It isn't well understood how to select the ideal battery for your drone. Maximize range - yet ensure performance in windy conditions, that there's enough power for takeoff, the voltage sag isn't too much - the list goes on.
Batteries are always Ying and Yang - add capacity - lose current output. There is no way a cell can maximize both parameters without affecting the other. An engine with the most horsepower will not also have the best MPG.
This frames the question: How to maximize capacity, ie flight time or range, while having acceptable aircraft performance. Typically, the starting point is to evaluate aircraft performance requirements. An example of requirements are:
800W needed for takeoff
500W needed for steady level flight
400W needed for landing
We put these parameters in terms of watts as a watt is a fundamental unit of energy and can be translated to a variety of voltages and current.
To set the experiment up, it is easiest to start with a battery which can provide more than enough current for the scenario, a high power (high C rate) battery ideally with the maximum capacity allowed by weight for the flight. From here, you will perform a flight with the battery under a standard flight condition and log the results. This will provide the initial data as seen above to begin down selecting options to maximize the aircraft range and ensures the motors and ESCs can safely operate with the greatest weight possible. This will determine the maximum weight of the battery selected. Extra weight can be added to the aircraft if it is not possible to put a maximum size battery on the aircraft. The battery is unimportant at this stage provided it can give enough current and the aircraft has safe performance with it.
The watts, or power, can be easily derived from the flight log of the aircraft. Watts is Volts * Amps. So while looking at the flight log, multiplying these two values provides an accurate representation of the instantaneous of energy required for the given scenario. Visually on the drone OSD the battery voltage is 20.5V and the current is 30.2V, the power used is 619W at that point in time. These points can be collected and used as an alternative to a flight log. We can discuss energy used over time in terms of watts per hour, abbreviated Wh. An example would be:
800W for takeoff which takes 2 minutes: 800 * (2/60) = 26.66Wh
500W for steady level flight for 10 minutes 500 * (10/60) = 83.33Wh
400W for landing which takes 1 minute 400 * (1/60) = 6.66Wh
These values can be added up to determine the total amount of energy used while in operation. In this case:
Total energy utilized during flight: 26.66 + 83.33 + 6.66 = 116.65Wh
PX4 and Ardupilot both can log watts and can be reviewed in online or desktop programs. Betaflight, etc, has some capability of this but may require the OSD approach.
The following process will then be to optimize a particular parameter, say steady level flight parameter, we need to increase this 10 minute flight to say 15 minutes. We will again add up the Wh required to evaluate: 26.66 + (500 * (15/60) = 125) + 6.66 = 158.32Wh
At this point I must explain that Upgrade Energy has already done much of the work for you. We rate our products in an easy to use an accurate Watts figure. This is the number of watts the battery can output for the entire capacity of the battery. Using the sliders on the product selection page, limit the slider to the greatest allowable battery weight and as a starting point, select a battery which has a continuous power rating of the average power used during flight. With this information, you will be able to determine the maximum flight time. Say we have a maximum battery weight of 950 grams and an average power draw of 500W. We might select the 6s 12Ah GREEN battery with a Wh of 259.2 as it fits the parameters of providing 600W of continuous energy. We convert this into a simple algebra problem:
Takeoff = 26.66Wh
Landing = 6.66Wh
259.2 - 26.66 - 6.66 = 225.88Wh left for remaining flight.
Remaining flight requires 500W so: (225.88 / 500) * 60 = 27 minutes possible for this section of flight.
Add in a 30% overall safety margin: 27*.7 = 18.9 minutes of flight, we have achieved more than 15 minutes of steady level flight!
Thus we have established with the 6s 12Ah pack that under ideal conditions we could achieve up to 27 minutes of steady level flight and can consider the battery to be highly optimized for the flight conditions.
In future blogs we will dive into further into optimizations and constraints when selecting the ideal battery. For now, this will provide a good starting point for most users.
If you have any questions, feel free to reach out to our sales team on our contact page, we are happy to help work these numbers out for you!
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