Monitoring of motor battery in flight

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rcgyuk
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Monitoring of motor battery in flight

Post by rcgyuk » Mon Jul 29, 2013 9:27 am

Not sure if this is possible or not, but here is my scenario:

I fly several different models, which all use different cell counts on the main battery.

I can use telemetry, to monitor the voltage under load, and I can use 'play voltage' to tell me the voltage at any time in flight.

I can also set a warning to tell me when voltage drops to a value I choose.

Problem is trying to remember what constitutes 3.3 volts per cell for the plane I am flying at the time, and then work out in my head how close I am to that figure.

Suppose I have a 5 cell LiPo, and when the voltage under load falls to 16.7v I want to start my landing approach.

What I want is for 'play voltage' not to tell me e.g. 17.5 volts, but (17.5-16.7) volts, i.e. tell me I am 0.8 volts away from where I have decided I want to land.

Any thoughts?


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jhsa
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Re: Monitoring of motor battery in flight

Post by jhsa » Mon Jul 29, 2013 9:45 am

Program your alarm 0.8V higher. we should always consider the voltage under load.

This works on the 9x. I suppose that for the taranis is the same..
Custom switch:
CS1 a<x A2 (or whatever sensor) 16.7V

Function:
CS1 Play track (flight battery alarm track number/name) repeat (seconds)

Like this you will have an alarm everytime the voltage goes under 16.7V. That might be for example when you increase the throttle for climbing but not at level flight.. At the end of the day you are the one who decides when to land but bear in mind that the voltage shouldn't go under a certain voltage under load or it could damage the cell/ cells. I always give a certain margin in case the cells are not well balanced for some reason..

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MikeB
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Re: Monitoring of motor battery in flight

Post by MikeB » Mon Jul 29, 2013 1:09 pm

What sensor are you using I'm now using a FAS100 which gives me the current as well as the voltage. With this, the Tx will also tell me how much of the battery capacity has been used, and I can set an alarm when it reaches a certain point.

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jhsa
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Re: Monitoring of motor battery in flight

Post by jhsa » Mon Jul 29, 2013 1:33 pm

that is assuming that your battery still delivers it's full capacity.. If not, the voltage will be too low before the capacity alarm sounds..
I've seen it happen from one flight to the other.. :o
It happened to one of my cr***y nanotechs that are supposed to be good.. Yeah right.. :(
For me mAh monitoring is nice, and having an aproximate battery capacity monitor is also very good, but I think that what really matters on all Lithium batteries is the voltage.. it should never go under a certain limit under load or no load at all.. If it does, the battery will be destroyed..
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gohsthb
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Re: Monitoring of motor battery in flight

Post by gohsthb » Mon Jul 29, 2013 1:52 pm

Being able to see the capacity AND the voltage under load gives an indication of how the C rating of a lipo is doing. I think what really goes on a lipo is the c rating. As the pack ages or gets abused the c rating drops, and the pack can't deliver the same number of amps under a load. This causes the voltage to drop more, which may drop the voltage too low on older packs. Causing abuse of the packs and further degradation. I attribute some of the longevity of my lipo packs to having the telemetry which tells me when the voltage is dropping too low.
-Gohst


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ShowMaster
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Re: Monitoring of motor battery in flight

Post by ShowMaster » Mon Jul 29, 2013 2:32 pm

I agree. I have packs that read 12.4 no load, they charge ok. In the same plane I always fly I can see them drop in flight quickly where others don't. Really a good test of the batteries capacity. I number and date all my packs and that usually helps decide it's time to retire a pack when the capacity drops.
I like the telemetry feature.

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Re: Monitoring of motor battery in flight

Post by G550Ted » Mon Jul 29, 2013 2:43 pm

Perhaps that's more of a reflection of increasing internal resistance and not a loss of capacity.

Ted

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ShowMaster
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Re: Monitoring of motor battery in flight

Post by ShowMaster » Mon Jul 29, 2013 3:29 pm

Does it really matter what we call it? To the best of my knowledge there's no way to reverse an internal resistance rise in a lipo so as it rises the packs posted capacity at the required current decreases.
Wattage delivery P= E squared/R. As R (internal resistance) goes up power delivery goes down. Assuming the plugs, wiring, esc remain the same, more internal R the less power delivered to the motor.
In the end our motors are rated at so much wattage to a prop to develop the thrust we need without overheating the motor.
So to simplify my comment. As the battery ages and has been repeatedly charged and discharged , and maybe abused, it's internal resistance will rise and it's posted capacity will no longer be valid.
The results will be a faster sag of the packs voltage under actual load resulting in much shorter flight times if used with the same plane and power requirement. The only option is to use it in a plane that requires less power to the motor or to replace it. I know of no way to restore a lipo batteries internal resistance lower once it's on the rise?

So... That's why I like the lipo telemetry display. When I see a battery I know was just charged drop in voltage rapidly under load in flight, it's time to replace it.
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