Reserve/Redundant Battery Switching

Good Morning!!

I am writing this morning about a different topic than my build because I am mad at the post office. My RVJET should have arrived two days ago and I am still without a plane. It is in the teens outside today so I thought it would be a perfect assembly day. Instead, I am going to talk about a safety system I have been thinking about for our drones.

Have you ever had a lipo suddenly dump its charge in the air? Maybe, you have had a particularly good day cloud surfing and lost track of time or distance? I would like to come up with an emergency battery switching system for such occasions. This post will be more of a brainstorm and ideas of what capabilities would be needed. I can get into hardware/firmware specifics as it is developed.

I think the first thing people are going to ask about is weight. People are pushing X-8's to 3.5kg, I think the RVJET can push it close to 3-3.5 with batteries and cameras for long flights. The Skyhunter claims 20-25,000 mah worth of battery capacity. So I would think that you would want at least 20% of main pack capacity in your reserve pack. The test plane that I will use for this is an old 8ft Telemaster. These are docile craft with very predictable flight characteristics. I have modified this with motors in wing pods that will have 10000 mah capacity each (two 6s 5000) I would have to add a 2000 mah battery to each wing to give me the reserve battery power. In a plane that can easily go all up at 7kg we have some room to add some batteries and hardware for a switcher. I think the last time I had it all up with mapping cameras it was around 5kg. With two 2450mah packs at 400g each I should be able to keep the necessary items well below the two kilogram cushion I have.

We have an air frame and an idea. I have not thought of all the parameters or what all needs to be measured to indicate a possible failure. The first and easiest is when you have discharged to 20% of main power that batteries could switch to auxiliary power. Severe voltage fluctuations, amperage flow drops, internal resistance spikes, overheating are few of the possible indicators of a faulty cell or power system. This will be an open project and I welcome any and all collaborators. If you have other ideas of what is needed in the firmware or the hardware, please comment. I think this would be a system independent of autopilots and osd's for cross platform integration. I think that you would be able to see when it switches to auxiliary power by a spike or drop in the battery level indicator on osd's. Another use would be a battery cutoff. If you are running multiple parallel packs. If a single pack is faulty it can cut it off from the system so it doesn't draw down the good pack. These are all initial thoughts. Thank you for reading and I hope that you have happy and safe flying.

Toby, I think you have to look more carefully at the failure modes you are trying to protect yourself against.

The most likely power problem you are likely to encounter is the speed controller failing. The next is a wiring problem. Both of these are most likely to be downstream from any battery switching arrangement.

Battery problems - a LiPo does not suddenly go dead in flight and draw all the power from any parallel connected battery. The worst that can happen is an intercell connection failing, causing the battery to go open circuit. This is where it really is worth having two batteries in parallel. An intermittent connection in a battery can be one of the most confusing and destructive bugs in a system you can come across, especially when this connection failure is subject to vibration.

Monitoring - if you are into logging data and analysing performance it might be a good idea to have a current monitor for each of the two batteries. In the case of a short flight due to unexpected short battery life, you can determine if one of the batteries is faulty, or if the excess current draw is down to a problem downstream.

Silicon diodes - most silicon diodes exhibit a voltage drop of 0.7 volts acroos them. 0.7volts x 40 amps means you have to dissipate 28 watts!! There are shottky diodes available which have a lower voltage drop, but with high currents, you are still looking at high wattages. The better way to go would be with some FET transistor arrangement, but now you are into development and getting well away from keeping it simple.

Connecting batteries of different charge in parallel - some upmarket battery chargers allow this. They use a polyfuse to limit any high current between the batteries. This is a device that increases its resistance when conducting a current over the threshold it is designed for.

Fuses - wire fuses typically do not blow at an accurate current reading. They can fail at up to three or more times their rating. There are "slow blow" and more accurate fuses available, but often these are in bigger and more expensive packages. You are back into research again.

Advice - Two batteries in parallel, then look to your wiring, checking procedures, using a dedicated battery/UBEC for your electronics etc. Use battery voltage monitoring to detemine when to land instead of relying on your speed controller to save your batteries.

There is nothing so simple that it cannot be improved without being more complicated.

Jack.

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Comments

Bill Salopek March 11, 2019 at 9:02am

Hi folks...I just thought I'd drag up this old thread to see if there's been any new thoughts or developments about an emergency backup battery, and whatever switching is needed. (Like Tom said, backup power supplies for computers exist, so it shouldn't be much of a stretch to apply whatever they are doing to drones.)

I typically fly with 2, 3S1P batts in parallel...really like that setup for the stated advantages...though I saw discussion of fuses and diodes, which I don't use...could some explain those more fully? And where to buy and how to install?

Anyway, any new thoughts on this matter?

Thanks...

--

Bill
Tom January 25, 2014 at 1:02pm

Toby,

Have you looked into super capacitors? I know that with that tech you could get the weight/size down to almost nothing and still accomplish what you are seeking to do. As for the "switch" from bad battery/good. I would look at how some of the higher end UPS's (for computers) are doing things. I have messed around with bad UPS's in the past and know that the control board for those things are not very big. So, if you combine a super cap and a small control board from as UPS (mod'ed for DC input/ output) you might be able to achieve what you are looking for.
MR60

Hugues January 25, 2014 at 1:28am

Regarding FC redundancy it is very much complex to produce for a simple reason: for dual redundancy you need at least three FC and a voting system. Indeed if you just have two FC you cannot always define which FC gives the right output. So you need a voting system: if two FC produce the same output then the third is most probably the failed one.

Conclusion: what Leonardo said: ultimate sophistication is simplicity.
Hendrikus Kampman January 24, 2014 at 3:44am

How about using two batteries each connected to a smart microcontroller based DC-DC converter? Then using the DC-DC converter outputs in parallel? Like any solution it has pros and cons. But you'll be able to monitor both batteries individually and shut one battery down if it dies; without causing any power disruptions or discharging the good battery into the dead one.

Hendrikus
Wanabigaplane January 23, 2014 at 2:28pm

Toby, I think you have to look more carefully at the failure modes you are trying to protect yourself against.

The most likely power problem you are likely to encounter is the speed controller failing. The next is a wiring problem. Both of these are most likely to be downstream from any battery switching arrangement.

Battery problems - a LiPo does not suddenly go dead in flight and draw all the power from any parallel connected battery. The worst that can happen is an intercell connection failing, causing the battery to go open circuit. This is where it really is worth having two batteries in parallel. An intermittent connection in a battery can be one of the most confusing and destructive bugs in a system you can come across, especially when this connection failure is subject to vibration.

Monitoring - if you are into logging data and analysing performance it might be a good idea to have a current monitor for each of the two batteries. In the case of a short flight due to unexpected short battery life, you can determine if one of the batteries is faulty, or if the excess current draw is down to a problem downstream.

Silicon diodes - most silicon diodes exhibit a voltage drop of 0.7 volts acroos them. 0.7volts x 40 amps means you have to dissipate 28 watts!! There are shottky diodes available which have a lower voltage drop, but with high currents, you are still looking at high wattages. The better way to go would be with some FET transistor arrangement, but now you are into development and getting well away from keeping it simple.

Connecting batteries of different charge in parallel - some upmarket battery chargers allow this. They use a polyfuse to limit any high current between the batteries. This is a device that increases its resistance when conducting a current over the threshold it is designed for.

Fuses - wire fuses typically do not blow at an accurate current reading. They can fail at up to three or more times their rating. There are "slow blow" and more accurate fuses available, but often these are in bigger and more expensive packages. You are back into research again.

Advice - Two batteries in parallel, then look to your wiring, checking procedures, using a dedicated battery/UBEC for your electronics etc. Use battery voltage monitoring to detemine when to land instead of relying on your speed controller to save your batteries.

There is nothing so simple that it cannot be improved without being more complicated.

Jack.
Rob_Lefebvre January 23, 2014 at 1:53pm

I'm not entirely sure if there is a dead period, I haven't honestly looked at it that closely. I don't believe there would be a dead period.

Currently, the Pixhawk actually uses something like this (but much smaller, built-in FET's, 3 sources) to switch between the different supplies (USB/Input Rail/Battery Input). You can hot-swap no problem.
Moderator

Dwgsparky January 23, 2014 at 1:49pm

Hi R . and Toby

I also prefer the simplicity of just putting the packs in parallel ,yes it helps for the duration but it is inherently safer than one pack . Regarding the diodes there is a simple solution, yes you will lose a little power due to the forward volt drop on the diode (lets say 0.2v approx) so a 100Amp system will lose about 20watts out of 1600 watts in use on a 4S system. the heating effect can be greatly reduced by putting 2 or more diodes in parallel .

So for the same 4S system at 100Amp lets put 3 diodes each will take 1/3 of the current = 33amps or 6.6watts / diode. nice and cool now. !!

R_L .. does that device switch without loosing the Vs out? or is there a small dead period?

I still prefer (and use) the simple 2 battery / 2 fuse approach, it works very well. Good luck to Toby , I wish you success.
Toby Lankford January 23, 2014 at 1:47pm

I think that is a wonderful idea
Rob_Lefebvre January 23, 2014 at 1:45pm

Ok, well, my suggestion for your research moving forward, would be to look at the idea diode controllers such as the one I linked. These have a number of benefits. Also, they would allow us to "see" and control what is going on better than passive diodes and fuses would.
Toby Lankford January 23, 2014 at 1:28pm

I just think that the technology is on the cusp or already here and not implemented yet to add a degree of safety. Honestly, when I first saw multicopters several years ago I said many of the same things. It is inherently unstable. That the flight algorithms were too complicated for the available processors. There are too many moving parts for fail points, etc. Truth is I love my multirotor and it is the absolutely simplest thing I have ever flown.

I don't have a solution, yet. I just think it is the next natural progression of flight safety. I am going to work towards it and I hope that people way smarter than me jump on board. The thought is not for endurance. I do the same as everyone else right now. I just load it with more fuel. I am thinking that a reserve 2000+ mah system on a 10000 mah main power system would be handy in an emergency. I think complete dual path is going to be a requirement from the FAA to try to freeze out small UAV operators. If we can come up with an inexpensive solution then all the better for everyone.

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