Best Practices – Batteries

Electrical, part 3.


We conclude this series on aircraft electrics by looking at batteries, backup systems, and electrical circuits. Even with this third article on best electrical practices, we have only scratched the surface of what there is to know. If you are getting ready to wire up your project, please seek out other qualified resources and get whatever training you can to make yourself proficient at this most important phase of airplane construction. The EAA SportAir workshops are great for this, as are the various workshops and forums offered at AirVenture every year. And don’t forget those “Hints for Homebuilders” on the EAA website. Lastly, I continue to be a big booster of Bob Nuckolls’ The AeroElectric Connection, available to download on the web at no cost. There is also an excellent online forum dealing with aviation electrics.

This Bob Nuckolls circuit shows an essential (endurance) bus and a battery bus that you would want for an electronic ignition system. It would be up to you to decide what circuits should be connected to the essential bus.


The days of debating the merits of wet lead-acid batteries versus sealed AGM batteries are long gone. Everyone is using sealed batteries, with the choice now being between batteries such as the Odyssey PC680 and the new lithium-iron-phosphate (LiFePO4) batteries made by companies such as EarthX. Earlier lithium batteries were very simple and lacked the built-in circuitry they needed to really be safe for aviation use. Those batteries are still available, but EarthX now makes batteries with overcharge protection, built-in cell balancing technology, overdischarge protection, and a warning light connection to alert the user to the state of health of the cells and the state of charge. There’s also short-circuit protection and excessive-heat protection. There may be others who are doing the same or soon will be. This added circuitry is a must if you are considering a lithium battery for your airplane.

The EarthX ETX680 lithium-iron- phosphate battery is very popular among weight-conscious airplane builders. It can save 10 pounds or more compared to the PC680 battery and increases cranking power from 170CCA to 320CCA. The ETX680 also contains the protective circuitry necessary to make it safe.

It is important to note that overvoltage protection in the battery needs to be supplemented by external overvoltage protection circuitry to be totally safe. Of course, all planes should have overvoltage protection, regardless of what type of battery they employ.

The weight savings a lithium battery provides is impressive and hard to ignore, but many builders will still not be sold on the idea. I plan to use a lithium battery in my next project, but I would not criticize anyone who wanted to hang onto their PC680 battery.

The other big battery question is, how big a battery do I need? A PC680 with its 17 amp-hour capacity will start even an IO-540 engine as long as it is fully charged. The problem comes rather quietly with age as the amp-hour capacity of the battery degrades. That 17 amp-hour battery may still start your engine, but how long will it last if the alternator fails at a time when you really need to have electrical power and can’t easily land? The hour of reserve electrical power you thought you had may end up being half that.

Periodic capacity testing makes a lot of sense. A capacity tester is available for a very reasonable price from Harbor Freight Tools, or you can simply conduct your own test by loading the battery and seeing how well it holds up over time. The important point to make is that simply cranking over your engine is not a very good test of battery condition. The more dependent you are on electrical power, the more important it is to really know the condition of your battery.

This schematic shows what Bob Nuckolls considers to be the ideal arrangement for the vast majority of builders. It employs the B&C SD-8 backup alternator and a single battery. This alternator needs no battery power to operate. You could eliminate the battery bus if you did not have an electronic ignition system or electronically controlled fuel injection.

Backup Batteries

In an age of all-electric panels, the backup battery is no longer an option. If you have one of the electronic ignition systems that relies on power to operate, the backup battery could save your life. How much of a backup battery you need and how to connect it depend on your specific situation.

Your ignition system will be a major driver in your backup decision process. If you have two magnetos, you do not need any power to keep your engine running. With some effort you can even start the engine without electrical power by hand propping. If you have one magneto and one electronic ignition you can always limp home on one magneto. However, if you have dual electronic ignition systems, you absolutely must have reliable power at all times. Somewhere in between, if you have two P-MAGs, you can keep running if you are already under way, but you will not be able to hand prop your engine with a dead battery unless you have some emergency means of energizing the ignition system. It goes without saying, if you have electronically controlled fuel injection, you will also need to be very concerned about backup power.

Next to ignition system considerations, the type of flying you do is a big factor. Lots of IFR flying calls for a backup alternator and possibly a backup battery. Day VFR only pilots will have less need for much backup power. If your engine will run without power and you are comfortable landing without flight instruments (I’m not), you may elect to forgo any backup battery, but you won’t see me calling that best practices. Most avionics manufacturers can supply you with a small backup battery for their equipment at a reasonable cost that will give you plenty of time to find a safe place to land. It doesn’t make much sense to forego the added safety this affords.

Here is the ultimate backup wiring layout with dual batteries and dual alternators. Most builders will not need this level of redundancy and complexity.

It is important to keep in mind that as batteries age, they tend to lose amp-hour capacity. They may still start your plane well, but instead of that 17 or 20 amp-hours you had when the battery was new, you might now only have two-thirds of that or less. A small backup battery may keep your electronic ignition system alive longer than your fuel will last, but when you add in radios, cabin lights, electric landing gear, and pitot heat to the equation that changes quite a bit. It is something to think about. Bob Nuckolls’ suggestion is to have two batteries, one of which is replaced every year. Thus, you will always have one battery that is less than one year old and a backup that is less than two. This is very conservative, but if you really need electrical power for night or IFR flight, you should give his recommendation strong consideration. In any case, there is no substitute for knowing the condition of your battery by testing it regularly.

I know that many people say that their PC680 battery lasted seven or eight years, but you should not confuse this with being smart if your life may depend on reliable electrical power.

Students at this EAA SportAir workshop get valuable hands-on experience with electrical tasks as well as a solid grounding in basic electrical theory. Similar workshops are offered at AirVenture every year.

Backup Circuits

If you only need backup power for your electronic flight instrument system (EFIS), the small battery that the EFIS manufacturer sells is sufficient and by far the easiest to install. This is something you should include without a second thought in almost every case, especially if you do not have any “steam gauges” as backups. The EFIS unit should have provisions for connecting the backup battery and instructions in the installation manual. A great advantage of an EFIS backup battery is that it maintains 12 volts to the EFIS unit during engine startup, when the main battery voltage may drop too low to keep the EFIS working.

If you have two electronic ignition systems that are totally power dependent, then you will want two batteries, one for each ignition system. There should be a provision for switching on and off the connection from each battery to the main electrical bus. Each ignition system should be wired directly to one battery and kept hot all the time. If that sounds dangerous, keep in mind that the installation instructions for the ignition system will describe how to prevent the ignition system from energizing the spark plugs unless it is turned on.

This RV-8 under construction has a typical layout for the master relay and starter relay. Note how the starter relay is mounted upside down; the ammeter shunt is being installed below and to the left of the relays, but the work is not yet complete.

Since an alternator failure is our main concern, many people will be well served by merely installing a second alternator. Here there are two basic choices. B&C makes a small permanent magnet alternator that needs no outside power to energize its field. If the engine is turning, this will produce power. The downside is that it is limited to an output of about eight amps. Both B&C and Plane-Power make backup alternators that will produce up to 30 amps but need some electrical power to energize the field. All of these units are made to be installed on the pad at the rear of Lycoming-type engines where the vacuum pump used to go.

Backup alternators are wired to the main bus through a voltage regulator that may be internal or external in the case of the B&C units. The small B&C alternator is simply on all the time, but the other units will be switched on or off with a separate alternator switch on the panel. This switch actually controls the power to the alternator field to turn it on or off.

Avionics Bus

A separate switchable avionics bus has been popular for a long time and remains so, but in this time of all-glass panels, it may have outlived its usefulness (Bob Nuckolls thinks so). The avionics bus groups radios, audio panel, transponder, GPS receiver, and ADS-B equipment onto one electrical bus that is switched on by means of a relay and a switch on the instrument panel. This protects these items from power surges that can occur during starting without having to turn each piece of equipment off for starting and then back on. It also saves wear and tear on the older-style on/off switches of your avionics. Many avionics shops continue to use the avionics bus in their panels.

Essential Bus

Nuckolls is a fan of the essential bus (which he now calls the endurance bus), as are many airplane builders. The reasoning behind the essential bus is that only the items that are absolutely essential for continued flight are on a single bus. With this arrangement, all other electrical power uses can be switched off with one switch. This instantly sheds unnecessary electrical load in case of an alternator failure. The concept is sound, but different people will have different ideas of what is essential and what is not. For example, a transponder is essential in congested airspace but not elsewhere.

My personal preference is to simply pull breakers in an emergency for items I don’t need at that time. They can easily be restored by pushing the breakers back in if needed. Admittedly this takes a bit more time and distracts from flying the plane, which can present problems in stressful situations. That approach, however, would not be very effective if you had fuses instead of breakers. In such a case the essential or endurance bus would be very handy. With circuit breakers, the argument is somewhat less compelling. It is worth your consideration in any case. Read The AeroElectric Connection for a longer discussion of the endurance bus.

This is the basic layout for a traditional electrical system with two magnetos and no backup power. Two possible ammeter shunt locations are shown as A and B. Note the jumper from R to GND that is used to ground out the right magneto for starting. (Schematic: Dave Prizio)

Basic Power and Ignition Circuit

The basic power circuit used in many planes consists of a master relay switched on by completing the circuit to ground. This relay controls all power to the master bus and the starter. The exception to this (not shown on the basic schematic) is for electronic ignitions and electronic fuel injection systems that are wired directly to the battery and not through the master switch.

The starter relay, on the other hand, is activated by running positive voltage through the ignition switch to the starter relay. The heavy current of the starter motor is thus routed through a relay instead of a switch directly. The alternator is activated by a switch that sends positive voltage to the field terminal of the alternator, often through a voltage regulator, but sometimes directly if the alternator is internally regulated.

The alternator feeds directly to the main bus (labeled positive bus bar in the schematic). An ammeter shunt or Hall effect sensor may be placed in the line from the alternator to the main bus (Shunt B) or in the line from the master relay to the main bus (Shunt A). They are both acceptable, but the Shunt B arrangement is more common. With this arrangement, you will be measuring alternator output. With the Shunt A setup, you will be measuring current in or out of the battery.

The shunt is a device used to sense current without running heavy current through the ammeter. A Hall effect sensor does the same thing but in a different way. The wire carrying the current passes through an induction coil that senses current flow. By the way, both sensor wires from the shunt need to be protected with one-amp inline fuses.

Wrapping Things Up

Use aircraft components and materials to build your airplane’s electrical system. It will be safer and more reliable that way. Always strive for light weight and simplicity whenever possible. There are exceptions to these two general rules, but they are relatively few in number and should only be made after careful consideration. When it comes to redundancy and backups, tailor your design to meet your actual flying circumstances. Try not to get carried away in either direction. Anyone who relies solely on an EFIS needs a backup battery for the EFIS. Not everyone needs two alternators and two batteries. For most people a small backup alternator will do a perfectly adequate job.

Many decisions to add redundancy should be tempered with a sober evaluation of your real needs. Nice-to-have extra stuff can lead to a plane that is 100 pounds overweight when it is all finished, which in turn leads to the temptation to arbitrarily increase gross weight. This is never considered a best practice.

Protect your wires. A smoking radio can be turned off, but a short in a wire can start a fire that can kill you. Every circuit needs a fuse or breaker, and that fuse or breaker needs to be sized to protect the wire being used.

Consider the use of a ground bus. Many electrical problems are caused by poor ground connections. This is the best way I know to help avoid them.

I hope these three articles serve as a jumping off point for learning more about electrical systems. We will leave electrical systems behind for now and delve into other best practices next time.


  1. I am just beginning this journey into homebuilding a Spezio TuHoler and I really enjoyed reading this article and see the value of this type of information. Since this is a Part 3 article, what are the names and dates of the previous 2 articles? Thank you for taking the time to educate us neophytes.

    David Kopanski


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