A low cost battery back-up system - RC Groups
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May 01, 2015, 01:38 PM
Cognitive dissonance
kcaldwel's Avatar
Discussion

A low cost battery back-up system


After a recent disaster, I no longer trust the motor LiPo as the sole source of power for my airborne systems. I actually had just received the final parts to build this system the day before my crash, but hadn't installed it yet.

I too have had success with a dedicated airborne pack, but the added weight and space requirements, and maintenance of yet another critical battery while still having only one power source didn't seem like the best option for me. Some of the commercial back-up units look quite good. The better ones are quite expensive and involve a lot of electronics. I don't really want to spend $200 dollars on the 4 or more I really should have.

The only RC power system analysis I can find indicates that the battery is by far the highest failure rate component. I have no idea if it is correct, but it is the best information I have seen, and does seem to correspond to my personal experience. It may be that a dedicated airborne pack has a lower failure rate than the stressed motor battery, but is it an order of magnitude better when maintenance issues are included?

https://www.rcgroups.com/forums/show...39&postcount=7

There was a good thread about using a simple, cheap, Schottky diode as part of a back-up system on RCGroups awhile ago. I decided to implement something like this:

https://www.rcgroups.com/forums/show...&postcount=120

http://www.ebay.com/itm/5-X-MBR1545C...E:L:OC:CA:3160

The diode will automatically switch to the highest voltage source. The question I thought about for awhile was what to use for the second power source?

In the end, I've decided to use a 1S LiPo as the secondary power source. This minimizes the added weight and space requirements, and will give a clear indicator for my telemetry system that there is a problem. Setting a telemetry voltage alarm for 4.5V will clearly indicate if there is a failure of a 5V or 6V BEC, and that the 4.2V back-up 1S LiPo is providing the power.

The DLG guys have been successfully using 1S LiPos as their primary power for some time now. Most systems run quite happily on 1S LiPo voltages. Most modern Rx seem to run fine down to 3V, despite what their specs might say. Most servos will operate down to 3.5V or so, but of course their speed and torque falls off. It isn't too difficult to try out a 1S battery at 3.8V to confirm a given system operates.

There is an additional voltage drop through the diode, but in practice it seems to be quite small. I have measured 0.10V no-load, and from the RCGroups thread it seems the drop is quite small for the loads anticipated. The diodes also isolate the back-up cell from feeding back though the BEC.

I did find another thread about a different type of back-up system successfully using a 1S, 200mAh back-up LiPo in a large pattern aircraft (Post #20):

http://www.rcuniverse.com/forum/elec...l#post11102126

I realize leaving a LiPo fully charged all the time may not be ideal. On the other hand, a fellow I fly with just uses and old Triton charger with no storage charge capability, and he leaves his LiPos fully charged all the time and has for many years now. Perhaps the lifetime of the batteries is impacted, but it doesn't seem super critical. The 1S packs are cheap enough that replacing them every year shouldn't be a major issue.

This is the 750mAh, 18g cell I plan to use because that is what I have in my DLG:

http://www.ebay.com/itm/Lectron-Pro-...E:L:OC:CA:3160

Even a 300 to 400mAh cell may be enough to provide the few minutes of power needed to land the airplane, and would be lighter and smaller.

This set-up won't be everyone's cup of tea. For a total cost of about $9 per airplane and an added weight of about 27g (less than 1oz.), it does provide a back-up power source and a clear warning from my Taranis that I have had a primary power failure.

There is an additional lead to plug-in every flight in which isn't optimum. I can check the back-up battery voltage under load on my Taranis telemetry before connecting the motor battery. Hopefully the low voltage warning from the Taranis when the motor battery is disconnected will remind me to to disconnect the back-up battery as well.

A system with the primary BEC set at 5.5 or 6V, and a small back-up 2S LiPo with a reg set at 5V would also work, if the 1S voltage was an issue for the Rx, servos, or comfort level.

Kevin

Edit: a HK 1S cell could take 5g and $5 off the system:

http://www.hobbyking.com/hobbyking/s...ound_Cell.html
Last edited by kcaldwel; May 01, 2015 at 02:16 PM.
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May 02, 2015, 08:51 PM
E sailplane thermal hack
That's GENIUS!!! KC,, super simple,, super Cheap!!
Think I might try that in my latest e sailplane.
Never thought of a shottky diode.
May 04, 2015, 05:06 AM
Registered User
Welcome to re-discovering the basics of a power-box

However... I am a bit puzzled, what kind of issues could led you to permanent loss of propulsion power source, IF the lipos were maintained as should be.

For example, the routine you described above, i.e. keeping a lipo fully charged for long time, is against anything I know as good practice.
We conducted an experiment at our club, using two sets of two packs each, on an Acromaster. One set brought in storage status, the other fully charged, after each weekend flight session. After 6 months both packs who were fully charged puffed, a coincidence ?

So I would also invest little time in packs maintenance, including regular measurement of internal resistance, this parameter also give a good clue about cells aging.
May 04, 2015, 10:48 AM
Cognitive dissonance
kcaldwel's Avatar
I never said I leave my LiPos fully charged. In fact, I always storage charge my batteries after flying.

The two Thunderpower 850mAh 70C batteries I was using in this airplane were run at about 42A (static, probably less in the air), two flights max 150m climb, each charge since new. The two flights always left them near storage charge anyway. I had done three flights once or twice, but usually after very short climbs of 50m or so practising F5J. They were always storage charged after flying. They balanced easily and had no signs of puffing.

The voltage stayed fine during the last flight until it dropped suddenly to zero. It could have been the 10A CC BEC, but it was working fine after the crash with another battery, and has continued to do so.

I have used $12 Nanotech batteries harder, with 3 flights per charge on a series of 1000mAh 4S batteries at 40+A for well over 1,000 flights, with a HK outboard BEC in another glider.

I'm not 100% sure it was the battery, since it was damaged in the crash.

I did have another LiPo that had one cell go open circuit in the air, so I can say for certain that LiPos can die suddenly even when not abused. That battery had been storage charge and used well within it's specs while I had it. It had about 5 flights in the same airplane before I got it. Yet it still went open-circuit while in moderate use.

I was not recommending leaving cells at full charge if you re-read my first post. I was merely stating I have seen batteries treated that way and survive, so I am hoping my back-up cell will last a season left fully charged.

Thank you for your input.

Kevin
Last edited by kcaldwel; May 04, 2015 at 11:39 AM.
May 04, 2015, 11:47 AM
Cognitive dissonance
kcaldwel's Avatar
I've done a bit more testing, and I do get the full 0.40V drop across the diodes under load. That means the systems see 3.8V from my fully charged single cell back-up, and 4.6V from a BEC set at 5V.

The BEC is easy to turn up to 5.5V so there is still 5V during normal circumstances. Both the airborne systems I have tested operate fine down to at least 3.4V, so my single cell back-up would still provide ample time to land the airplane.

The 13g 650mAh HK round cell seems to be the lowest cost and is quite light and small. I don't think many DLG pilots are storage charging their HK round LiPo cells regularly, and I have not heard of any issues so far. The whole package would weigh about 22g (less with shorter leads), take up minimal space, and cost about $5 with the extensions.

I certainly never claimed to have invented any of this and have provided links to the original threads. It seems like it would have avoided the loss of my glider. I hope this might be of some use to others that weren't aware of the simple diode back-up system.

Kevin
Jun 05, 2017, 10:02 PM
Oleg Golovidov
olgol's Avatar
Kevin,
have you been using these diode backup systems in your planes? Any more comments or new experiences? Any saved planes?

I bought a pack of 5 diodes a while ago and finally decided to build one. I just followed your description picture and made me one and will be using it it. Hope it never becomes needed in flight
The diode+wires+connectors weighs about 7.5g. The 650 mah battery about 13.5g. Total about 21g.

The voltage drop (no load) is about 0.2-0.22 V.

--
Last edited by olgol; Jun 05, 2017 at 10:23 PM.
Jun 05, 2017, 11:10 PM
Cognitive dissonance
kcaldwel's Avatar
I've been using them in all my e-gliders and a couple of pattern airplanes with a 1S back-up. So far, I've never needed it since I've started installing them. I could have used a back-up battery pretty badly a couple of times right before that though...

Last year, I only flew 1000mAh and larger batteries in my e-gliders, and had no battery issues like I did with the smaller high C rated packs. Unfortunately Revoelectric seems to have stopped making the 1000mAh , 82g, 70C pack I flew last year, so I have some 80g 750mAh packs I'll have to switch to when the Revo packs start to fade.

I haven't been flying this year, so that means less battery problems.

Kevin
Jun 05, 2017, 11:18 PM
Registered User
I use this setup too after Kevin suggested it. Works great. I use 1S as well as 2S for backup batteries.
Jul 31, 2017, 12:56 PM
Oleg Golovidov
olgol's Avatar
This backup gadget saved my plane yesterday.

Either I forgot how many times I launched off the battery, or it did not charge well between the flights. I thought I only launched twice on it... The launch was limp and I got only about 50m altitude. I thought I had an issue with the new motor (again). Then a minute or two into the flight, my low voltage alarm triggered, which told me that the main power source was no more, and the plane was powered by the backup system. I continued flying for another minute maybe, going up in a good thermal, but then came to my senses and brought the plane to the ground as carefully as I could. There seemed to be some reduced flap power at higher speeds due to lower voltage, but otherwise I probably could have flown a full 10 minute task on the backup. Very glad I had the backup power system...
Jul 31, 2017, 01:09 PM
E sailplane thermal hack
If you have receiver telemetry then if you use a separate receiver pack you'll ALWAYS know the state of your receiver pack. It's isolated from BEC and it never has to strain. Even though TWO power sources are obviously better,, it doesn't seem to me your gaining much here,, a 2s 1200lipo receiver pack only weighs 1.7oz ,, and with the backup redundant system you have more failure points,, would be easy to become complacent about maintenance on backup since most times would never b needed. Plus I believe the 2s lipo as receiver would give linear discharge curve,, you'd know WELL in advance when receiver pack was starting to get low.
But this backup system IS more redundant .
Just my 2cents
Last edited by Airman74; Jul 31, 2017 at 01:15 PM.
Jul 31, 2017, 01:28 PM
Oleg Golovidov
olgol's Avatar
Jerry, there is certainly some truth you what you say. Still I like this setup more.

With a separate Rx pack, it needs to be 5V for me (low voltage servos), and it needs to be big enough to last a few 10 min flights, preferably a full day of flying. Otherwise it needs to be charged during the day. That's another charger, and more hassle.

With this backup system, the secondary battery does not get used at all, stays fully charged forever. I have not charged it for many weeks now, just check the voltage every time. When connecting before a flight, I connect the backup first, I see the voltage on my radio, I hear the "low voltage" alarm, then I connect the main battery and verify that the alarm is off now. No complacency, very convenient and simple.
Jul 31, 2017, 01:50 PM
Cognitive dissonance
kcaldwel's Avatar
Glad it worked for you Oleg!

I just got one of these very cool little adjustable voltage and current supplies, that will make it much easier to test the low voltage performance of airborne systems for 1S operation. I always wanted one for home, but they used to be huge and expensive.

http://www.ebay.ca/itm/DP-30V-5A-Con...72.m2749.l2649

Kevin
Jul 31, 2017, 01:56 PM
Adam
xStatiCa's Avatar
That is exactly the reason I bought one of those a couple months ago. Well worth the money to me. I got tired of draining batteries on my charger to test at different voltages.
Jul 31, 2017, 02:37 PM
E sailplane thermal hack
Quote:
Originally Posted by olgol
Jerry, there is certainly some truth you what you say. Still I like this setup more.

With a separate Rx pack, it needs to be 5V for me (low voltage servos), and it needs to be big enough to last a few 10 min flights, preferably a full day of flying. Otherwise it needs to be charged during the day. That's another charger, and more hassle.

With this backup system, the secondary battery does not get used at all, stays fully charged forever. I have not charged it for many weeks now, just check the voltage every time. When connecting before a flight, I connect the backup first, I see the voltage on my radio, I hear the "low voltage" alarm, then I connect the main battery and verify that the alarm is off now. No complacency, very convenient and simple.
Yes,, points WELL made,, duly noted. Maybe I should switch too ;-)
Jul 31, 2017, 02:41 PM
Registered User
Quote:
Originally Posted by kcaldwel
Glad it worked for you Oleg!
I just got one of these very cool little adjustable voltage and current supplies,...
Kevin
I don't quite follow how these are being used, can you elaborate? Are they indirectly able to apply a fixed/known load so you can see voltage (suppression) vs. nominal? That my only beef with all the lipo field testers out there. They are handy & useful for cross checking cell voltages through the balancing harness. But they are glorified volt meters that presume a discharge curve to yield % used/remaining indication. This can be misleading & lull you into false security, especially low maH capacity cells. I still have an old school Ni* RX battery tester, JR maybe? When push the load button & see the LED's cascade down you knew pack was unsafe under typical servo load, whereas nominal voltage looked ok.


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