Solar Airplane Preheater

I park my airplane at a tie-down where electricity isn't available. For those not familiar: airplane engines need to be warmed up in the winter before they're started. Many airplanes have onboard electric engine heaters, such as those made by Reiff or Tanis.

By using a large power inverter and some golf cart batteries I can preheat my airplane for 2-3 hours. The batteries are recharged over a few days using a solar panel. By using a remote relay controlled via text message, I can start preheating before I leave home for the airport.

I'll take you through my build.

• 100W Solar Panel (amazon, $80)
• RTU5024 GSM Relay (ebay, $25)
• Prepaid GSM SIM card ($5, amazon)
• Two sets of 4 AWG (or lower/larger) inverter cables (ebay, $8 per pair)
• Solar Charge Controller (ebay, $15)
• Cheap 1000W 12V Power Inverter with Switch (ebay, $50)
• Large Plastic Outdoor Tool Box (Home Depot, $60)
• Golf Cart or Solar Batteries (Read more for detail, $325)
• 80A Inline Fuse (ebay, $8)
• 12/3 lighted extension cord (amazon, $25)

Here's a diagram of how it's all put together.

We make use of a prepackaged remote control relay that can be operated via SMS messages to turn the power on and off. There are some tricks needed to make this work with the large amount of power that's needed to run the preheater, which we'll talk about.

A solar charge controller is used to interface the solar panel to the batteries. There's a few important differences between this and a standard solar setup. Most notably: usually, the charge controller switches the load, but our inverter draws far too much power for the charge controller to handle. Instead, the inverter is connected directly to the batteries via a fuse.

How, then, is the inverter switched on and off? A relay on the 12V side would be too large to be practical, and a relay on the 120V side would leave the inverter running constantly and drain the battery, so both of those are out. Instead: we control the inverter's electronics directly. This is easier than it sounds.

On the inverter is a small power switch that operates the inverter's brain. Remove the switch from the inverter and extend the two small wires found on the back of the switch. Connect these to wires to the C (common) and NO (normally open) terminals on the remote relay. Now, the remote relay can act like it's flipping the power switch on the inverter.

What, though, is lost by bypassing the load switching on the charge controller? An important job that the charge controller has is to shut down the load if the batteries are discharged to the point that they could be damaged. In order to retain this important function we power only the remote relay from the charge controller's load terminals. That is: don't power the remote relay directly from the battieries. When the batteries become depleted the charge controller will de-power the remote relay which in turn will de-power the inverter.

The airplane's onboard preheater remains plugged into the (usually inactive) inverter via an extension cord at all times.

Home Depot sells plastic toolboxes that are fairly weatherproof- mine has been outside for 3 years with no trouble. They even have handy spots for wires to enter and exit.

Put some heavy batteries inside and it's not going to move! I mounted the inverter and charge controller to the side of the box with screws.

The position and angle of the solar panel matters: ask the internet for tons of information on this. The short version is that it needs to face south, be at an angle, and be unobstructed. I attached mine to the side of the side of the plastic toolbox with stainless steel hinges and screws.

Prepaid GSM SIM Card

You want the cheapest plan possible, which is probably $5 per month and meant for alarm systems. SpeedTalk seems to be the dominant vendor here and has worked for me.

RTU5024 GSM Relay

This is a little box that you can send a text message to in order to turn a small relay on or off. They actually work. Setup is a little frustrating because the instructions were probably google translated from some other language.

Activate your prepaid SIM then follow the directions (heh) to set up the relay box. When you configure the time the relay stays activated, choose the special number that means “stay on until I turn you off again”.

If you’re really into electronics and software and have WiFi available, you can instead build your own switcher that does fancy stuff like battery monitoring. Intended for nerds only. :)

About Cheap Power Inverters

Cheap inverters make horrible dirty awful power that you should almost never use… except for with heating elements, for which dirty power is perfectly fine. They’re reasonably efficient, even.

Assume the power ratings of cheap inverters are quite optimistic, so buy one rated for more than you need. So: if you have a 600W preheat system, buy an inverter that claims at least 1000W of continuous power. Read the specs carefully to find the continuous power output. The number written on the box will probably be some larger number that says “peak” or “max”, which means approximately nothing.

You want an inverter with a switch! This is important; see the "How it Works" section.

About Solar Charge Controllers

This is what lets your solar panel nicely deliver power to charge the battery. A cheap PWM unit should be fine and cost $15. Search ebay for “solar charge controller”.

If you want to spend a bit more, get an MPPT charge controller. You’ll get a bit more power out of your solar panel. Be warned that there are an endless number of fake MPPT charge controllers on ebay. A real one will have some weight to it (it must have a large internal inductor) and cost at least $50. The cheapest acceptable one I’ve found is an EPever Tracer2606BP or Tracer1206AN (ebay), the latter of which costs more and has a display.

Read the instructions for whatever charge controller you buy. You’ll at a minimum need to tell it about the type of battery it is charging.

Extension Cord

12 GA or lower/larger. Get one where the end lights up when the power is on (this really comes in handy).

What Kind?

You want solar or golf cart FLA (flooded lead acid) batteries. These are meant to be discharged and recharged repeatedly. Don’t use car batteries, they can’t handle this kind of charge cycle and will fail quickly. You could use lithium chemistry batteries but they’re a lot more expensive for the same capacity, and we don't care about weight anyhow.

Capacity

How big of a battery? Let’s calculate a scenario.

Say I have a preheater that’s rated for 600W, an inverter that is 85% efficient, I want to preheat for 2 hours, and I assume that battery manufacturers lie about specs by a factor of 2.

600W / 12V = 50A

50A / 85% = ~ 58.8A

58.8A x 2hrs x 2 (lie factor) = 235Ah

So in this scenario I want 12V batteries that store 235Ah (amp-hours) of power. Many batteries of this size are 6V, meaning you’re meant to connect 2 of them together to make 12V. Pay attention when ordering in case you need 2.

Getting Really Big Batteries

They’re very heavy, so having them delivered to you via freight is not friendly to your wallet. Instead, find a local store to who will order them, then pick them up yourself. Here are a few sources, each listed with a sample battery choice (all 6V batteries, so you’d need 2).

• Batteries Plus: 6 volt Duracell Golf Cart and Scrubber deep cycle battery (SLIGC115), 230Ah
• Interstate Batteries: GC2, 225Ah
• Lowes: Deka GC15, 230Ah

As far as I can tell these are all the same battery with a different sticker on them, probably all made by East Penn Manufacturing / Deka.

After preheating your engine, fly safe and warm! See you out there.