A123s/Taranis/Battery Alarms/Noob

[SecretSquirrel]

I have several A123s and I absolutely love them. I recently purchased a Taranis and have been playing with the setup of the radio.

In setting up my radio, I set an alarm for the onboard voltage at 6.1 volts. I read somewhere that a 6 volt alarm was recommended and I added a safety margin. I totally understand the voltage curve of the A123s and realize volts don’t play the same role as they did with NiMh/NiCd. I have only flown the Taranis a couple times and didn’t have any issues with my settings

Tonight I started adding a couple other planes to the Taranis and during setup I noticed something strange. When some of my control surfaces were at full deflection, the onboard A123 would drop below 6 volts, sometimes to the 5.8 range and my low voltage warning would trigger. Of course when the control surface went back to neutral, all was good. I realize a servo at full deflection is going to take more battery power but I guess I am not sure if what I am seeing is normal or I should be concerned.

I guess now I have a couple questions:
1. It didn’t appear as if the control surface was binding but they were pretty maxed out. Could this be what is causing the battery to drop below 6 volts or is this a normal voltage drop during full deflection? I ask because it will drop on say the rudder but not the ailerons , both at full deflection.
2. Also, again fully understanding the A123 voltage curve, is there a number that is safe to put in for a low onboard battery voltage? Is it really 6 volts?
3. I am debating setting a 5 second delay on the Taranis meaning if the battery drops below 6 volts, for more than 5 seconds, the alarm will trigger. Allowing the 3d flying without the alarm but should the battery truly drop below 6 volts for an extended period, having enough time to safely land. Does this make sense?
4. Any other advice would be greatly appreciated.

Knowing me and how particular I am about my batteries, I have a feeling actually seeing the onboard voltage is going to cause me more concern than benefit. In the past, I charged, flew a few flights and noticed a minimal 300 Mah usage and went on with life

Thank you in advance, I know I kind of rambled, I hope it makes sense.

[Kilrah]

Your A123 at 6V is completely flat, as can be seen almost nothing is enough to draw it further down.

Li-Fe's have the worst possible discharge curve from a measurement point of view, as in they have a 3.3V nominal voltage (6.6V for 2 cells) that they will keep for 95% of their discharge curve, only the very start and the very end are steep drops from charge end voltage and to discharge end voltage.

So basically at 6.6V your battery can be 95% full or only have 5% left - at maybe 6.4 it's pretty much dead. The levels being that close causes problems since if your wiring is thin it can cause drops that make your perfectly good battery sound a low alarm.

With Li-Fe's you should put a voltage alarm only as last resort and if it sounds you want to be on the ground in seconds. Best is to never go any close to draining it by keeping a more accurate record of usage time or just charge it often and verify the capacity you put back in is safely far away from the battery's capacity.

I do not love them at all for that reason, especially for powering a receiver setup where it's less easy to keep track of usage than propulsion (here a current sensor / mAh count does the job without issues).

[Daedalus66]

What I'm wondering is how you charge your batteries, and how you ever let them get so far below the normal operating voltage. Fully charged, LiFe chemistry for a 2s pack will read about 7.2v. When in use the voltage will drop fairly quickly to 6.6v and stay there, or very close, for most of the useful discharge curve. By the time it gets to 6.4v you would already be recharging it. At 6.3v, if you ever got that low, the low voltage alarm would say "Land right now!" And you would do so. But you would take the alarm as a lesson never to push that far again.

Used that way, LiFe chemistry can be a good source of power. If you "fully understand the A123 voltage curve" you will see that you absolutely must not rely on a low voltage alarm to tell you when to land. Rather, you need to adopt old fashioned conservative techniques of charging before going flying, keeping track of flight time, checking voltage periodically, and above all not pushing into the region of rapid decline in voltage. Your voltage alarm is strictly a last ditch protection and should never be heard. If it is, you need to revise your battery management practices.

You also need to ensure that your on-board battery has ample reserve capacity for times when you stay longer than usual at the field. I suggest it provide at least twice the flight time you think you might need. That's to 6.4v.

As Kilrah says, it is hard to measure at what point on the discharge curve a LiFe battery is right now. That just means they have to be managed properly with large safety margins. Adopting conservative techniques, lots of people use LiFe batteries very successfully and safely to power their on-board radio systems. But relying on telemetry is NOT the way to go.



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[SecretSquirrel]

I agree with everything both of you are saying. I wonder if something is off in my Taranis settings. I always balance charge my A123s and they "peak" or end the charge routine at 3.6 volts per cell.

Typically I take 3 or 4 planes to the field and I have 4 A123s, so at most I would fly each battery 20-30 minutes. I never really paid attention to volts, until the Taranis and Telemetry, because of the discharge curve. I agree and understand Mah usage and understand the proper way is to fly paying attention to time, recharge, and pay more attention to Mah usage than volts. Where I have probably failed is paying attention to each plane. typically the recharge results in a mere 300 Mah going back into the battery so I have never been concerned.

Admittedly, I have been messing with several planes, on the ground, using the same battery for the past three weeks. I also need to check and see if the signal wire is plugged into the Rx as I have batteries wired to not use a balance plug for charging, specially made by Steve over at NoBS.

So if nothing else, I have learned that a battery alarm of 6v is absolutely incorrect. Understand a battery alarm is absolutely just a fail safe measure in case something goes terrible wrong.

Back to the drawing board I guess. The big question in my mind at this point, is how many Mah go back into the pack I was using for setup. That should be very telling.

[Daedalus66]

Sounds as though you're on the right track. The mAh in will indeed be telling. Good luck.


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