Secondary EFIS Power

This entry is part 62 of 67 in the series 13 - Electrical / Instruments

Because I have an “all electric” instrument panel, there are some challenges. One of those is powering the basic instrumentation during engine start. When you hit the starter button, the battery gets loaded down and the output voltage can (and usually does) drop enough that the EFIS reboots. It takes the HXr about a minute to boot up. So for that first 60 seconds after starting the engine, all I’ve got for engine health is the “Low Oil Pressure” warning.

To resolve this, I installed the TCW Technologies Intelligent Power Stabilizer (IPS).  This small, lightweight box gets power from the battery and provided a constant 24 volts even when the input voltage drops to as low as 9 volts. It can only output 24 volts for a couple of seconds when the input power drops but that is sufficient to keep the EFIS up and running during engine start.  The HXr has three separate power inputs.  The primary power input is connected to the battery.  The secondary power input is connected to the IPS.

With the GPS swap requiring me to move things around, I had to eliminate the IPS. To keep the HXr powered up during engine start I will have to go with a backup battery.  I would have done this originally except that the optional backup battery is only available for 12v HXr’s. I looked for a 24v backup battery, but they were either too big or too expensive. This is what happens when you go with a 24v electrical system.

So I decided to think outside the box.

I call these “Barbie Batteries”.

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I found a store that sells “Scooter Batteries” for things like mobility scooter, small electric cars… You know, those little cars that parents get their little kids?


I found that while the 24v batteries were big and pricey, I could get a pair of small 12v batteries that were very affordable. The batteries I got were 5ah SLA (Sealed Lead Acid). By connecting them in series, I would get 24v, 5ah.  Not only would this power the primary EFIS, AHRS and magnetometer during engine start, it would also keep the devices powered for at least an hour if the main electrical system failed.  An added bonus is that since the Magnetometer and EFIS will have an independent power source, I can eliminate the compass in the panel.

The down side is that they weigh 3.5 pounds each. Once I’ve verified this works, I may look for some new, fancy, hi-tech, low-weight batteries.

As for where to put these, I decided to put them in the nose. I noticed when flying alone that I required quite a bit of nose-down trim for level flight. Seven pounds in the nose should help that.

The first step is to make a tray for the batteries. So I wrapped the batteries with duct tape, took a piece of spare fiberglass, cut it to size and then applied some fiberglass strips to create the sides. Once it had cured, I removed the batteries and trimmed to size.

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Mounting in the nose was a bit of a challenge.  First I had to find a spot that was as far forward as possible but not interfere with the nose gear or anything else. Then I had to fabricate the supports and hold it in position while in glassed everything in place… And it had to be level.

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Once that was bonded in place, I had to fabricate the hold downs.

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Then I just had to wire everything up.


The primary power comes on whenever the master switch is on.  I’ll have a second switch on the panel to apply the backup battery power to the secondary input of the EFIS. So I’ll throw the backup battery switch while I’m doing the pre-flight check, Then when I’m ready to start the engine, the EFIS will be up and running.

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