battery charge so fast? Of course not. You see, when you pushed the little button, some very low-tech electronics measured the voltage of the battery and compared it with some pre-established thresholds. This is what lights up those indicator LEDs. The whole thing works by measuring the voltage. This simply does not work. A lead-acid battery state of charge is notoriously difficult to estimate. The voltage method fails most of the time because the voltage of the lead-acid chemistry depends on many factors: partly the state of charge, temperature, acid concentration, battery load, acid stratification, battery internal resistance, battery age, and so on. The voltage only depends partly on the state of charge when it is measured at rest. This means no charging or discharging for at least 6 hours or so. The water pump running for two minutes will already lower the voltage significantly, to the point where it is not a reliable indicator of the state of charge anymore. The battery would need to stay at rest for another few hours before the voltage starts reflecting the real state of charge. For lithium batteries, the voltage method is completely useless. The reason for this is that the voltage curve of lithium batteries during a charge/discharge cycle is very flat.

    Ok then, how does a cellphone estimate the remaining battery? The circuit inside a cellphone, tablet, or laptop that does this is called a battery fuel gauge. I’m going to get a bit technical now so if you are not interested in this you can skip to the next paragraph. A battery fuel gauge works by integrating the current over time. Let me translate that. If we have a battery with a capacity of 1 Ah (pronounced Ampere-hour or Amp-hour) it means that it can provide a current of 1 Ampere continuously for one hour before becoming discharged. Similarly, a car battery that has 80 Ah can provide a current of 80A for 1 hour, 40A for 2 hours, or 1A for 80 hours. Knowing this, we can look at the current that flows in or out of a battery and add it up to get the total charge that flowed in or out. This is equivalent to counting how much water we put inside a tank to estimate how full the tank is. Let’s suppose we cannot see inside the water tank. If the entire capacity is 100 gallons and we know that we poured 10 gallons into the tank, we can estimate that the tank is 10% full. Now if we use water at a rate of 5 gallons per hour we can also estimate that it will take 2 hours for our tank to become empty. The same idea applies to batteries. Let’s take the cellphone again. The power a cellphone consumes is not constant. Sometimes the current may be 0.8A during a call, sometimes may be 0.3A when browsing the Internet, and sometimes can go as low as milli-amps (a milli-Amp is a thousand of an Amp), for example when it is idling with the screen off. The fuel gauge circuit is sampling the current at very short intervals of time, probably in the range of a few thousand times per second, and adds up all these samples. This is called integrating over time. The idea is that the current does not change much in that short period, so we can assume that it is constant. This is how a laptop, smartphone, or tablet works and this is how Thornwave Labs’s PowerMon, the Bluetooth Battery Monitor / DC Power Meter works too.

    PowerMon samples the current a few thousand times per second and adds up all those chunks to come up with an accurate estimate of the charge that went in or out of the battery. Knowing the initial battery capacity, it estimates what the state of charge is. Also, knowing the remaining charge in the battery and the current power consumption will also estimate for how long the battery will be able to provide power. Until now I covered the battery monitor feature but there’s more this Bluetooth device can do for your RV. PowerMon will show you a lot of information in real-time: voltage, current, power, temperature, the power meter (energy in Wh), charge meter (in Ah), and a lot of statistics. The charge meter and power meter are integrating the current and the power respectively. These will give you an idea of how much energy was flowing through the device. You can reset these counters and use them to check the power different appliances consume. They are like the trip odometer on your car. 

    You probably heard of ghost loads. These are loads that are on even when you think they are not. TVs for example, even when turned off will consume very little power to keep some internal electronics running, ready to react to the remote control when you wish to turn them on. Wall cubes, phone chargers (that you typically leave plugged in), laptop chargers, smoke detectors, and alarm clocks are all examples of ghost loads. Individually they don’t draw very much power at all but they add up. In your house, they are not an issue since 5 Watts of ghost load does not mean much in the grand scheme of things. Your electric bill shows how many kWh (kilo-Watts-hour) you used in one month and it’s typically in the hundreds or even thousands of kWh. In an RV, this is not the case. There is about 0.1A to 0.3A of ghost load in a typical RV, mostly from the smoke detector, gas detector, thermostat, and entertainment center. This is 0.2A every hour so in 24 hours this means 4.8Ah (just multiply). This is a lot! This means that in only 10 days it would drain 48Ah from your battery. In 20 days your battery may be completely drained and you wouldn’t even know it. Unless you have some solar panels on the roof you should check your RV every 2 weeks or so and recharge the battery even when it is in storage.

    Some fixed this issue by adding a battery disconnect switch that can completely disconnect the battery from the rest of the RV. PowerMon can do this too! Just add an external relay and the Bluetooth battery monitor can control it. You just got yourself a remote ON/OFF switch that you can control using your cellphone. Just don’t forget to turn it off when you are not using the RV.

    The external relay I mentioned can help in other ways too. Have you heard of a low voltage disconnect? This is basically disconnecting the battery when a low voltage event is detected, which most likely means that the battery is discharged. Over-discharging a battery, keeping the loads on until the voltage drops very low has a very negative effect. You probably heard that lead-acid should not be discharged below 50%, even the deep cycle ones. This is in order to prolong its life. Completely discharging a lead-acid battery will significantly damage it and doing this a few dozen times will ruin it completely. An RV furnace during a cold night, while you are sleeping, tends to do just that: run the battery down until the voltage drops to a few volts. I know this because it happened to me once. I had to replace the battery because it would not hold much of a charge anymore. It was used on only three trips before that.

    This is where the low-voltage disconnect feature comes in handy. When the voltage drops below a threshold, PowerMon will automatically disconnect all the loads, hence avoiding an expensive battery incident. Once the battery is re-charged, it will automatically re-connect all the loads.

    Other features are high-voltage disconnect (just what the name suggests), over-current disconnect (works like a circuit breaker), disconnect on fuel gauge (can disconnect when the state of charge reaches a specific percentage), and timers (can control the relay based on timers). Other than the battery fuel gauge and the disconnect features, the PowerMon device measures a multitude of parameters. 

     Now that I mentioned relays let me get a bit more in detail. This will be useful, so stay with me. The PowerMon can drive a relay. This is how the disconnect features work. Any 12V / 24V / 48V relay that has a coil current of less than 2A will work. When the power is on, the relay will be kept engaged and this is where we have a little bit of a problem. Because the relay stays engaged at all times while camping, there will be an extra current drain on the battery: the relay coil. It’s not much but it easily adds up, just like the ghost loads I talked about earlier. For example, a 0.25A relay coil (pretty common on the automotive-style relays) means that in 24 hours it would drain 0.25A x 24hr = 6Ah from your battery. When your entire battery is only 80Ah, that’s a lot. Luckily, there is a solution for that. A solid-state relay is an electronic device that does the same thing as a relay but achieves that by electronic means instead of electro-mechanical. It has a few advantages over its mechanical counterparts: no contacts to wear out, an almost infinite number of on/off cycles and the best of all, the current required to hold it engaged is in the milli-amps range instead of hundreds of milli-amps or even amps. Most SSRs you find on the market are only working in AC, not DC. There are a few that you can find (mostly Chinese) but the quality is not there. I personally tested a few and they drop a very high voltage (80mV at 15A), they need heat sinks and I easily burnt one at only 30A, when it was rated at 100A. For this reason, at Thornwave Labs, I decided to create one and so we did. The TL-SSR12100DCU is a 12V 100A DC unidirectional SSR that works directly with the PowerMon device. Unlike other SSRs on the market, ours really works at 100A continuous current, requires no heatsink, and drops an almost unbelievable, 30mV at full load (100A). At a typical load seen in an RV, 20-30A, it drops less than 10mV. If you have an inverter that needs more than 100A you can easily parallel a few of these and get a solid electronic switch that will replace a pretty big relay and save battery power.

   I left the most interesting part at the end: data logging. As of 2019, PowerMon can log all the data is measuring. This allows you to see how much power you are using and when, how quickly, how often is your fridge running and for how long, the furnace, and so on. Unlike some competitors, PowerMon is logging that data in internal FLASH memory so it doesn't matter if your phone is connected to the battery monitor or not. It always logs data. The phone will download that data every time it connects to the device and you can visualize it as a chart. You can choose the sample rate from once every second to once every minute. This will also dictate how much data can be stored. If PowerMon is logging once every second you can expect about 18 days worth of data to be stored. Going to once a minute will extend this period to over 3 years. This means that even at the highest resolution, once a second you will be able to see everything that happened with your RV in the past 18 days (from an electrical standpoint)

    What do you think about adding a piece of high-tech to your RV? Knowing how much power is left in your batteries and being able to see that on your cellphone will make those camping trips more predictable and your RV battery happier.

    Oh, and I almost forgot the coolest feature of them all. It’s wireless! You don’t have to drill through your RV and install new wires. Simply mount the device close to the battery (can be inside the battery case), connect it in line with the existing wires, and forget about it. It is that simple!

Razvan Turiac
Razvan Turiac


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