Battery Voltage for Failsafe

Hi,

AFAIK, a Li-po cell should not be discharged below 3V but usually its recommended to go upto 3.3V only.
Now my question is that are these values taking into account the voltage sag that is happening due to current draw? For Eg: if the cell voltage is 3.5V when no power is being drawn but as soon as the drone starts to climb the current draw makes the voltage 3.3V which is failsafe voltage. And when the copter lands the voltage again rises to 3.5V, so does that mean this 0.2V/cell offset needs to compensated and the real failsage voltage should be 3.1V instead of 3.3V so that when copter lands, the voltage recovers to 3.3V ?

Im working with a 12s 44000mAh battery which are fairly in their low cycle count in lifespan. I usually land at 43.5V. But when i look at the consumed mAh, its about 32000mAh. So am I leaving a lot more endurance in the battery or this is normal case scenario ?

Regards
MindProbe

I think you have to be 100% sure of the accuracy of your BATT_AMP_PER_VOLT multiplier to make this assumption…

Also, I don’t think you can drain near 100% of the battey each flight without risking thermal problems or reducing its lifespan.

And you also have to consider the kV relationship for your BLDC motors, as the voltage is getting lower and lower maybe your motors will not have enough RPM (and consequently thrust) to keep your drone flying back to home, specialy if you far far away from it, so you have to consider this safety margin also.

Thanks for answering @BrunoBagarini

I have Battery Monitor from MAUCH, these come with a factory-calibrated A/V and V Multiplier Value which makes it very accurate in my assumptions. So my mAh Consumed and Voltage numbers must be quite accurate.
Yes, fixed KV makes sense but the flight controller would simply command a higher throttle to take care of that. And I’m thinking to discharge up to 40Ah out of 44Ah which seems not so bad for battery health but the voltage achieved to reach that point suggests that I will be overstressing the battery. That’s why I’m confused and need more opinions on this matter.
I want to discharge up to 40Ah/44Ah but that would make the voltage go lower than recommended.
What is the usual failsafe voltage set for Critical and Low Parameters in case of a 12s setup ?

You can use the raw voltage measurement or the sag compensated voltage. Just change the voltage source parameter

…until it can provide enough thrust to maintain your multi-rotor flying. Be careful about it. And if the FC have to command almost 100% of the PWM allowed, you may incur in some stability issues.

I have always use the values recommended in the initial parameter tuning page.

I want to add the comment that 3.3v/cell on a LiPo battery is pretty low. Yes, you can do it, but if you routinely draw your battery down to that level the longevity of the battery is going to be significantly reduced. I’ve found that if the battery starts going below 3.5v/cell the voltage drop is sharp. Landing with the battery between 3.5 and 3.6v/cell I am still getting 80-85% of the rated battery capacity. If you feel the voltage sag you are experiencing is too much you may need to look at higher C rated batteries.

Edit: This might be a better way to explain what I’m talking about. You can see where the voltage of the battery is starting to take a real dive right at the end before I landed. This example was an autotune flight so the current draw for the flight would have been reasonably stable overall. You should consider if the extra moments you’ll get running to 3.3 are really worth it.

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This is pretty much all I have been using for many years spanning loads of batteries. Sure, when a battery is old or failing keeping an eye on voltage is important but it’s not the metric I use. I have only crashed one time due to a low battery and that was returning from a 14km Mission* that I miscalculated.
*This was in the cowboy days of RC craft when there were no rules.