Battery reading at Tower app

Hi guys,

Need advice here. I wonder why at Tower app, before fly, the battery reading is 13.9. But right after take off, the battery reading drop to 13.0. After landed and props stop, the reading goes up again.

Why is this? I can’t be sure my battery left while flying in this case.

I’m using 3DR power module. Please advice, thanks in advance.


This is completely normal. When a load is applied to the battery, the voltage will drop. Higher quality and higher C-rated batteries should drop less when the load is applied, but they all will drop.

On my Hexacopter my 4S battery reads 16.8V, and drops to about 15.8-9 as soon as I take off. Then it slowly drops as the capacity is depleted. I land at between 14.0-14.2V. I have a few lower-quality batteries that will drop to 15.2-3 right after takeoff when fully charged.

For my racing drones, the 4S batteries drop from 16.8 to about 16 on takeoff, but on really hard punch outs or full throttle runs the battery will drop as low as the high 13V range and then bounce back to 15V when the throttle returns to the middle.

Also, after landing and letting the battery rest, the voltage will “rebound” and come up a bit. I just have all of my timers and battery warnings setup so that I land when the resting voltage of the battery returns to about 3.6-3.8 volts per cell after landing, which on my racers is about 14.5 under load and the hexa is about 14V.

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Then I was wondering how Arducopter Battery Failsafe work. Won’t it estimate the required return home battery and try RTL at low battery?

How can I set Alert Low Battery accurately? Please advice.


You need to figure out the voltage drop of your battery under load. As an example, if the cells are 4.2V prior to arming, and they drop to 3.9V in hover, then the voltage drop is .3V per cell times the number of cells under load. Take the instance of a 3S battery, the voltage drop is .9V under load.

Once you know that, use the loaded voltage figure for battery failsafe and if the lowest voltage you want your battery to be is, say, 3.5 vpc (10.5V for a 3S) after the flight set the battery failsafe to .9 volts below that, or 9.6V.

Adjust those figures for however deep you would like to discharge your battery.


Thanks so much for the detail explanation. Appreciate very much.