Thread: Telemetry?
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Old 01-07-2017, 12:06 AM   #24 (permalink)
Thunder Fighter
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Posts: 5,247

Join Date: Jul 2007
Location: Melb, Oz

Originally Posted by extrapilot View Post
Yea, but who cares, if the use of that metric is to determine when to land? You have what amounts to a fuel totalizer- a device which tells you have many gallons you have burned, but not how many were in the tank to begin with; you cant determine how much remains.

You want to kill a pack- run on an mAh sensor, with that pack at 3.8v on departure, or with too high a capacity defined for it. Heck, just a real change in temperature has significant effect on battery capacity, and that requires some testing.

It probably works OK for people who fly all the same packs on a given machine, at a consistent temperature, and who check their pack voltage before departing. But what do you do if you find a pack in the bag that shows 4.05v/cell? What do you set your mAh limiter at? There are a slew of ways to get in trouble here. If you use pack voltage- there are no ways you can get in trouble, so long as you fly a given cell count on a given model.
As someone who has flown with mAh telemetry for several years with heli's I can say it is the most reliable method available.

The issues you bring up are easily solved. I have a voltage alarm that activates when I first plug in the pack so I always know my packs are above 95% when I put them in the heli, I also always have a low voltage alarm set so if by some case something goes wrong with the mAh measurement I have a backup that should stop me damaging my packs.

For the multiple size packs I can setup a switch to take account of the pack capacity I put into the heli but now with LUA and the Jeti and some great work by Teros on his RC-Thoughts web page you can automate it so the capacity limit is set by the RFid tag attached to the pack that updates the application on the Tx via telemetry. The Vcontrol also has the capability to do this in an automated way with RFid tags.

You can tweak the capacity measurement to bring the packs down at a consistent level every time and adjust over time if you have packs coming down lower as they age. With the RFid method you can burn a new tag when required at a lower capacity so it is all automated.

With all of this it is a pretty foolproof system, I always leave a little in hand and alarm at 70% used so by the time I land I am well within 25% remaining and this little buffer takes into account any issues with small capacity differences between packs and temperature over the range I fly in.

Voltage measurement in flight might be fine for sports or scale pilots where the load is fairly consistent but it just doesn't work for flying 3D flight and especially when you fly very different headspeeds. Even with averaging of the voltage, if I set my alarm where needed when flying in my fastest head speed and throwing around the heli the voltage drops are large, so will need to be down in the low 3v range, then if I take my lowest head speed which cuts the current draw and load (i.e. voltage drop) on the pack down by 75% (even doing 3d) the cutoff will be way too low.

I could say the same thing about the voltage method in that this might work fine for someone who always flys the same head speed and style in a flight, but I can tell you that when flying very different head speeds (e.g. 1350/1650/1900rpm) and styles of flight within one flight the mAh method is by far the most foolproof and workable solution.

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