What’s in a watt?

airstream light switchesWhen you turn on a light in your house, the light goes on. That’s simple. But when you turn on a light in your Airstream while on battery power, it may seem like you’re suddenly expected to have an advanced degree in electrical engineering.

That’s because managing power is one of the biggest challenges new owners face when they start roaming away from reliable campground electricity. Volts, amps, watts, and strange jargon quickly become part of daily conversation.

As Airstreamer said to me the other day: “I don’t know what any of it means, and yet I’m expected to know just to operate my trailer.”

Discussing electrical systems in an RV can be endlessly complicated, but let’s just keep it simple for now.

AC and DC—Your Airstream has two electrical systems, one for 120 volt AC power (just like the power in your house) and one for 12 volt DC power (from the battery).

Think of volts as a measure of pressure, like water pressure in a pipe. The higher the volts, the more pressure. North America uses 120 volts for ordinary outlets, and Europe uses 240 volts.

The 120 volt system in your Airstream is functional when the trailer or motorhome is plugged in. It powers the air conditioner, microwave, television, standard electrical outlets, and the refrigerator (when running in electric mode). It also goes to the power converter, which turns some of that 120 volt AC power into 12 volt DC power. This is used to recharge the battery.

When the Airstream isn’t plugged in, those circuits are off—in most cases. Why “most cases”? Because some Airstreams have generators which can produce 120 volts, and some Airstreams have inverters.

An inverter takes 12 volt power from the batteries and turns it into 120 volt power (the exact opposite of what the converter does). Because this will drain the batteries pretty quickly, inverters are usually wired only to a few things like TV and microwave, just so you can use them while you’re boondocking. You can’t run the air conditioner through an inverter because it draws too much power.

So how do we know much power something consumes? That’s where “amps” and “watts” come in. Every household appliance has a label printed on it somewhere that shows how much power it uses. For example, take a look at the power adapter for your laptop converter, or the charger for your tablet computer. In very fine print it will say something like this: “Input 100-240v~ 1.5A     50-60Hz Output 20v 4.25A max.”

That means this device can accept a range of voltage from 100 to 240 volts of alternating current (AC) at frequency of 50 to 60 Hertz. In other words, it can be used on both North American 120-volt and European 240-volt electricity. That’s nice to know in case we decide to use it in Europe, but we are really interested in the amount of that power it consumes.

This part tells us the rest: “~1.5A”. That means the laptop power adapter requires up to 1.5 amps. Let’s go back to the water pipe analogy. If 120 volts is the pressure, 1.5 amps is analogous to the diameter of the pipe. More amps means a bigger pipe, which of course can carry more water.

The total amount of energy consumed by this device is the product of the pressure and the diameter of the imaginary pipe. This device consumes 120 volts X 1.5 amps, which comes out to 180 watts. That’s the number we were looking for.

(By the way, if you’re wondering about the “Output” numbers, you can do the same math.  This adapter puts out 20 volt power for your laptop at a maximum of 4.25 amps, which is 85 watts.  The difference between 180 watts input and 85 watts output is lost mostly as heat.  That’s why the adapter gets warm when you’re using it.)

From light bulbs you know that a higher wattage bulb is usually brighter. That’s because it’s using more power. Watts tell us the total electrical consumption of anything, and we can use watts to compare different devices.

So why do we talk about 30-amp or 50-amp power cords? That’s the maximum your power cord is rated to carry on a continuous basis. A 30-amp power cord is really a 3,600 watt cord (120 volts X 30 amps) and we could refer to it that way but it’s standard to talk about it in terms of amps.

adapter

“Aha,” you’re thinking, “so a 50-amp power cord is really a 6,000 watt (120 X 50) cord?” No, because there’s a trick. 50-amp power in North America is supplied at 240 volts. So it’s really a 12,000 watt cord. I know, it’s not fair, but that’s the way it is. Now you know why the 50-amp cord is so much heavier: there’s a lot of copper in it to carry all that power. A smaller cord would melt!

That means an Airstream with a 50-amp connection has more than three times the power available to it compared to a 30-amp connection. Even if you’ve got dual air conditioners, you’ve got plenty of extra juice. Those air conditioners will pull up to about 4,800 watts at full tilt, leaving you with 7,200 watts for everything else. Our example laptop at a mere 180 watts is hardly even noticeable in the overall scheme of things.

Once in a while you may need to plug the Airstream into a regular household outlet, which generally provides just 15 amps at 120 volts (which is 1,800 watts). This is fine for keeping the Airstream charged and powering low-wattage AC appliances, like portable fans and laptop computers. But it’s a very bad idea to run the air conditioner on 15-amp power. It might seem to work, but long term it’s likely to damage the air conditioner’s compressor or cause overheating or even melting at the plug.

Now let’s look at the other power system in your Airstream. The 12 volt DC system is driven by the batteries, and it is responsible for powering everything else in the Airstream, including lights, water pump, furnace, “cigarette lighter” 12 volt outlets and USB outlets, fans, refrigerator (when running in gas mode), propane leak detector, stereo/DVD player, breakaway switch, etc.

By running most things on the 12 volt system, most power consuming appliances in the Airstream can be used anytime, which is convenient for roadside stops and overnights without hookups. Since the 12 volt battery is kept topped up by the 120 volt power converter, it will never run out of power as long as the trailer is plugged in. Only when the trailer is unplugged will appliances be working solely on battery power, and with a little conservation, battery power can last for days.

batteryYou might be wondering why we don’t have 120 volt batteries so that everything can use the same voltage. The reason is simple: 120 volt batteries would be extremely heavy and expensive. It’s much more practical to use a 12 volt battery, charger, and appliances—much like your car does. It’s also easier to design solar panels and generators that produce 12 volt power.

In the end, it doesn’t make much difference to most appliances. A light, whether running on 12 volts or 120 volts, will produce about the same amount of illumination for a given wattage. In other words, a 12 volt light that consumes 1 amp is about the same as a 120 volt light that consumes 0.1 amp. Both consume a total of 12 watts and both will be about the same brightness.

So a watt is a watt, whether that power is supplied at 120 volts, 240 volts or 12 volts. If you ever get confused about comparing power from your solar panels, generator, or the needs of various appliances, figure the watts (remember, volts X amps = watts) and you’ll have a fair comparison.

Rich Luhr is the author of Airstream Life’s (Nearly) Complete Guide to Airstream Maintenance,  available at the Airstream Life Store. There’s a lot more about Airstream electrical systems, including maintenance tips, in this 220-page book.

Options for preserving your Airstream batteries during storage

Ed B. from Washington posed a question that might be on your mind, too—

“I’m wondering about replacing the factory converter/charger unit with a ‘smart charger’. Battery management seems to be my problem just now. When in storage, the batteries go down to zip or overcharge, both of which are hard on the batteries. We have no problems while underway.

I tried putting a small charger unit on the batteries while in storage, but the small constant draw seems to fool the charger in to delivering full charge all the time without activating a trickle delivery as it is designed to do. Is this why Airstream has not put ‘smart charging’ units in at the factory?”

The converter/charger that Airstream installs works very well for the vast majority of cases. It is perfectly adequate unless you (a) want integrated inverter capability; (b) switch to AGM or other type of batteries; (c) want a faster charge rate, or some bells & whistles; (d) add a lot more battery capacity.

Before you spend money on an upgrade, try a few other basic steps:

Battery disconnect1. During short term storage, use the “Battery Disconnect” switch to cut off demands on the battery (except the propane leak detector).

2. During long term storage, disconnect the battery cables and put the trickle charger on. Make sure what you have is really a smart trickle charger designed specifically for storage of batteries and not simply a battery charger with a “float mode”. If the latter, then all you’ve done is replace your Airstream’s charger with another one that is doing exactly the same thing.

3. As an option, if your Airstream is stored outside, consider a solar panel and solar charge controller. A 100 watt panel would be plenty to keep your battery happy under average conditions.

4. Make sure you are maintaining the water level in the batteries during storage. Consider switching to AGM batteries. These will not “boil off” (lose water) during charging cycles so you don’t have to check the water level during storage, and they last longer.

5. If you have the ability to measure current draw on the battery while the trailer is stored and the Battery Disconnect is switch to “STORE”, do so. Parasitic draw may account for up to half an amp, but if it’s higher than that you have a voltage leak or device that isn’t working properly, and that will kill the battery pretty quickly.

Secrets of Power: The Xantrex TrueCharge 2

Most travel trailers are built for weekend use. That mean the factory equips them with a battery or two that is sufficient to power the trailer for about two nights without being plugged in. After that, it’s time to find a source of power.

So it’s not surprising that not long after an Airstream owner begins to travel a bit more away from the KOAs and toward bucolic and private boondocking sites, they begin to look for ways to extend their power capacity. The next step is often a generator or solar panels, and a new set of batteries with greater capacity. Absorbed Glass Mat (AGM) batteries such as Optima and Lifeline are a typical choice because they last a long time, can be deeply discharged, and are maintenance free.

But then an interesting problem crops up, one that most Airstreamers—and even many RV repair technicians—don’t know about. The power converter built into your Airstream isn’t optimized for charging AGM batteries. Often it will undercharge them, particularly in cold weather.

People tend not to notice this unless they have also installed a highly accurate battery monitor, because the factory installed monitors are only rough estimators of the battery state of charge. So they are often misled into thinking the batteries are being fully recharged. But incomplete charging results in shortened battery life. Batteries will only have their longest possible life if they recharged to full after every use.

To truly get the most out of your new AGM batteries you need to replace the factory installed power converter too.

Only a few brands of power converters have the built-in capability to provide the correct voltages needed for AGM batteries. For this review, we upgraded an Airstream Safari to a Xantrex TrueCharge 2 (60 amp model) to see the difference in performance and the ease of installation.

The conversion job is fairly straightforward for a handy person, taking about 2-4 hours. Installing the Xantrex is easy; it’s removing the old converter that gets into a little mechanical surgery, because the factory charger is built into the same case as the fuse panel and circuit breaker panel. Separating the lower charging unit from the rest requires a drill, wire cutter/stripper, screw drivers and Torx drivers, a nut driver, and a few other basic tools. (Documentation for this job with photos can be found around the ‘net by Googling “replace power converter Airstream”.)

Once the old power converter is removed, installation of the Xantrex is painless. The Xantrex TrueCharge 2 is a beautifully crafted device that looks like it should be on display somewhere. As one friend commented, “It’s way too pretty to be installed inside,” but indeed that’s where it goes. It screws down to the floor securely, and then connecting it is a matter of butt-splicing six wires (ground, DC – and DC+, and three wires for AC), which takes only a few minutes. Since it’s designed for marine and RV use, everything on it is marine grade. Providing rubber covers for the wire connections is a nice touch, typical of this high-end unit.

The TrueCharge 2 is pre-programmed for 3-stage charging of “wet cell” batteries. Changing settings to provide optimal charge for AGM batteries is a matter of pressing two buttons. Once done, the output voltage jumps to the correct level for AGMs, which means those batteries will be charged to their fullest capacity. This was the feature we were looking for in our test Airstream, since other brands such as Parallax and Intellipower provide just one output voltage for all types of batteries.

The TrueCharge 2 can also support up to three separate battery banks, and can even run in parallel with a second charger for really large installations, but most Airstreamers won’t need those features. A single 40-amp or 60-amp TrueCharge 2 will be sufficient to replace the original factory unit.

TrueCharge Remote
TrueCharge Remote

The LED display on the case shows exactly what the unit is doing (rate of charge, type of battery, etc), but since it will be hidden out of sight, adding the optional Remote Panel is a wise choice. This panel connects with a simple telephone-style cable, and can be mounted on the wall anywhere within 25 feet. As a bonus, the Remote Panel adds a few features that the onboard TrueCharge 2 display doesn’t provide. If we are to be totally honest, it is also just plain cool-looking. You might never touch it again, but all those colorful LEDs certainly make for a nice display.

Battery Temperature Sensor
Battery Temperature Sensor

A particularly useful option is the Battery Temperature Sensor. This measures the temperature of the battery and allows the charger to compensate. Warm batteries need lower charging voltages; cold batteries need more voltage. If you simply slip the sensor over the negative post of the battery and run the provided wire back to the TrueCharge 2, it will figure the optimal temperature compensation automatically. If you don’t use the Battery Sensor you can still manually compensate for temperature using buttons on the Remote Panel.

Interestingly, since the TrueCharge 2 was designed for worldwide markets, it can accept input voltages of 90 to 265 volts AC. With that and other protections built into it, it’s unlikely to be damaged by excessively high or low voltage at the campground. It also has a battery equalization (or “de-sulfation”) mode that helps maintain the batteries.

Once the TrueCharge is in place and power is applied, it checks the batteries and then starts silently maintaining them. Although it has a built-in fan like the original converter, it rarely runs and when it does it’s much quieter. It’s also considerably lighter than the old converter because it isn’t mounted in a bunch of heavy sheet metal. Overall: it looks, feels, and operates like the major upgrade that it is.

Xantrex is known for premium products, and the TrueCharge 2 doesn’t disappoint. Running about $600 online for the 60-amp model, Remote Panel, and Battery Temperature Sensor, it’s a good investment for Airstreamers who like to get the most out of their power system. Smart, well-designed, flexible, and in every way an improvement over the original gear, it’s a device befitting an Airstream.