Challenger Ultralight Aircraft Maintenance

Tip #1 – The Thirsty Battery

I noticed two seemingly conflicting trends occurring concurrently on my instrument panel voltmeter.

The voltage reading with the engine off was slowly dropping over time. It was usually reading about 12.8 volts. Over a couple of months it had dropped to 12.3 volts.

Yet when the engine was running at cruise setting, the voltage reading was getting higher. It had usually stayed around 13.2 volts but was now reading up around 13.8 volts.

What was happening? Was my voltage regulator slowing dying?

Finally, one day I went to start my engine and it did not have the power. The battery was not dead but it was not up to the task of starting the engine either. I went home and got my battery charger and recharged the battery for a morning.

Again, I noticed something odd. The ammeter on the charger was not indicating a very large rate of charge. It started charging at about 2 amps but after a few minutes was down to a 1/2 amp. I left it charging while I did other tasks.

But I was puzzled. It was a relatively new battery. It is a Canadian Tire lead acid utility type for small tractors. I have it mounted in the nose of the Challenger for ballast as I am a relatively light pilot. It has always had lots of guts. It could not be in need of replacement so soon. What had caused it to lose voltage? I had not left the master switch on or any other equipment plugged in that would have drained it.

When I went to start the engine again after hours of charging, it would not start! The voltage was still reading really low.

Finally, in desperation, I removed the plastic covers that seal the 6 individual battery cells and took a peak inside. (This was no small task due to the location of the battery in the front of the aircraft.) I could see the tops of the metal plates inside the cells. There was the answer!

It had been a very hot summer. Some of the water inside the battery cells must have evaporated. The battery was low on electrolyte fluid. I went off home again and retrieved a jug of distilled water and topped off the fluid levels in each cell. Again, I connected the battery charger to the terminals. I was gratified to see it accepting a charge, although still not a very large one, about 2.5 amps.

When I returned after about 6 hours I removed the charger, flipped on the master switch and checked the voltmeter. It was reading almost 13 volts. A healthy battery. It had simply been thirsty. This battery is not a sealed unit and the water that forms the electrolyte fluid is prone to slowly evaporate over time especially in hot weather.

The output voltage of the battery had fallen with the level of the electrolyte. Less electrolyte meant less output voltage.

But why had the voltage level during cruise flight gradually increased over time? The battery acts as a sink to absorb the additional voltage produced by the alternator on the Rotax engine. At cruise settings, the Rotax 503 puts out as much as 40 volts. Having a battery in the electrical circuit absorbs this extra voltage. As the capacity of the battery was reduced due to lower electrolyte levels, so was its ability to absorb the additional voltage from the engine. Hence the voltage level at cruise slowly climbed over time.

The one good thing from the experience is that the voltmeter helped me to see the problem develop and indicates also that the problem is solved. It will also help me in future to see if the issue reappears. But next time I will be much wiser.

• Lesson 1: Periodically check the electrolyte level in the battery.
• Lesson 2: A voltmeter is a handy gauge to have on the aircraft instrument panel to detect and analyze maintenance issues.

Checking the electrolyte level is important in a lead acid battery because the plates will get permanently damaged if they are exposed to the air for too long. This will inhibit the power output of the battery even when distilled water is added to replace the electrolyte. It may also reduce the battery’s ability to hold a full charge for very long. A robust 12 volt battery will usually show 13 – 14 volts on a voltmeter. A damaged battery will show a lesser voltage and will not produce the current for starting the aircraft for more than a few seconds. In this instance, the battery must be replaced.

In wintertime, a battery with low electrolyte may freeze and become permanently damaged. This will be evident if the sides of the battery are bulged out. Such a battery will need to be replaced.

I ended up replacing the above mentioned battery with a new one. The problem of poor starting slowly returned. The battery was about 6 years old. That is about the age limit for a typical lead acid battery. The new battery starts the engine with zest. And it only cost about 50 dollars. A small investment for a big return in performance!

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