Power on the Go – Part 1
June is almost here and my fun projects have been stacking up as I’ve been busy at the office or doing yard work. Brenda and I are heading to Montreal in 2 weeks for the Grand Prix so I am finally taking some time to work on the Cabin A LLC travel trailer. (The yard will still be here when we come back.)
This will be the longest trip on the Cabin A so far and we plan to spend at least one night on the road “dry” camping at a Walmart parking lot or rest stop. This means that we will not be connected to an Alternating Current (AC) power source. I would still like to be able to watch a movie before going to bed, charge up the computer and keep a small fan running at night in case it gets too hot. We have a portable inverter that we use in the car to charge up the cell phone and power the computer; however, it is fairly low power and we do not have a 12 volt outlet on the camper.
Another reason why I want an AC power source for the trailer while on the road is to power the stereo. The Cabin A came with a stereo system that for some reason runs only on AC. I would have rather installed my own stereo since I would have bought a better quality head unit and speakers and one that ran on Direct Current (DC) so it could be powered by the 12V battery on the trailer.
It wasn’t too long ago that getting 120V AC on the road without a generator was almost impossible; however, the proliferation of integrated circuits and more efficient power systems has made power inverters commonplace in vehicles of all types today. An inverter basically converts Direct Current to Alternating Current. Inverters are rated in Watts, which is a measure of power. Typically, an inverter will list two power figures, one for continuous power and one for “surge” power. The surge power figure will always be higher than the continuous power figure. When electrical devices start up (especially those that run electrical motors), they require more power during the first few seconds of operation in order to overcome the forces of inertia. The surge power rating is meant to account for this; however, the inverter will not able to power devices at the surge power level for very long.
If you are unsure as to whether an inverter will supply enough power to run all your devices, add up the surge and continuous power consumption of all the devices (typically listed in the back or in the specifications included in the owners manual) and compare the total to the surge and continuous power rating for your inverter, respectively. If you exceed the maximum surge or continuous power rating for the inverter, it will probably shut itself down or burn a fuse (which can be internal thus requiring the unit to be opened for replacement).
You must also take care not to overload the circuits on the vehicle or run down the battery too quickly. For this you need to calculate the current draw of the inverter. To find the current draw in Amperes, divide the power in Watts by the Voltage. Since the input voltage for automotive inverters is around 12V, you must divide the inverter’s power rating by 12 to obtain the maximum current draw. For example, if the inverter’s power rating is 750 Watts, the maximum current draw is 750 / 12 = 62.15 Amps. This means that if you plug in the inverter on the cigarette lighter of your car and draw the maximum power it will probably blow a fuse as accessory fuses on cars are rarely more than 25 Amps. For this reason, larger inverters should be connected directly to the battery using the appropriate gage wire as per the instructions that came with the inverter.
Another reason to calculate the current draw for the inverter is to make sure that you don’t run down the battery two much. Generally, vehicle batteries are classified as starter batteries or deep cycle batteries. Starter batteries are meant to provide a large amount of current over a short period and be recharged almost immediately. This is the type of batteries typically found in passenger cars. Deep cycle batteries are more often found in RVs and boats and are meant to supply steady power over long periods of time without being recharged right away. Some batteries used in boats can serve as both starter and deep cycle batteries.
Deep cycle batteries are rated in Ampere-Hours, which is a measure of how many amps a battery can provide in an hour under normal conditions. This will give you an idea of how long you can run a particular load before completely discharging the battery. For example, if an appliance draws 20 Amps and you run it for 15 min, that is the equivalent of 20 x .25 = 5 Amp-Hour. If a battery is rated at 100 Amp-hour, you could theoretically run that device for at least 5 hours.
On the Cabin A, I plan to connect a 750 Watt Vector inverter directly to the battery and mount it to a custom platform located under the bed between the battery and the freshwater tank. The stereo is currently wired directly into AC distribution panel so I plan on cutting the cable and installing a male plug so it can be connected into a regular wall outlet. I will also mount a power strip to the tabletop where we normally set the TV. I will then run the cable from the power strip along with the cable from the stereo to the inverter. The power strip will be used to power the TV as wells any other devices we want to power off of the inverter.
In addition to the inverter, I also purchased a ProMariner® Promite 5 automatic battery charger from Consumer Marine Electronics. This unit is quite reasonably priced, fully automatic (so it won’t overcharge the battery) and rugged. It will allow me to keep the battery charged while we are connected to shore power even if I still run some things off of the inverter. (I haven’t done the math yet to see how much current I can draw before the charger won’t be able to keep up.) I plan to install a new wall outlet under the bed hooked up to the same circuit to which the stereo was wired before. I will connect the charger to this new outlet so that, whenever we connect to shore power, the battery will be charged. This new outlet will also come in handy if the devices connected to the inverter start running down the battery too much while we are connected to shore power. I can simply reach in, disconnect whatever is connected to the inverter and reconnect it to the new outlet, which will be located nearby.
On part 2 of this article, I plan to post pictures of the installation and described what was involved.
Disclaimer: I am not an electrician. If you try any of the things described here it is at your own risk. For more information on DIY vehicle electrical projects see the book “Car PC Hacks†by Damier Stolarz.
