[Scerbina]

Hi All,
I just wanted to share my DIY power redundancy setup. The idea is not novel, but it is implemented a bit differently. The beauty of this solution, and the reason I am so excited about it is that I do not have to recharge the RX pack. Maybe top it off a few times in a season. It is like a spare tire, tucked in there just in case. I don't want to bother with charging RX packs to be honest.

Here are the advantages of this simple solution:
- Have 2 power sources for the RX and servos. If the main BEC is fried, the spare one will kick in automatically.
- Spare batt pack is very small/light. Add very little weight. (about 30 grams or so total, battery, BEC and cable)
- Do not need to charge the RX pack on a regular basis. Very rarely top it off.
- The spare pack will not just add redundancy if the main power source is down, it will also share the load if the main BEC cannot handle the load on it's own.
- Fly off the main battery pack, but if it gets disconnected, it is nice to have a security of a separate RX pack.
- The radio can be programmed to inform the pilot that the RX is running on a secondary power.

I stopped worrying about my SBECs frying and brownouts, after implementing it.
The basic idea is - to have 2 power sources for the RX and servos, just in case. One of them running of a very small and light battery pack, that almost never needs to be re-charged.
It is a simple custom Y cable with a diode inside. Very easy to make.

1) First power source is your main BEC
2) Second is a spare, running off of an ultra small battery pack.

Main BEC supplies 6V, while the secondary supplies 5V. This way I can program my radio to warn me when it switches to the alternative (spare) BEC. The spare bec runs off a small 2S - 300Mah (or so) battery, only 17 grams. Mine is easily capable of providing 10-15A if needed.

The best part is that with 6V/5V voltage difference, the higher voltage gets passed through, while the 5V source does not. This means your small battery pack does not get discharged, and you only need to charge it maybe once every few weeks. My secondary BEC has a LED, and it consumes around 16ma, which should last 10-15 flight hours or so.

OPERATION: Here is how it works. Both BECs are connected to the RX through a diode. If the main (higher voltage) BEC stops supplying power (voltage drops below the other BEC's voltage), the secondary BEC kicks in. The radio (if programmed) tells you that you are running on the secondary power source (your RX voltage is lower). Also, if at any time the power draw is increased significantly (servo jam for instance), the voltage of the main BEC starts dropping and once it reaches the voltage of the secondary BEC, it will kick in to help share the load (sort of). So practically speaking, the second BEC will also help supply more power if needed.
So now you have the security of a separate battery RX pack, without adding much weight (about 25-30 grams total) and most importantly without having to constantly recharge your RX pack. Plus there is the redundancy of 2 BECs and the additional power when needed.
For extra security you should also disconnect the power of the retracts from your RX. Connect retracts directly to its own BEC (say the unused one in your ESC). This will significantly decrease the chance of your BECs ever running into a low voltage/high power draw situation.

This is very easy to implement, and the diode is readily available at any electronics source store, or ebay/amazon for a few bucks. The only downside is that the diode will drop the voltage by about 0.5V, and can be as high as .7V if the current gets high. So, you need to choose the BECs accordingly. After the drop I get 5.5V on main, and 4.6V on spare. This is fine for my receiver (min 4V), but you would have to check the specs of yours, and possibly get a different BEC. Hobby King's 5V BEC for example is 5.5V in actuality, so after the voltage drop it will be 5V.

It took me 15 minutes to solder it all nicely. Now I am worry free.

Andrew

[bigdbc]

Seams like a good idea and fairly easy to do. Thanks for sharing.

[Scerbina]

Originally Posted by bigdbc

Seams like a good idea and fairly easy to do. Thanks for sharing.

Thanks! So far I only installed it into one of my EDF Jets, and it works great. Most importantly it's the confidence and the piece of mind I have now, while flying it

[Max]

Excellent!

That is exactly what I am looking for for my larger electrics. Up to now I was using a separate battery to power the RX and servos so that if anything happened to the ESC it would not cause me to lose my model.

This redundant system would give good peace of mind and allow a smaller battery on board.

Do you have the part number for the diode beside what is shown on the picture? Would be great to be able to get the Sayal part number too If anyone knows it, share it here

[Scerbina]

Originally Posted by Max

Do you have the part number for the diode beside what is shown on the picture? Would be great to be able to get the Sayal part number too If anyone knows it, share it here

Max,

This is the actual part number in the picture. I have attached the datasheet. But, don't limit yourself to this particular diode. Any similar Schottky Barrie Rectifier would work. I would get one with 15A or higher rating (7.5A per side). This is basically 2 diodes tied together on one IC package. If you show the picture to the sayal sales rep, they will point you to the part of the store where they keep these types of diodes. This is a very common type, that is widely used in the power supplies (PC power supplies for instance). It is very important that it is a Schottky type, since the voltage drop in them is minimal. What I would suggest is order some from Amazon for the future, I only paid $4.74 CAD for the lot of 5 shipped.
https://www.amazon.ca/gp/product/B00...?ie=UTF8&psc=1

And get any similar diode from the local electronics store if you need one right now. The summer is short here, and there is no time to wait for 2-4 weeks while it is delivered
That's what I did. I went there, and they had a MBR2045CT, which is the same diode, just rated for 20A. Set me back $3 each. Buying online is way cheaper.

[cravenjw]

Don't forget the heatsink! your voltage drop across the diodes will range from 0.5V - 0.6V. Your going to need to dissipate a far bit of heat. About 10W at 15amps!

Also keep in mind that your devices (RX/servos) will not see 6V, they will see 5.3V-5.4V on your primary 6V feed. If you "fail over" to the second 5V feed, those consumers will only see 4.3V-4.4V. Make sure that is enough to operate correctly. Many servos will not.

[Scerbina]

Originally Posted by cravenjw

Don't forget the heatsink! your voltage drop across the diodes will range from 0.5V - 0.6V. Your going to need to dissipate a far bit of heat. About 10W at 15amps!

Also keep in mind that your devices (RX/servos) will not see 6V, they will see 5.3V-5.4V on your primary 6V feed. If you "fail over" to the second 5V feed, those consumers will only see 4.3V-4.4V. Make sure that is enough to operate correctly. Many servos will not.

Yes I warned about the voltage drop at the bottom of my post. Like I said, check your equipment and get the BECs that would work with your equipment after the .5V voltage drop. HobbyKings 5V BEC for example, has an output of 5.5V, so that ends up being 5V after it passes through the diode.

In regards to the heat sink. There was an extensive discussion on the other forum about using it on diodes. If you feel you need one, sure, why not. If you know your current draw is a constant few amps. Most setups draw up to 0.5A. I have tested this cable in my Su-35 EDF jet, with 11 servos and retracts, 3 of them are digital servos and are gyro controlled. The consumption reaches 0.3-0.5A when I move all of the control surfaces at the same time. The diode does not even get warm. If I simulate normal flying, the current flow is even considerably lower.
Check the specs of your servos, BECs, RX. Test your system

[kcaldwel]

I've been using a dual Schottky diode back-up set-up for awhile now in my sailplanes, total cost for parts per airplane is about $6US.

https://www.rccanada.ca/rccforum/show...7&postcount=27

https://www.rccanada.ca/rccforum/show...5&postcount=29

I use a 1S LiPo as the back-up battery, without another BEC, to save space and weight. I've been using either a 750mAh flat cell, or a 650mAh round HK cell, whichever fits in best. A 1S LiPo provides about 3.7V fully charged at the Rx and servos. My two large sailplanes that I have tested so far, with 6 digital servos each, work fine down to 3.3V. One has a FrSky SBus system that also works fine at 3.3V (3.7 to 3.8V at the battery). I have also tested the system in my 62" Osiris pattern airplane, and it also operates down to 3.3V.

I run the BECs in the airplanes at either 5.5V or 6V, so the normal voltage is 5 or 5.5V. My Tx gives a warning if the voltage drops below 4.7V. The 1S LiPo gives lots of time to land the airplane if I get an alarm. I fly my DLGs on 1S all the time, without the drop across the diode of course, but they last for hours on a 750mAh cell with 4 digital servos. Of course the servos are a bit slower on the lower voltage, but they work fine.

The current draw in most airplanes is actually quite low most of the time. I've attached a chart of a pattern airplane doing the intermediate sequence with 4 digital servos.

I've had two LiPo packs go open circuit on one cell, so zero volts at the connector, in the air on me now. An outboard BEC didn't help in those circumstances. One was a 5S 5000mAh pack in a pattern airplane, which foturnately kept the controls working for 30 seconds after the motor cut out, and I was able to land it safely. The other one cost me a very nice 4m Maxa e-launch sailplane. The battery dropped to zero volts suddenly, and not even when it was under power. All my airplanes I care much about no have a 1S battery back-up. For $6US and less than 30g, it is worth it to me.

Kevin

[kcaldwel]

Duplicate

[Scerbina]

Thanks for the chart Kevin. I would say that is about the same current draw I experience in my EDF Jet. I never tried it with 1S battery. I assumed 3.3V was way too low for my Frsky 8XR receiver, which is rated for 4V. But I didn't actually try to push it down to 3V and see if it still operates that low. I am afraid is a bit too low for it. The IC inside it I believe can go as low as 2V, but if they utilize a high voltage drop inexpensive voltage regulator, than it can get too close to browning out.
I am glad 1S works for you. I would probably still keep mine on 2S with BEC on mid-large size aircraft, just to be on a safe side. It is still very very light. But that is just personal choice