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2 Battery Redundency

Old 01-29-2003, 01:50 AM
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Default 2 Battery Redundency

I have seen posts pertaining to 2 batteries & 2 RX with the batteries wired together for redundency. (In the case of battery falure you dont have to land the plane with one half of the controlls working )

I assume they are wired after the switches, with possably a fuse or a fuseable link.

What size fuse or link are we talking about?

Or am I being WAY to paranoid?

TIA
Old 01-29-2003, 03:04 AM
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Default 2 Battery Redundency

You're not being paranoid. There are two ways of doing this.

You can just run them as separate systems, and if one system fails, the other system leaves you with half the airplane to fly. Not a bad backup, and half an airplane can be controlled, its just more difficult. There are numerous stories with multiple eyewitnesses to attest to the fact that the solution works.

Another way to run the dual receivers, and give you an additional level of backup at the cost of about $2, insignificant additional weight, is to use a jumper from one of the receivers to the other receiver. This adds an additional level of redundancy, no significant downside, and does away with the half an airplane mode except for a receiver failure. In the end, the only thing that isn't redundant is the pilot and transmitter.

The jumper should only have the two leads for the positive and negative, and not the signal wire. On the positive wire, insert a fuse of about 15 or 20amps. The fuse is only there to prevent a hard short on one side of the airplane to short out the other side, thus bringing down both systems. If the jumper or fuse fails during a flight (vibration on fuse, solder joint, etc), the only thing lost is the redundancy, as it will not cause any other failures. When you land it will be obvious when you check your packs for the next flight, and you can replace the jumper or fuse.

From what I have been told by multiple RF engineers, is that there is not exposure to increase interference with the receivers power bus tied together, and it might actually help to share a common ground. But in either case, it won't hurt.

The only issue that you need to think about is that the jumper will be supplying all the current for both systems in the event of a failure on one of the systems. If you airplane is overloaded to begin with (say 14 servos on two 7 channel receivers), you might be pulling a lot of current across the jumper. It should still be sufficient for the duration of a flight, but not something that you would want permanently. If you are running 14 servos on two 10 channel receivers, you are in much better shape and need not worry about it.

The good thing about this setup is this. You can loose a receiver, battery, switch, servo, broken connector, hard short, open wire.
regulator (if used), etc, and still land the plane. In most cases, you won't even know it happened until you land.

Paranoid. Nah. Just cautious. Why let a failure take your plane down, and possibly hit someone, when you can do so much to avoid it from the beginning with a little thought, and some extra cash.
Old 01-29-2003, 03:12 AM
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Default 2 Battery Redundency

I meant to mention this, but forget. Its the configuration I think you mentioned.

Another method is to open up the battery packs, and add another set of leads. These can then be soldered to a set of switches with double poles, so that each battery feeds both systems. They would need to be wired such that they will not share a common ground when charging.

The advantage of this is that in a failure, the current does not flow into one receiver, then just across to the other receiver via the jumper. If you are running an overloaded system to begin with, this is probably the better way to go. If not, the jumper is easier, and provides for a short circuit failure, which this one does not. If you try to add two fuses on the batteries to prevent a short, the fuse on the good system could blow first, leaving you with no power at all :disappoin
Old 01-29-2003, 06:47 AM
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Default 2 Battery Redundency

Very interesting info Stephen. Thanks for clearing up the fuse portion of my install.

As far as overloading the RX with 14 servos ( which I will have) in case of a failure: The only overload will be on the signal wire. 'Cause all my servos will be powered independent of the RX's. 2 battery packs, 1 supplying the power for each RX's servos. I was also thinking how to make the servo power reduntant.
That shouldn't be too hard, just jumper both power wires from a servo(s) to both packs.

"In most cases, you won't even know it happened until you land. " This is exactly what I am after!!!

The design is still on paper, so its easily changed at this point.

The paranoia continues....
Old 01-29-2003, 07:22 AM
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Default 2 Battery Redundency

stephen,
What guage wire are you using for the jumper??
Old 01-29-2003, 08:08 AM
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Default 2 Battery Redundency

Stephen

I recall all the discussion of late regarding the power jumper RX to RX... Most agreed no issues would present themselves...

While all this was going on I was trying recall of a situation that was brought to lite buy another modeler. Unfortunately I couldn't muster the memory. Low and behold the I recently saw the information.

As for "servo generated interference", it has happened to me, and was isolated to a single servo causing a total lockout of one receiver. How do I know it was the Ground circuit? Because, before I isolated the receivers form each other, I had a common ground between them, and during that time they would BOTH go into PCM failsafe/lock at exactly the same time, even though they were on separate battery packs with no common power. After removing the commonality of the ground, only the one affected by the bad servo would go into PCM lock, which led to me finally isolating out the bad servo.

Now there are some distinct differences with this installation and subsequent problem it does bear consideration as to what may possibly occur...
Old 01-29-2003, 01:12 PM
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Default 2 Battery Redundency

As for "servo generated interference", it has happened to me, and was
Michael, Good info and glad you brought it up. That is exactly what I was after when I posed the question on the IMAC/SA list. It doesn't sound common, and the installations may differ, but the exposure is there nonetheless on both single and dual receiver installations. Worth keeping in the back of ones mind.

Stephen,
What guage wire are you using for the jumper??
You don't need anything larger than what you are currently using for your batteries and switches. If one side dies, and you have one battery supplying all the juice for the whole system, the battery will be handing twice the load, since it is now supplying current for both receivers, but the jumper will only be handling a normal load for the second receiver. So, if you are using 22ga from your batteries to the receivers,the jumper can be the same size.

Where you will need the increase in size is from the good battery to the first receiver. It will be doubling the current that it would normally supply. So, I would recommend 18ga for the battery to the receiver, and 22ga for the jumper.

I tested my 40% Extra with all 22ga wiring for the batteries and switches, as well as the jumper. I switched off one battery and tested the voltage levels with a simulated full load on the airplane. Everything worked, and the voltage was within safe limits. This was for dual 5cell 2700mah NiMH packs, 12 servos, and dual 10 channel JR receivers. I ran it for a full 15 minutes to simulator a normal length flight.

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