RV Battery Monitoring

Boondocking or “dry camping” in an RV without a battery monitor is like driving a car without a gas gauge.

If you fill your tank up everyday and never drive further than to the corner store, this strategy can work just fine. But if you are going to do any sort of serious camping without electrical hookups or a daily dose of generator, it is essential that you keep a close eye on your batteries. This is particularly important when relying on solar, because you will rarely know for sure what state of charge the sun has left you with.

Batteries will last longest if you never cycle them below 50% capacity, and going over 80% drained starts to seriously damage even the best “deep cycle” batteries. If you use the “lights are getting dimmer” method to tell that your batteries are running down, they will almost certainly be permanently dead within a year. Not good – particularly if you’ve invested in expensive AGM batteries.

Voltage Metering:
The simplest way to keep an eye on your batteries is via a voltage meter. A fully charged 12V battery will read 12.73 V, at 50% capacity 12.10 V, and when you have just 20% left (critical territory!) 11.66 V. The Oliver trailer includes a system status panel that will show you your battery voltage, and by keeping watch you can get a sense for how you are doing.

But there is a BIG catch. To get an accurate and meaningful voltage reading, your batteries must have been sitting idle for at least six hours, and preferably twenty four. That means no lights on, no charger connected, no solar running – essentially no use whatsoever.

If the battery is not well rested, particularly if it is currently being used (even if just to power a light), the voltage reading is going to be off – and therefore nearly useless.

Imagine if your car’s gas gauge was only accurate after you had been pulled off the highway for six hours. That is not particularly conducive to getting anywhere….

Consider the voltage meter to be essentially just a “guess gauge”, and not a gas gauge.

Specific Gravity Testing:
You can also very accurately test the state of charge of well rested flooded lead acid batteries by using a hydrometer to measure the specific gravity of each individual battery cell. But doing this involves sucking up battery acid into a glass tube. Not fun, or typically practical.

In a car, this would be akin to pulling off the road, letting your engine cool, and then sticking a hose into your gas tank to see how much fuel remains. You really don’t want to be doing this. Trust me.

Battery Monitors:
A real battery monitor works by measuring the current flowing into and out of your battery via a very accurate shunt. Once the monitor detects that your battery is full, it keeps track of every amp of outflow, and it gives you a percentage remaining readout or an easy to read empty to full bar graph.

This at last gives you a usable “gas gauge” view of the sate of your batteries.

Battery monitors are not cheap, but they are worth it. If you are actually going to be using your batteries for more than a day or two of disconnected camping, I consider a proper battery monitor to be essential equipment.

There are only a few options on the market however:

• TriMetric TM-2020: The TriMetric has been on the market forever, and it is a trusted and well proven design. It is also the most affordable battery monitor around, available for about $165. The TriMetric has a simple (though unattractive) LED display that shows you the battery % full readout, and which can also display battery voltage and current. The maker of the TriMetric also has available the much more expensive PentaMetric unit which does data logging and which can be interfaced with a PC. The PentaMetric is overkill for all but the most extreme RV installations.
• BlueSky IPN-Pro Remote: The IPN-Pro remote requires and works in conjunction with a BlueSky solar charge controller (like the 2512iX). The IPN-Pro provides a very attractive display that combines both solar charging status with real battery monitoring. It even learns your batteries internal charge efficiency, becoming more accurate over time. If you have a BlueSky solar controller – this $180 option is an ideal match.
• Magnum Energy Battery Monitor: If you have one of the higher end Magnum Energy inverters (expensive), and the Magnum Energy inverter remote control interface (also expensive), you can add real battery monitoring to your system via this $160 upgrade. I was disappointed however to discover that this battery monitor will not function when combined with the more affordable Magnum MM series inverters.
• OutBack Power Systems: If you are looking to spend a LOT of money and you have a much larger RV than the Oliver, OutBack Power Systems produces a line of high-end inverter / chargers and solar controllers that can be combined with their FLEXnet DC battery monitor and MATE remote display to produce a very snazzy looking integrated system.
• Xantrex Link 10: The Link 10 is a basic battery monitor with a simple LED display and red, yellow, and green status lights to indicate how close to critical you are. The Link 10 does not display a percentage battery remaining view, but it will tell you how much run time you have left at your current load. Aprox $200. The similar Link 1000 incorporates remote controls for certain Xantrex inverter / chargers. The Link 20 and Link 2000 allow monitoring of multiple battery banks.
• Xantrex Battery Monitor: This $230 monitor is very similar in functionality to the Link 10, but it does offer a nicer display, a proper battery percentage readout, and an option for data logging to a PC.
• Xantrex LinkLITE: The Xantrex Battery Monitor is currently being discontinued and replaced with the new LinkLITE and LinkPRO models, which are due to ship in August. The LinkLITE will cost $270, can monitor two battery banks, trigger a generator to start when the battery is getting low, and it looks to have a very clear and intuitive display readout.

Current Awareness:
One of the nice side effects of using a real current measuring battery monitor is that it shows you exactly how much power you are using at any given time. When you flip on a light switch, power up an inverter, or turn on the radio, you will see exactly how much additional current that new load is drawing. Without this sort of awareness, you may never even realize which things are the major drains on your battery.

Once you have a battery monitor installed, it is a good idea to work your way through your RV – turning each light and gadget on and then off, one at a time. Make a note of the current draw of every item. This information will be invaluable later, particularly if you are trying to be miserly and maximize your run time on batteries.

Concluding Thoughts:
If you are using cheap Walmart special batteries, it is actually not unreasonable to abuse them till they drop, and to just plan on replacing them every year. If this is the case, a real battery monitor is probably overkill for you. Pay attention to the battery voltage, never ever run things till the lights get dim, and you’ll be fine for a while.

But if you are going to be doing a lot of off grid camping, or you are planning to spend a lot of money on expensive batteries, you are a fool if you do not also invest in an accurate way to track their condition. You really should get a battery monitor of some sort.

If you have a BlueSky solar controller, the best choice is obviously the IPN-Pro. If you are building a Magnum or Outback complete system, you also have an obvious choice – though certainly both of these are overkill for something as small as an Oliver.

If you are getting a standalone battery monitor, I’d base my choice primarily on aesthetics – since you do NOT want your battery monitor mounted in an out of the way place.

You do want your gas gauge to be readily seen, after all.