Electrical Charging System



As I said in a previous article, unless you’re hand-propping a tube & fabric airplane that’s older than you are, you’ve got a battery in your airplane.  And if you’ve got a battery in your airplane, you’ve got an electrical system to charge the battery.


Actually, that’s only true 99.999% of the time.  I know of a certified aircraft which was shipped with a battery and a starter, and no charging system.  However, that is extremely rare.  The owner carried around an extension cord and a little charger, and every time he landed, he plugged it in to recharge the battery.  Saved a few pounds of weight, but pretty weird.


The electrical system in your aircraft is composed of a battery, lots of wire, some solenoids or relays, a starter, a generator or alternator, something to control the generator or alternator, and probably some lights and radios and maybe some pitot heat.


Let’s start at the battery.  It will have a little thing nearby, which will click when you turn on the master switch.  That little thing is called a “master solenoid”, which is simply a big switch which is controlled by the little master switch on the panel.  The little master switch on the panel can’t handle 50+ amps of current, so it controls an electro-magnetic switch called a solenoid, next to the battery, which can easily handle 50+ amps of current, because it has much larger contact area.


The big positive wire from the battery goes through the master solenoid to the “main bus” of the aircraft.  Notably what comes off this is a huge wire – obviously designed to carry lots of current - which goes to the engine starter.  Again, it must go through another (starter) solenoid, because the starter switch (again) can’t handle the current to power the starter, which draws an awful lot of current for a very short period of time.


Ok, that’s the electrical starting system of your light single engine aircraft.  Off the main bus will be a bunch of circuit breakers and fuses which power all the electrical equipment in your aircraft, like the lights and radios, etc.


However, you need something to recharge the battery, which powers all this stuff.  And that’s your electrical charging system, which keeps the aircraft electrical system between 13.5 and 14.5 volts.


Again, your electrical charging system can usually be ignored when it’s working well.  But sometimes it doesn’t, which can leave you without electrical power at the most inconvenient of times, such as at night inside a cloud.


I should mention that twin engine aircraft have very complicated electrical charging systems, which are outside the scope of this humble note, and generally above the comprehension of most AME’s.  A story for another time.  We are only looking at light single engine aircraft electrical charging systems right now.


Historically, light single engine aircraft didn’t even have electrical systems – they were hand-propped which kept the expense, weight and maintenance costs down.  However most people don’t like hand-bombing the prop these days.


Older aircraft – the first with electrical systems – used something called a generator to recharge the battery.  Like a starter, a generator is bolted to the engine, but unlike the starter, which only turns sometimes, the generator turns all the time.


The problem with these older generators is that they don’t provide much current at low RPM.  Or at any RPM.  At night, you must be very careful about what lights and electrical load you have, to avoid rapidly discharging the battery when you are taxiing on the ground.


The newer alternators produce often twice as much current as an older generator, weigh much less, and they put out plenty of current at low RPM.  While this sounds really great at first, keep in mind that this will require an awful lot of torque at low RPM, to make that power, and that can cause problems with some direct-drive alternator couplings, often found on Continental engines.


Lycoming engines generally have an alternator at the front, driven by a belt, like a car.  You should check the tension of the belt every once in a while.  You don’t want it too tight (which can damage the bearings – one failure mode of the alternator) but you don’t want it too loose and slipping, either.  Changing an alternator belt will require removing the prop – because the belt is smaller than the prop diameter – which is annoying.  I have seen people strap extra alternator belts onto their engine.  While this is fine on a homebuilt, I have no idea as to it’s legality on a certified aircraft.  If you know the answer, please don’t tell me.


When your cowlings are off, check the wires on the back of the alternator.  The little wires can often vibrate and come loose, which is bad.  If they disconnect, the alternator will stop charging.  This is not good, as I found out one day in a retractable-gear single which used an electric motor to drive the landing gear.


Another failure mode of the alternator is the diodes.  Diodes are one-way electrical “check valves”.  Your alternator uses six of them to convert the AC power that it produces, into the DC power that your aircraft wants.  The diodes take a lot of current, and heat, and vibration, so it’s not surprising that they fail.  Generally one will fail, then another, etc.  You can spot this by a whine in the radio which tracks the RPM, and by a reduction in alternator output.  If you pull the alternator field breaker and the whine goes away, you’ve got bad diodes in your alternator.  Nobody fixes anything any more, so you will probably need a new alternator.  Get an FAA-PMA’d lightweight alternator.


If you look at your panel, you will see two circuit breakers.  One will be a high current circuit breaker (eg 50 amp) and one is a low current circuit breaker (eg 5 amp) which is for the alternator field.  The field uses a little bit of current to control the alternator’s output.  It’s the little wire on the back of the alternator.  At the other end, it’s connected to a box called a “voltage regulator” and on older airplanes, an “overvoltage relay”.


The overvoltage relay takes the alternator offline if the system voltage goes too high, to protect it.  This feature is often built into the newer voltage regulators, which are solid state things of beauty.  Three wires.  Just try to mount it aft of the firewall, to keep it cool, especially after engine shutdown, when it gets cooked in the cowling.  Electronics don’t like heat.


Anyways, I hope this brief overview of aircraft electrical charging systems helps.  As if often the case, I get the uneasy feeling that I have raised more questions than I have answered, but oh well.  That’s what the wiki is for.




Sept 2014