sUAS Batteries

All sUAS use lithium polymer batteries, even MOST gas-powered aircraft have backup batteries. As of today, these are the highest energy density batteries at a reasonable price. Lithium is one of the lightest metals, so it is perfect for sUAS.

• It is very important that lithium polymer batteries obtain the proper care. These batteries are considered hazardous materials. There are restrictions to shipping them and carrying them onto airplanes. They cannot be shipped in the cargo hold of an airplane as checked baggage!
• Higher capacity batteries will give longer endurance up to the crossover point when the battery weight pushes the airframes power system over the limit.
• Batteries wear out. They lose a small percentage of their total capacity after each use. The amount lost depends on how the battery was used. Using up more than 85% of the battery’s storage capacity is detrimental. Leaving the battery fully charged between uses is detrimental.
• Keep in mind that your home’s circuit breakers are 15 amps at 110v. Lipo batteries, can put out 228 amps at 24v, so it can really kick out the amps. This is why they are so tightly controlled. They are very dangerous at full charge. Do not leave the battery at full charge unless it will be used soon, and do not charge the battery unattended.

Keeping batteries at the correct state of charge at all times is quite the task. Some sUAS will have a multitude of batteries that need to be maintained simultaneously, many requiring different chargers, and most requiring 110v to perform the charging. Some systems may have:

• Multiple flight batteries
• Camera battery
• Remote control battery
• Monitor battery
• Laptop battery
• Tablet battery
• Smartphone battery

A checklist is imperative to avoid forgetting to turn off one of the batteries between flights. The camera battery if separate is most common to be left on.

The DJI Inspire 1 for example is one of the best thought out battery and charging systems today. Most sUAS manufacturers will adapt these features.

• It has an integral battery for the camera and airframe.
• The transmitter can be charged while it is in use (not true with many others).
• The transmitter will charge the battery in the tablet or smartphone it is connected to when the transmitter is on except for Apple products such as iPhones and iPads.
• Batteries will self-discharge if they are stored to increase storage life.
• Battery level is easy to check. Just touch the button and the charge state is revealed.

Battery labels – the labels have a few designations which are important. For example, below is the Inspire 1 battery. All batteries have similar labels. There are 2 versions of the DJI Inspire 1 battery, the TB47 and the TB48. As shown below the TB48 is:

• 5,700 mAh – This is the storage capacity when the battery is new. The TB47 is 4,500 mAh.
• 129.96Wh – Wh is watt hours. This is found by multiplying the voltage by the amperage. The maximum allowed to be carried onto most airliners is 100Wh, so this battery cannot be legally transported on a commercial airline. Some will allow up to two batteries over 100Wh but not exceeding 160Wh with special permission.
• The TB47 is conveniently 99.9Wh. This is the one to choose if you plan on traveling on commercial airlines.
• 22.8v This is the nominal voltage which is 3.8 volts per cell. Some batteries will state 22.2v for a 6-cell battery which is 3.7 volts per cell. This allows a lower stated number of Wh. The actual fully charged voltage is 4.2 volts per cell, or 25.2 volts. The minimum recommended voltage for a LiPo cell is about 3.7v.

A couple of designations which are missing here are the number of cells. In this case it is 6 cells in series which is usually designated 6S. Another missing designation is the nominal and maximum discharge rate of the battery. This is expressed in terms of the capacity of the battery. This is not disclosed by the manufacturer. It is probably about 40C which means it can continuously discharge at 40 times its capacity – in this case 5.7 amps for the TB48 which calculates to 228 amps at 24 volts.