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.

Views: 3193

Comment by Rob_Lefebvre on January 23, 2014 at 9:40am

What exactly are you trying to accomplish?  Reliability, or flight time increase?

Simply plugging 2+ batteries in parallel is a very easy way to gain both.  Complicated battery switching circuits will not give you any extra duration increase, and can actually hurt reliability.

All my high-value machines use 2 batteries in parallel, and telemetry and flight planning solves the problem of accidentally flying too far.

Comment by Toby Lankford on January 23, 2014 at 10:27am

Yes simply plugging batteries in parallel would give you more capacity.  That is not what this post is covering.  This is a secondary emergency/reserve system seperate from the main power.  One of the things the FAA is looking at is how to be safe in the NAS.  That is why several commercial and now hobby class systems are actually moving to on board diversity flight controllers.  A backup power system would seem a natural extension.  If you have a short, a loose cable on you main power system, or any other manner of problems that can occur.  This would be able to get your plane home.

Comment by Rob_Lefebvre on January 23, 2014 at 10:48am

What I'm asking is... how would this actually make the entire system more reliable?  How would you get a short on a battery lead?  Wouldn't a simple fuse handle that easily?  If you have a loose wire, the dual-battery system handles that easily.

I'm not telling you not to look into this.  I've been interested in it myself.  I'm just asking, have you figured out exactly what problems you are looking to solve, and is there an easier way to solve them?


Moderator
Comment by Dwgsparky on January 23, 2014 at 1:11pm

Hi Toby 

I understand what you are trying to do and I also agree with R_Lefebrve that simplicity is best, A Lipo cannot just dump charge in midflight, the energy has to go somewhere usually as heat as a short circuit, A cell can fail (see below) 

The option to switch batteries is dangerous. For it to work you must at some point have both the bad and the charged battery (different voltages) connected at the same time to the same point and then disconnect the bad one.The resulting almost instant transfer of charge to the bad battery will be limited only by the internal resistance of the good battery, It can easily be 10's or 100's of Amps especially on a 6S or 10S battery.

If you want a comparison try connecting a good battery to your flat car battery to boost it, the spark is the same transfer of energy to the flat battery. 

I always use 2 batteries connected in parallel for the whole flight, Each battery has a suitable fuse before the junction of the two packs and if I want to be ultra careful I will fit a 50Amp silicon diode in series with the fuse. this will prevent any charge passing into a battery with a bad cell during the flight and draining the good battery.

This configuration also allows you to monitor your entire system from the telemetry on a single channel and is considered a failsafe redundant system . very easy, very safe and very economical.  

Dwgsparky

Comment by Rob_Lefebvre on January 23, 2014 at 1:35pm

Sparky, your diode needs to shed a bit of heat doesn't it?  

Unfortunately, most of my setups would need to be fused for 100+ Amps, and things start to get a bit big, and heat from diodes, etc. starts to go up.

From what I can see, 99% of failures are a simple open-circuit on a battery.  Or, a battery simply draining much faster than it should.  Having two batteries simply wired in parallel handles both of these cases pretty easily and reliably.

Trust me, I'm interested in reliability and redundancy. I just haven't found anything that makes sense yet.

Comment by Toby Lankford on January 23, 2014 at 1:59pm

I also have flown many times with two batteries in parallel.  When I watch people showing youtube videos of their flights they too are using two or more batteries.  Generally we do this more for endurance than reliability or a combination with endurance being the forethought.  I watch the labels on some videos and they are running 20000 mah in batteries and using 19000 or more out of the batteries.  The multiple batteries are for endurance and pushing the limits not always for safety. On single rotor helicopter with cameras, I used parallel batteries for endurance and to split the amperage draw between the batteries so that they would have a longer utility life.  As opposed to my 3d helis pushing every limit and the batteries having a shorter life.

That being said I love the idea of the silicon diode.  I think that would be the same answer for ensuring that your emergency battery system would not be in danger if the main power failed.  It would prevent that spark or the weak system drawing on the emergency system.  I will be interested in exploring that option.  Thank you for the suggestion.  

As far as a switching system.  What makes me think this is now possible is that they have now flown UAV's with hybrid lipo/ fuelcell systems.  They use lipo for periods of high current draw, such as takeoff and high speed runs, etc.  These systems would obviously have high speed switching and current feedback protections.  I appreciate this is new and different but that is the point.  You have not found things more reliable than the current because we haven't thought of it yet.  I think that is the spirit of this whole website.  Thank you for the input gentlemen.  I will definitely take your ideas and thoughts into account as we move forward.

Comment by Rob_Lefebvre on January 23, 2014 at 2:10pm

But it's not new and different.  That's what I'm saying.  This has been looked at before, and found to not actually be beneficial.  I am interested in seeing what you come up with.  I'm just asking some questions to see where you're going with this.

Better than diodes would be an ideal diode controller:

http://www.linear.com/product/LTC4416

Comment by Toby Lankford on January 23, 2014 at 2:13pm

Well, I am sure it is not my last bad idea.

Comment by Rob_Lefebvre on January 23, 2014 at 2:17pm

Toby, please, I'm not trying to say it's a bad idea.  It's not.  Maybe you have some new solution.  That's what I'm trying to find out.

Comment by Toby Lankford on January 23, 2014 at 2:28pm

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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