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.
Understanding Battery Voltage
The most common way of determining when to land is by checking battery voltage on the fly.
Your LiPo battery is made up of cells, each ranging in voltage from 3.3V (absolute lowest voltage that should never be exceeded with risk of damaging the cells) to 4.2V (Maximum rated voltage, above which they may spontaneously catch fire), with the storage charge being 3.80V to 3.85V.
As 3.3V per cell is the lowest voltage, it’s good practice to stop discharging your LiPo further when it reaches 3.5V just to be safe. Note that when you are flying, the battery is under load, and so the voltage will sag. When you land, the voltage will come back up a little.
- 1S – 3.5V
- 2S – 7.0V
- 3S – 10.5V
- 4S – 14.0V
- 5S – 17.5V
- 6S – 21.0V
However, for tiny whoops, because they use little 1S batteries, the voltage will sag more than bigger batteries. So it’s normal to land at 3.2V or even 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?
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 “mAh Consumped”
A current sensor allows you to see how much current is being drawn in real-time, and your flight controller can gather this data and give you an estimate on how much battery capacity has been consumed. Since you know the capacity of the battery, you are able to work out exactly how much juice is left in the pack.
“mAh consumed” is 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 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.
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.
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.
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.
- Dec 2014 – article created
- Aug 2016 – Updated info about telemetry
- Dec 2017 – Updated info about current sensor
Hi mate, I’m flying a 1S Mobula7 tiny whoop. I’ve noticed that the battery voltage remaining displayed in the OSD of my goggles is always 0.2V lower than displayed on my TX16S RxBt home screen widget. Any idea why? Thanks!
Hello Guys, I have notice that in my OSD say3.3 or 3.5 and I land immediately… But I check the same battery and checker say taht voltage is 3.7 o 3.8… does it normal?…. or do you know what happen?
Yes it’s normal.
Hi thanks for generous info.
I find the mah used the most useful indicator . I fly in thermals so flying time varies a lot and I need to know when to leave a thermal , some times from some altitude. One plane configuration is light with a small motor ( power 500 mah and camera /vtx 370 mah ) and no osd or gps or battery sensor . This is very good flying ( I am often joined by hawks and kites ) but some lightweight equipment would be helpful. I dont want to get into a negative spiral of adding weight to cope with weight .
Super light jst connector battery sensor for consumption please!
I just commented on your article about my Battery Buzzer, at the bottom of the page on “Get Longer Flight Times” I found this article of yours: “Monitor battery Voltage”…! The first Buzzer is just like mine, so I’m sorry to bother you with my question, also my Pixhawk PX4 I think Monitors Voltage, I also have Telemetry, and I’ll be adding OSD later…! Plenty of ways to skin a Cat..!
What do You think of this product?
I’m from europe and shipping nearly double the price.
I don’t like the talking Taranis telemetry and my simple osd is influencing my video quality.
Wondering if the FLEA would also have a negative impact on the video or not.
Great work you are doing here…
I noticed there’s no mention of current here. I find a current sensor to be more valuable than voltage, as I can sag voltage pretty quickly while still having current remaining. Current gives me an idea of how much longer I can fly (even if I have to back down a little). Voltage can do similar, but is far more crude and depending on too many things.
You mean battery capacity? yes it might work too, but capacity used/remaining is an estimation which depends on a lot of factors, so i won’t use it alone as a signal to land. I think voltage still a primary indicator.
Thanks for this Oscar. I have a buzzer like the one shown in your picture, however my balance lead is already being used to power my FPV gear. Do you know of a way to split the balance lead so i can also attach a buzzer? Thanks again.
sure, maybe some DIY an adapter for the balance plug or the XT60 plug to power your FPV gear? while available for voltage alarm?