lowest safe battery voltage?

[merk]

Hi All,

Getting some pretty cold weather here and just wondering what the safe zone is in terms of battery voltage? I turned on the option to display that for the last few times I've taken it up. And if i remember correctly, it fluctuated between 4.1 at the start to around 3.7 or 3.5 when i landed.

At what voltage is it considered unsafe?

 

[slup]

With a LiPo battery you shouldn't go below 3,3V/cell in order to not damage the battery. Furthermore DJI have it inbuilt in their firmware that when a cell goes below 3V the AC will force land immediately ... but those low voltages you will normally only see if a cell fails. I would recommend to aim for a landing around 30% ... that will usually mean that you're on ground shut off at 20% ... and that would be around 3,6V/cell.

When flying in cold temps that will increase the internal resistance in the battery ... meaning it will be much more prone for major voltage drops when put under load. If you're unlucky & don't fly with a pristine battery the cell voltage can momentarily fail & sink below 3V ... & start a forced landing. So pay close attention to the cell voltages & maneuver the AC soft without a massive power draw.

Usually the battery management system (BMS) in the battery will calculate 3,6-3,7V as close to 30% in the GO4 app.

If looking into a DAT log from one of my flights when I really pushed the flight time it looked like this (my battery is a 3 cell... yours is a 4 cell).

The Blue graph shows the battery percentage calculated by the BMS
The red graph the lowest single cell voltage recorded over time
The bright green graph shows the total battery voltage (in my case all 3 cells together)
And the dark green the rpm from one of the motors for reference
The blue background color is a low battery auto landing
The light green background is the landing phase

As you see exactly when the BMS says 30% the lowest cell voltage is down to 3,605V & the total voltage is 10,868V (divided by 3=3,622V/cell in average).

When the AC have landed & the motors have been shut off (where the markers are placed in the chart) the BMS says 5% with a lowest cell voltage on 3,511V & a total battery voltage of 10,655V (divided by 3=3,552V/cell in average).

Also note what happens with the total battery voltage after the AC have landed & the motors have been terminated... the voltage rises up when the load is gone, it bounces back to 10,89V (cell average=3,63V).

(Click on the chart to make it larger)

 

[Camino Ken]

It seems to be common knowledge that LiPo’s don’t work well in cold weather. Based on some quick research I found that two things happen at cold. The chemical reactions slow down and the effective series resistance (ESR) increases, and there is a time dependent component. I found this graph below that shows the effect on cell voltage versus time at different temperatures.



You can see in the graph that cell voltage drops a lot in the first minute of operation and then improves. I suspect this increase In cell voltage is due to self heating caused by the heat dissipation by the internal resistance.

I am wondering if this phenomenon might be one of the exacerbating factors in some of the recent cases of drones losing power soon after takeoff.

 

[merk]

wow thank you for that detailed reply slup. From what you have said, it sounds like my batteries were working ok. I'll keep an eye on them during this cold snap and if they go below 3.5 it'll be time to land. Thank you!