Sailplane Power Supply
Contrary to popular belief, modern gliders are not really autonomous flying machines. Pilots very much rely on carrying some amounts of stored energy with them in flight. The energy might be stored in batteries to feed the electronic equipment or in chicken sandwich to feed the pilot. And while consuming a sandwich usually does not present a big problem, organizing power supply to electrical devices might appear trickier than one would expect.
De-facto standard for supplying power to avionics and other devices in the cockpit is 12V. Usually it is provided by one or more lead or (newer and lighter) lithium iron phosphate (LiFePo4) battery. The capacity is typically in range of 7 to 12 Ah. Often a single battery is fully sufficient for all day flight. Yet, many pilots carry two batteries for additional capacity and for added insurance against unexpected power loss.
This article focuses on different ways of connecting devices to one or more batteries.
Please note, that real-world wiring are somewhat more complicated then schematics below. Complete solution should involve circuit protection (in form of fuses or circuit breakers) for each battery and preferably each connected device. Capacitors for smooth battery switching might also be helpful. This article omits these important details for the sake of simplicity. You will find a lot of useful information on these topics in Resources section below.
We will consider options to wire power supply to feed four electrical devices, typically found on modern sailplane: a radio, electronic variometer, FLARM and USB power supply for our flight computer PDA.
Single battery
Single battery setup has one serious advantage: the simplicity. Power bus, that distributes current to devices is connected directly to the single battery and provides consistent power until the battery is depleted.
Pros
- Simple
- Single circuit
- No attention required to operate
Cons
- Limited capacity
- Lack of redundancy
Two batteries in parallel
This seems to be the most obvious choice. From the physics class we all know, that when you connect two of the same batteries in parallel, you get the same voltage as each of them provides and double the capacity. That seems to be exactly what we want.
In practice, however, this approach has inherent problems: batteries rarely provide exactly the same voltage and capacity. If voltage is different (for example if one of the batteries is undercharged), electric current starts to flow from one battery to another. The battery with higher voltage starts to charge the one with lower voltage. In most extreme case, when one of the batteries dies, the faulty battery may easily draw all the charge from the healthy one, quickly discharging it. It is not possible to replace malfunctioning battery with new one — you have to replace both of them with new ones of exactly same model.
One can remedy the cross-charging problem by putting diodes into the circuits. Diodes pass current only in one direction, effectively blocking it from flowing between batteries. This solution also have drawbacks though: diodes cause small voltage drop in the circuit. Typically one can expect about 0.7–1V drop for semiconductor diodes. This is effectively lowers usable battery capacity: instruments will stop functioning after voltage drops to some level due to battery discharge.
There is some treatment for this problem as well, however. Special kind of diodes, called “Schottky diodes” have fairly low voltage drop that makes this approach practical.
Pros
- Simple
- Single circuit
- No attention required to operate
- Capacity easily scaleable to include more batteries
- Can operate on single battery
Cons
- Requires identical batteries
- Allows for cross-charging
- Faulty battery affects healthy one
- Lack of redundancy
Second battery as a backup
This is very popular setup. Instead of connecting batteries together, all instruments are powered from the single one. When the primary battery is depleted, pilot just switches the circuit to the secondary one by toggling the tumbler.
This eliminates all the problems of parallel battery setup, but unfortunately brings it’s own. This approach requires otherwise busy pilot to constantly monitor the state of primary battery and switch to secondary one when charge goes to dangerously low level. Otherwise they risk to get a power outage, which in the worst case can result in data loggers stop working, forcing unlucky pilot from the first place in a prestigious contest!
Another inefficiency of this setup lies in the fact that single battery powers the all the instruments at a time, therefore taking the full load, lowering its performance.
Pros
- Independent batteries
- Single circuit
- Good redundancy
- Can be scaled for more batteries as long as pilot is able to switch between them
- Can operate on single battery
Cons
- Requires manual battery switching
- Reduced battery performance due to higher load
Independent circuits
Another solution involves creating two independent electrical circuits with their own dedicated power supply. Each circuit powers its own set of instruments.
This setup doesn’t require any input from the pilot and both batteries are employed at the same time, lowering the load on each of them. Batteries are not interconnected, thus the danger of cross-charging is eliminated.
The advantage of unattended operation hides its own dangers, however: single faulty battery will cause its instruments to turn off and there’s little pilot can do about it. Also, since devices take different amount of current, it is very hard to achieve even discharge. This will cause one battery to be eventually discharged first. In this scenario the second battery cannot provide precious power even if it still has some charge available. This also somewhat lowers the effective combined capacity of the system. Needless to say, the pilot is required to always carry two batteries: full operation on one battery is not possible.
Pros
- Simple
- Independent batteries
- No attention required to operate
- Load is distributed among all batteries
Cons
- Lack of redundancy
- Separate circuits needs to be balanced
- Reduced total capacity due to uneven discharging
- Cannot operate on single battery
Commuted circuits
This somewhat unusual setup seems to combine most of advantages of other solutions and manages to avoid most of the pitfalls. Two batteries are powering two independent circuits, each of them is powering its own circuit, or both circuits in case the other battery is switched off. The pilot operates two toggle switches for battery A and battery B. If both of them are switched on, this results in “independent circuits” set up. If only one is switched on, the corresponding battery feeds both circuits, resembling “Second battery as a backup” solution.
The pilot can just switch both circuits on and leave the system unattended, but in case something goes wrong, any of the batteries can be switched off and the other will provide power for all the devices. It is also possible to carry only one battery if its capacity is sufficient.
Surprisingly, this logic doesn’t require any sophisticated electronics. It can be achieved by wiring two DPDT (ON-ON toggle with 6 legs) switches in a clever way.
The downside of the method is that it may look confusing to a person, not familiar with its mode of operation. Consider the (unlikely) scenario, when one of the batteries dies in the middle of the flight and half of the instruments goes down. In order to turn them back ON, pilot is required to put one of the switches to OFF position. And if the pilot toggles the wrong switch, they will lose the rest of the instruments. Similarly, if you connect only one battery, following the habit of setting both battery switches to ON position will only power half of the instruments. Obviously, this may seem counter-intuitive to unaware person.
This makes this solution somewhat less ideal for club gliders, flown by lots of people, not familiar with all the peculiarities of its power supply system. On the other hand, this seems to be the best solution for pilots, willing to give up some of the fool-proofness for the ultimate flexibility.
Pros
- Independent batteries
- Supports unattended operation
- Load is distributed among all batteries
- Can operate on single battery
- Good redundancy
Cons
- Separate circuits needs to be balanced
- Can confuse unaware users
- Does not support more than two batteries
Conclusion
As can be clearly seen, none of the described schemes can be considered “ideal”. Therefore choice has to be made depending on particular set of requirements.
For the typical club glider, that is usually has a fairly light power consumption, single battery is usually completely sufficient for all-day flying. For added insurance one can recommend “Second battery as a backup” scheme — it is rather popular and pilots should have no trouble understanding and operating it.
For private sailplanes with lots of electronic devices “Commuted circuits” setup might be preferred, as it combines advantages of many other schemes and avoids their worst drawbacks.
Resources
- Electrical Wiring Made Easy (pdf) — overview of hardware, wires, their type and size.
- Three Uniform: Battery Strategy — good hints on installing batteries.
- SBSS ➜ Sailplane Wiring 101 — video presentation about wiring and installation
