Long Story of Rx emergency Backup power supply

[PeteMitchell]

The loss of my Prestige 2pk due to a severed power supply lead, convinced me that I had to use an emergency backup power supply.
Much had been written about a light weight low cost backup unit in RCGroups, but it became more understandable to me when Colin Paddon  showed me his version of it, using a small 300mAh single cell lipo for power coupled with a 5V booster board.

This tiny unit seemed to work perfectly and after much testing on the bench I installed one in each of my models.

But the moment came when it was needed and it didn’t work.  

The occasion it failed was during a trimming flight.  It was after the second launch on the installed 4s850 lipo ( plus a few short motor runs back to height) and a longish glide flight with my Plus X.  I forgot about the state of the battery until telemetry and the tx voice warned me that I had ‘low voltage’. Fortunately I was at that moment already in my landing approach so did not panic, all was well and in control until I pulled full Crow breaking, at this point all contact with the model was lost. Fortunately as the model was only at about 4 or 5 ft off the ground  damage  was minor.
Investigation after showed that my 4s850 lipo had been run down flat ( less than 5% capacity). The backup unit was working perfectly; I came to the conclusion that whilst it worked when bench tested, it could not supply sufficient Volt/Amp when full Crow was used in flight. And  I assumed that this had caused the backup board to overload and it had cut power to protect itself. 

My Plus X has 6 HV servos in the wing and 2 more in the fuselage, so pretty obvious that it needed more volts. First step was to set my esc to output 8.2v BEC.  I found another type of booster board on eBay, one with a variable power output and another 1s/380 mAh lipo. This setup gave 8v. and has seemed to work well during the past year, plus I am more careful now not to exceed battery usage. But still when Crow Breaking has been used the tx always warns of low voltage. There has never been any loss of control, but the warning is always a worry. So I lost faith in booster boards and have changed again to now use a 2s 350 Li ion pack which I have made up my self. When fully charged these cells show 8v on my DVM, but when measured under load in the model this drops to a stable 7.4v So far this has been successful in that the cells hold  voltage far longer under load than the booster board setup and I do not get the ‘low voltage’ warning when Crow Breaking. And they weigh only 3grm more than the booster board/lipo. 
I also used to run a separate power supply from my esc/bec to the rx to bypass the Altis. But with this and  the backup unit there was so much extra wire in the cramped fus, which all had to be moved every time a lipo was changed, it gave to my mind so many extra possible fail points that I decided to find a more simple way.

I now power direct from the ESC/BEC to Rx via the Altis. And the backup Li ion powers directly to the RX via a spare port. As can be seen in the attached, there are no diodes etc
My Tx telemetry alarm is set to warn at 7.6v. When I plug the backup in to the model, telemetry warns ‘Low Voltage’. When the main power is also plugged in the warning stops, and the telemetry shows RX voltage at 7.9v. This drops to 7.8v if the backup is removed.
So far so good, the new setup works and no unwanted warnings are sounded, or escaped smoke.

But as an electrical dunce I would be very pleased to hear from others about what I am doing and any reasons for what and why I should

change anything.

[cirrusRC]

I'm rubbish at electric theory but I thought diodes were needed to isolate the power sources and stop them feeding into each other?

 

[PeteMitchell]

1 hour ago, cirrusRC said:

I thought diodes were needed to isolate the power sources and stop them feeding into each other?

 

Same for me I have tried to educate myself but its hard as you get old

I have an orriginal Astro Watt Meter that I used when flying indoor models. It measures small voltage. I put it inline between the rx and the backup and repeated this between the rx and esc/bec. Bench testing by spending 10mins twiddling the sticks and the result was that there was a measureable voltage but it was just .1 or .2v 

As flights are not that long and lipos are changed each flight I could not see what benifit there would be from fitting diodes, so didn't. 

But thats the part that concerrns me ?

[John H]

Pete, the arrangement in your dia connects the two batteries together in parallel, they will both discharge together.  Whilst this may be helpful if one battery is disconnected it is of no use if the batteries are flattened.  Another issue with this arrangement is if one battery is disconnected you will not get an alarm on the Tx.  

[Justin Mellor]

Monitoring the 4s pack voltage by reading the voltage from the ESC is not a good idea! By the time that the time that the 4s pack has discharged enough for the ESC voltage to start dropping, they will be almost completely discharged. 8V/4 = 2V/cell! Connecting another 2S pack in parallel is likely to confuse the situation. You may be charging the 2s pack from the 4s pack at some point .

At the 7.6V warning, you will have 3.8V/cell on the 2s pack and the 4s pack may already be dead at that point. You would definitely want to get down quickly.

To properly monitor the 4s pack voltage, the battery voltage before the ESC needs to be measured - ideally with a voltage checker connected to the balance lead connector, checking the voltage of each cell.

Hope that helps - Justin

[PeteMitchell]

35 minutes ago, John H said:

they will both discharge together.

Having used this setup with the boosterboard backup for the past year and many flights I have never found that the backup battery lost any capacity. At least when the 1s380 lipo was recharged it never took more than50mAh to get back to 100%  My test with the Wat Meter inline shows that only .1 or .2v is drawn from the backup battery when the main supply shows upto 7+v being drawn when powering the servos

35 minutes ago, John H said:

one battery is disconnected you will not get an alarm on the Tx.

In testing when the main lipo is removed from the esc, the tx warns 'Low Voltage'

[John H]

11 hours ago, PeteMitchell said:

Having used this setup with the boosterboard backup for the past year and many flights I have never found that the backup battery lost any capacity. At least when the 1s380 lipo was recharged it never took more than50mAh to get back to 100%  My test with the Wat Meter inline shows that only .1 or .2v is drawn from the backup battery when the main supply shows upto 7+v being drawn when powering the servos

In testing when the main lipo is removed from the esc, the tx warns 'Low Voltage'

 

11 hours ago, PeteMitchell said:

Having used this setup with the boosterboard backup for the past year and many flights I have never found that the backup battery lost any capacity. At least when the 1s380 lipo was recharged it never took more than50mAh to get back to 100%  My test with the Wat Meter inline shows that only .1 or .2v is drawn from the backup battery when the main supply shows upto 7+v being drawn when powering the servos

In testing when the main lipo is removed from the esc, the tx warns 'Low Voltage'

Pete, sorry for my hasty response, I was looking on my phone and thought you were connecting two batteries in parallel – I did not realise one was via a BEC. There are still issues with the design as others have pointed out that I will discuss later

[PeteMitchell]

11 hours ago, Justin Mellor said:

To properly monitor the 4s pack voltage, the battery voltage before the ESC needs to be measured

I do agree Justin, my packs are recharged and ballanced after each flight, and I know a 20/30sec motor run will consume upto 40% of a 750/850 4s pack.

The situation described came about because I was trimming the model, generally enjoying myself and forgot to keep track of motor run time.

Thats one of the reasons for a backup, but my main concern is if a wire should break or some other fault develope in flight, maybe the Altis or a servo.

[steve knowles]

I’m certainly not an electronics expert but at the risk of being well "wide of the mark" I thought it might be worth highlighting the following.

As others have noted, logic would suggest that the two batteries would feed into each other, hence why most backup solutions include a diode.

However, in this case this doesn’t appear to be happening, so is there something within the YGE BEC circuitry that prevents this happening.

The text below is from the YGE operating instructions. Although this refers to using a separate battery as a "buffer", does it offer any any explanation why Pete’s system appears to be unaffected by not having a Diode?

“The BEC-Voltage can be adjusted in 0,1V steps in the PC[1]Software. It is possible to use a buffer-battery (not 35LVT!), no diodes are necessary.

Caution! Make sure that the battery-voltage matches to the BEC-voltage. Too big deviations can damage the ESC or the battery. We recommend to use a 2S-Lipo for a BEC[1]voltage of 8,0V.  You can also use a buffer like the Ultra Guard from Optipower." 

[Dick Whitehead]

I think it just means that the YGE BEC output may already have a diode in it (I have Kontronik ESCs that are also OK with a backup battery in parallel). The suggestion that the battery voltage should be close to the BEC voltage does make me wonder a bit, but that could just be to stop people using silly combinations as the diode will have a max voltage/current rating.

I use a 2s Lipo in parallel with an 8.4v external BEC via diodes to provide a "backup" type dual supply. I also use the BEC via a diode and resettable fuse to keep the 2s Lipo charged up during flight, and it stays at about 8.2v so no need to worry about forgetting to charge the battery.

Dick

[John H]

A more considered responses to Pete’s idea.

In this text I have quoted some voltages that Pete has measured in his test circuit.  Note that as we are working to relatively small voltages there may be some error in these.  I have also used Steve’s quotes from the YGE instructions.

In its normal mode of operation Pete has his BEC (YGE68) output set to 8.2 volts.  This being supplied by the main Li-Po.  This voltage is connected to the backup Li-Ion battery via the receiver connectors.   This Voltage, being higher than the Li-Ion voltage, will cause a current to flow into the LI-Ion battery and charge it.  Unfortunately, the max specified voltage for a Li-Ion is 7.4V (7.6V in some cases).  I.e., the Li-ion is being over charged.  Pete has measured this overcharge current to be about 200mA.  This power will be dissipated by the Li-Ion as heat.  Pete assures me that the LI-Ion battery does not get warm. Whilst treating the Li-Ion like this is not a good idea, it seems that the short time it is in operation combined with the small numbers involved and the robustness of the Li-Ion battery means he can get away with it.  Personally, I would lower the BEC output to match the recommended Li-Ion max voltage. This is born out by the statement in the that Steve found in the YGE documentation “Caution! Make sure that the battery-voltage matches to the BEC-voltage. Too big deviations can damage the ESC or the battery. We recommend to use a 2S-Lipo for a BEC[1]voltage of 8,0V. “

Should the main LiPo fail (or go flat, or become disconnected) then the BEC output voltage will fall below that of the Li-Ion and the Li-Ion battery take over powering the receiver (and associated servos).  This operation relies on the BEC behaving like a diode, ie blocking the Li-ion current going back into the LiPo.  This is another concern as the reverse voltage characteristics of most BECs are not specified but, once again, it will more that likely be OK.  Indeed, Pete has tested this to be OK in his arrangement.  Note that the Li-Ion battery must be of large enough capacity to power the servos long enough of to land safely. This maybe larger that you may expect battery due to the high current taken by flap servos under load and the typically higher internal resistance of Li-Ion batteries to that of LiPos. The YGE68 ESC/BEC instructions do indicate that this type of operation is ok for there BEC “It is possible to use a buffer-battery (not 35LVT!), no diodes are necessary.”

In Pete’s circuit he has a voltage alarm set at 7.6V ie below the LiPo battery but above the Li-Ion battery.  This alarm would activate should the main battery feed fail.  Should you lower the BEC voltage as I would recommend this alarm voltage would also need lowing to say 7.2V, it would then activate when current is taken from the backup battery so dropping its voltage.

In conclusion this arrangement is not of the best engineering design although YGE do seem to have considered it and given it the thumbs up under the conditions discussed above.  This is a practical solution to protect against the most common faults of flat or failed or disconnected main battery.  If you use this idea test thoroughly on the ground first.

[PeteMitchell]

My thanks to all who have contacted me, privately and on the forum about this.

I am very happy that to learn that my emergency backup solution has been thought of before, and even referred to in the instruction manual for YGE esc’s which of course I have never read completely through before.

My way of arriving at my solution is not to be recommended as it involved limited knowledge of electricary, trial and error plus the assumption that no smoke is good.

But I would recommend any reader of this thread to seriously look at ways to protect your expensive model with an emergency power backup system of some sort.

[thermaldoctor]

Very interesting subject. I've never done it in RC cars or latterly gliders and not had a failure in nearly 40 years but doesn't mean it can't happen. Sh*t can and does happen as I have found out recently - not RC. So of course it is very wise to protect an expensive glider from a power failure whether connector, battery or BEC. 

Will check out the YGE claim with Heino Jung of YGE. If it is so, then that is a great and very easy solution.

Also consider Jon Wells Practical RC Red switch. Its a smart mag switch with 2 inputs and 1 output. It will feed whatever is the higher voltage input into the receiver. If the higher one goes lower voltage than the other input then it will automatically switch power supply. 

Regards

Neil

 

[MikevE]

I have to say it was horrible to watch Terry's Eternity smoke in at Horsham with a failed battery. That stiffened my resolve to sort out my own backup battery.

A key point is that there must be some sort of telemetry alarm so that you know if your system is running off the backup. 

[thermaldoctor]

Can now totally confirm that the YGE 65LVT has full battery redundancy built into its circuitry. You need to simply Y-Lead the main BEC power supply with a similar voltage back up battery. I did it at PCB level but of course it can be done very easily at any level.

You need to connect the main drive battery first, then wait a few seconds and connect the auxillary.

For example I run my BEC  at 6.0v. So I used a small 4 cell nimh back up battery giving approx 5.5v . Too much difference in battery voltages does make the connectors a bit warm and there is current flow. +/- 1v seems the tolerance. So stick to a reserve battery of similar voltage to what you run. Or...alter your BEC voltage to accommodate.

So it makes it very easy to do a redundancy with the YGE. But then the YGE has best in class really strong 8A constant BEC with 15A bursts so I have never needed it but........

For me, although it is more expensive, with YGE you have it all in one one complete small package: strong BEC and battery backer. So much better for me personally than a cheap drone esc with lots of wires, a separate BEC dangling about and then an extra set of battery backer components rattling around in addition.

For sticking in a £2k+ glider I only want the best

Best regards

Neil

 

 

[John H]

Thanks Neil, pleased to have my theory’s confirmed. Thanks also to Pete and Steve for the info that led to this conclusion.  But mostly well done YGE for correctly specifying there design.  I’m sure better documentation would have lead more people to adopt this method of back up and YGE components.  Hopefully this brings the long story and much misinformation about backup to a conclusion.  

[PeteMitchell]

Glad my solution is confirmed, at least for YGE users

But  I would not recomend a Y lead connection for the backup as this will still give only one power supply to the rx.

I prefer a dual supply as per my pic in the orriginal post. This would have saved my Prestige which crashed due to the single power supply wire being severed.

And my new emergency backup has already proved it works when I recently again forgot to keep track of motor runs when trimming a new Ultima2.

Lost another 4s850 lipo, but saved my model

 

 

[DaveH]

I have a little experience of a battery backup system and tested a few of the components on the bench. 

My plan was to power the Rx via an ESC/BEC device and to supply backup power via a small BEC/UBEC device (and small 2sLiPo).   I tested a bunch of small BEC/UBECs on the bench, checking what output they were capable of and also trying to determine how they might react to transient power draws as might be experienced when I hit the flaps or waggle all the sticks at once.    

Some of the U/BECs managed their rated current output with some drop in voltage, some were very poor and one was significantly better (two blew during tests).    The surprise was that a couple of the UBECs collapsed their output to zero when hit with a transient overload.  These UBECS did not restart until they had been powered off/on.  The (linear) BECs generally reduced the output voltage during the overload (if reduced too far, this can cause a brown-out in the RX).   These BEC/UBECs were 3 to 4A versions (hoping to stay small and light).  

I would suspect that those voltage boosters might have performance issues in much the same way. 

Several UBECs and BECs were destroyed during testing.   Better to burn them on the bench than in flight.

 

 

[Jon_S]

Are there any commercially available devices at all ?