Over-discharging your LiPo battery can lead to permanent battery degradation or even damage, therefore it’s important to monitor your battery voltage while flying quadcopter and know when to land. In this article we will explain how you can monitor your battery usage in a drone.

Make sure to check out this article to find out more about how to look after your LiPo batteries.

Checking Battery Voltage

The most common way of determining when to land is by checking battery voltage on the fly.

Avoid discharging your LiPo voltage below 3.5V per cell! Note that when you are flying, because the battery is under load, the voltage will sag. And when you land, the voltage will come back up a little.

• 2S – 7V
• 3S – 10.5V
• 4S – 14V
• 5S – 17.5V
• 6S – 21V

However, for tiny whoops, because they use little 1S batteries, the voltage will sag a lot more than bigger batteries. So it’s normal to land at 3.0V or 3.1V per cell, and after some 10/15 minutes of resting it will come back to around 3.5V.

There are many ways of measuring battery voltage in a quadcopter.

Using Lipo Battery Voltage Alarm or Buzzer

The most popular choice is a voltage alarm, which connects to the balance plug of a LiPo battery. They are cheap, and widely available. The buzzer is triggered when any one cell drops below a set threshold, e.g. 3.5V per cell.

Where to buy?

• Banggood
• Amazon

Built-in Voltage Sensor in flight controllers, FPV cameras and OSD

Many flight controllers, OSD modules, and even FPV cameras these days have built-in voltage sensor (a.k.a VBAT input). By connecting your battery directly to this VBAT pin will allow real-time battery voltage displayed in your FPV feed (OSD).

The biggest problem with monitoring voltage is that it decreases with fluctuations. For example when you do a punch out, the motors draw a lot of amps, and the voltage drops off much faster than under normal load, but when you lower your throttle the voltage will quickly recover again, this is called voltage “sag”.

Poor quality LiPo batteries tend to have worse voltage sag under heavy load, which leads to your low voltage alarms being triggered in error, before your pack is actually sufficiently discharged for landing.

Example of FC, FPV Camera and OSD modules that have voltage sensing capability:

• Matek F405 AIO FC (detect voltage via power input)
• Runcam Eagle 2 (detect voltage via VBAT pin)
• Micro MinimOSD (detect voltage via VBAT pin)

Checking your current

A current sensor allows you to see how much current is being drawn in real-time, and your flight controller can gather the data and give you an estimate on how much battery capacity has been consumed during flight. Since you know the capacity of the battery you are using, you are able to work out exactly how much juice is left in the pack.

“mAh consumed” can be a much more accurate indicator than voltage to decide when you should land, because it doesn’t fluctuate with throttle levels like voltage does.

The 90% rule… You should land when you have used 90% of your battery capacity! At this point the battery should theoretically be at around 3.5V – 3.6V.

You should not however, rely solely on “mAh consumed”.

The data from a current sensor resets to zero when the power is disconnected, or when the FC restarts. If your battery is disconnected during a flight and lost previous “mAh consumed” data, your won’t be able to determine the power left in the pack.

Therefore, this method only works with fully charged batteries. When using partially discharged batteries you won’t know the exact initial capacity, this is why we say it is best to have more than one solution in place, i.e. current sensor as well as voltage sensor.

Note: current sensor might be inaccurate upon first use, see this guide for information on calibrating current sensor.

If you don’t have a current sensor on your quadcopter, you can also set up a “virtual current sensor” as shown in this tutorial.

Timer

Timer is an extremely primitive method for determining when you should land. Remember battery usage can vary greatly between flights depends on how you fly. The result is the same as with a car, the faster you drive the more fuel you use.

If you do a flight time test “at a hover”, then recharge the same pack and go and fly an aggressive acro or racing session, you will often find that the battery provides a flight time of less than half of what was achieved during the hover test.

For me, timing flights is a last resort when there really are no better options.

The Different “Low Voltage Alarms”

There are many different forms of low voltage alarm you can have on your quadcopter.

Buzzer/Beeper

If you have a buzzer connected to the flight controller, or if you use one of the low voltage alarms that goes to the balance plug of your LiPo, it will beep when the voltage is low.

OSD

For pilots that fly longer range, one might get into situations where the quadcopter is too far away for the buzzer on the quad to be heard. The best option in this case would be to use an OSD to display the voltage in the FPV feed.

Personally I find using an OSD system ideal, mainly because I love seeing numbers! I can see in real time what the voltage is and plan my flight accordingly. I much prefer this over hearing the loud complaints of the dying battery from a buzzer.

Telemetry

Similar to OSD, Telemetry is another great way to monitor your battery usage, it can also be a backup system used in conjunction with OSD.

Many radio transmitters can display battery voltage on the screen. The Frsky Taranis can even be configured to speak vocal alerts and read out voltages.

Edit History

• Dec 2014 – article created
• Aug 2016 – Updated info about telemetry
• Dec 2017 – Updated info about current sensor