If you’re involved with a large-scale drone operation, you’ve likely encountered frustrations dealing with lithium-polymer batteries. Battery logistics are a common challenge for companies scaling up drone programs, and they represent a considerable hidden cost of growing a fleet. Besides the purchase cost of each individual battery, you also have to account for storage, usage and condition tracking, transport, and obsolescence.
Though improved battery design or other energy sources may be on the horizon in coming years, the lithium-ion or lithium polymer (LiPo) batteries we use today are sufficient for many use cases. By structuring your operations with some sound processes in mind, you can reduce many of the challenges of managing large numbers of batteries.
Safer Storage of Drone Batteries
Lithium-ion and lithium-polymer batteries can be highly combustible when punctured, overcharged, or otherwise mishandled. When the battery is damaged it produces heat and gas, and a single damaged cell can, in some cases, rapidly produce a violent chain reaction.
Proper handling procedures can help mitigate these risks. Here are a few tips that will help safeguard your equipment, facilities, and employees:
- Charging and discharging are particularly dangerous processes, and both should be done under observation.
- To prevent fire, ensure that there are no flammable materials within five feet of the charging station.
- The storage facility should be cool, dry, and equipped with a functioning smoke detector and fire extinguisher.
- When storing LiPo batteries for for more than 24 hours, be sure to discharge it to 50 percent or less if the battery is not equipped with protection circuitry.
How Many Drone Batteries Are Enough?
The short answer is that for an aircraft that uses one battery at a time, four batteries per aircraft is an absolute minimum for a commercial operator. Six batteries per aircraft may be acceptable if you can easily move batteries from one aircraft to another or charge in the field to meet surges. If it’s not easy to shift batteries between aircraft or charge in the field, you will need eight batteries per aircraft to cover 90% of use cases. 12 batteries per aircraft will likely cover 99% of use cases.
So how did we land on these numbers? From observations of our clients and our own experience, my recommendation is that at minimum you should have enough batteries to fly a total of 45 minutes without having to recharge.
45 minutes is a lot of flying on one job. You can map almost 200 acres at 1 in/px in 45 minutes. If you need to map a larger area than that, you should probably be using a fixed-wing UAS. To reduce anxiety, up the minimum by 15 minutes to 1 hour. For example, with a Phantom 4, which uses 1 battery, you should have a minimum of 4 batteries per aircraft.
Yes, I know that the published endurance of the Phantom 4 is more than 15 minutes, but that number assumes optimum conditions and no reserve. If you need to stay out in the field for an extended period without having to recharge, assuming 6 planned flights during that period, you will need 8 batteries per aircraft.
For example, a construction company UAS operator would probably not need more than 4 batteries 50% of the time. About 30% of the time they would need eight batteries. They would need 12 batteries perhaps 10% of the time. This works out to about 6-7 batteries per aircraft distributed across the fleet. If your aircraft are geographically isolated from one another and batteries cannot easily be shifted to cover surges, 8 per aircraft should cover 90% of use cases. You may have unique requirements that vary from this rule-of-thumb.
Incidentally, being able to shift batteries to cover surges is one of several reasons why many companies choose to homogenize their drone fleets.
Maintaining Your Stock of Drone Batteries
With a large fleet of drones comes an even larger stock of batteries, and performing usage and condition tracking on both is vital.
- Is the battery firmware up to date?
- Which aircraft is each battery compatible with?
- Are there any external indications of damage?
- Did an operator have a problem with a battery?
- Was it involved in an accident?
- Are you balancing the usage burden between batteries (recommended) or do you prefer to consume the useful life of a smaller set of batteries and replace them sooner?
- How many charge-discharge cycles has it undergone?
Tracking this data may help you detect a problem with the battery before it causes an accident or job failure. You should rotate battery use to distribute wear and tear evenly and not place a disproportionate load on a smaller set of equipment.
Skyward provides tools that businesses use to manage and share information about batteries in a clean, easy format. Skyward also offers Take Flight, a comprehensive resource for general operating procedures, including more detailed information on how to handle batteries.
Having robust systems of accountability across your entire operation gives managers, legal teams, and other stakeholders insight into the entire operation. And it can help lower drone insurance premiums.
Transporting LiPo Batteries on Commercial Airlines
News stories about exploding Galaxy Note 7 phones prompted the FAA and the DOT to ban them from all flights. Though your drone’s lithium ion or lithium polymer propulsion batteries do not contain the same manufacturing flaw that led to Samsung’s debacle, there are still safety restrictions on how LiPo batteries must be transported on commercial flights:
- They cannot be checked in the cargo hold of the plane, and must be carried on with your personal luggage.
- If a battery is rated at more than 100 watt-hours (Wh), and less than 160 Wh, each passenger is allowed to carry only two in their carry-on luggage—batteries less than 100 Wh face no such number limit, though.
- Though it’s only legally required for shipment of LiPo batteries, when traveling by air you should pack your batteries in a puncture-proof container and discharge them below 30 percent. Some people recommend covering the terminals (with tape, for instance) to avoid forming an accidental circuit.
- Finally, when flying outside the United States, you should make sure to comply with international regulations.
As you might already know from your own experience, dealing with these requirements when sending a crew out for a job can be a headache. Considering that each battery provides less than 30 minutes of flight time at best, you will likely need several for each job so that you can rotate fresh batteries into service as the spent ones charge and so that quality or safety does not suffer because of anxiety about having enough flight time available.
Take a lesson from Richard Lopez of Hensel Phelps, one of the largest general contractors in the country. “Our key has been to keep everything consistent. We all use the same DJI airframes and sensors,” Richard said. “Transporting batteries gets expensive, so each of our district offices needs to have them onsite. We can take aircraft with us if we fly somewhere but not the batteries.”
If you’re at a major firm with numerous locations, and you run your drone operations out of a few different offices, it might make more sense to have drone batteries at offices around the country. This allows the company to avoid the risks, expenses, and hassles of bringing potentially hazardous materials onto a flight, and lessen the need for investing heavily in carrying cases and other supporting equipment.
Building With the Future in Mind
We mentioned earlier that new battery technology is in development, but whatever form the new batteries take, it will likely be years before they’re ready for common use. We should get comfortable using lithium-ion and lithium polymer batteries because, like it or not, they’re going to be here for a while. Regardless, your stock of batteries will eventually become obsolete, either buy upgrades to the batteries themselves or to your fleet. This is one hidden cost of updating your aircraft, and it’s difficult to avoid.
Even though you will eventually swap out the batteries themselves, or their chargers, a well-designed set of procedures will stand the test of time. The culture of accountability and safety will pay its own dividends, and if you’re using Skyward’s equipment management system, it’s simple to incorporate new devices as they come along.
For a more complete set of resources, including a comprehensive general operating manual and other standard operating procedures, Skyward offers Take Flight. If you’d like to improve the safety and efficiency of your drone operations, using the documents and checklists offered in Take Flight as a foundation will do just that.