Battery Redundancy
by Branwell McClory

 

In this day and age, where plane's are getting bigger and more expensive, where the areas around our flying sites are getting more populated, some level of on board electrical redundancy is probably not such a bad idea.

There are various thought's and opinions on this topic, here are some of them and what I feel are some of the pros and cons.

 
Method One:

Two packs, two switches into one receiver.
This method is good from a simplicity point of view.

Pros:
Packs can be different capacities but must be the same number of cell's.
Takes into account cell failure on 4 and 5 cell systems.
Wiring failure.
Switch failure.

Cons:
Limits the choice of cells if weight is an issue.

 
Method Two:

Two 5 cell pack's, two regulators, two switches into one receiver.

The two packs protect against a cell failure and the regulators insure a closed short up stream of them wont cause a voltage drop.
The regulators will also keep the servo speed constant over the time of the flight, but wont hold the torque constant.

The two switches protect against a switch failure.

Pros:
Protected against cell failure.
Regulated voltage to the servos.

Cons:
Standard regulators are limited to 5 amp's continuous load.
Regulators become a possible point of failure.
Higher weight than "method one".

 
Method Three:

Method Two but adding a second receiver splitting the plane down the middle, i.e. one receiver for the servos on the left and one for the right.

Pros:
Some protection against receiver failure, it depends on how the receiver fails.
Double the load carrying ability of the receiver section of the system.

Cons:
More complex to inspect.

 
Method Four:
Battery Backers.

The idea is that if you have a battery failure, this switching device will cut in a back up battery.
It's an interesting idea but the device it self then becomes a single point of possible failure, personally this doesn't appeal to me.

Things to consider:

The servos we use today on the giant scale planes can draw up to 1.5 Amp's when they near stall.
Given that the average 35% plane has 8 of them, that adds up to 12 possible Amp's, obviously bigger planes will draw more as the servo count and load goes up.

Do we stall the servos ?, in a word, yes, every time we do a snap roll.
Combine that with the lack of mechanical advantage on a plane set up for 3D, and you have a situation where you have very heavy loads.
For example, the plane I currently fly, a 40% Aeroworks G202 set up for 3D, gets 3 fifteen minute flights from a 2000ma pack.

Wiring is another issue, the average output wire of a battery pack gets warm a 3 amp's and hot at 4. This is one of the reasons I double wire my packs.

Then there is the receiver it self, how much current can it transfer across its buss ?.
Most will carry 25 amp's peak, not constant. This is something that will become more of an issue as the planes get bigger.
Splitting out the servo power from the receiver power starts to become attractive much after 35% in my opinion.

Conclusion:

There are many different systems, some good and some not.
I'm sure there are calculations to figure out the probability of failure of one system over another.