“LiPo” has become a very acceptable name for Lithium-Polymer batteries, a relatively recent development in the history of batteries (circa 1980’s). They use polymer electrolyte instead of liquid electrolyte commonly found in earlier batteries. An electrolyte is merely the stuff inside batteries that contain the potential to generate electricity when cations and anions are drawn to electrons.
The first commercial application was announced by Sony in 1991, and using LiPo batteries in drones is common these days.
Design and components
There is an interesting chemical process that occurs within batteries. A LiPo cell consist of the Lithium polymer electrolyte, positive and negative charged electrodes, and separator. The separator blocks the electrodes from touching each other, but allows for the cations and anions to migrate within the polymer electrolyte towards the differentially charged electrodes. This movement of cations and anions represents an energy potential that can be used to fly drones (and for other lesser important applications, too!)
A LiPo can consist out of a single cell or multiple cells, each able to reach a maximum charge of around 4.2V. When in use, the charge should never fall below 3.2V. Therefore, a cell is typically regarded as having a 3.7V charge (halfway between the minimum and maximum charge boundaries). The number of cells is by an “S” printed on the battery (1S, 2S, 3S and so on).
Preventing Lipo’s from overcharging (beyond 4.2V) or falling beneath 3.2V is critical its safe operation. For this reason, chargers are fitted with sensors to sense when a charging battery approaches its upper limit, and cut the electricity flow. Similarly, electronic speed controllers are able to shut down the motors (or activate an autoland feature*) if the battery charge falls below a certain minimum.
*View our discussion about features in drones here
The Good and the Bad about Lipo’s
Essentially these batteries have an outstanding electrical storage capability for its weight, making these LiPo’s ideal to be used in drones and other airborne applications where weight is of critical importance. However, compared to other battery types, Lithium is a very expensive commodity mainly mined in Chile, Australia, China, Argentina and the U.S. (a few other mine locations are being developed presently). Moreover, the demand for Lithium is high with major industrial developments towards cleaner sources of energy (such as Tesla’s electrical cars) in progress. These factors contribute to the relatively high cost of LiPo’s.
A major drawback of LiPo’s is its limited lifespan- commonly around 200 cycles (a cycle is a charge/discharge occurrence). With further development, it is possible that this statistic might improve.
As drone flyers, we are mostly interested in how long the battery can keep our drones in the air. However, this is not always easy to determine and you find confusing information on the Internet, showing that many writers (especially in the hobby category) do not understand the basics of LiPo battery power well.
Note that it is necessary to consider both the “power supply” side (e.g. the LiPo batteries in drones) and the “power demand” side- in our case, a brushless motor.
“Supply Side” – the Battery
Firstly, just wrap your head around the concept of a “Coulomb” (denoted with a capital C, often printed on drone batteries), named after some clever Frenchman from the 1700’s. A Coulomb is the amount of electric charge that a 1 Ampere (a measure of electricity force) flow delivers in 1 sec. (It is also equal to 1 Watt per Volt (W/V), but do not worry about that for now.) It is a rate concept as opposed to a quantity unit of measurement. A 50C LiPo would, similarly, describe an amount of 50A delivered in 1 sec, or 1A delivered for 50sec.
In the picture herewith, this single-cell 900mAh (0.9 Ampere-hour) drone battery would deliver 0.9C in 1 sec, or 3,240C in an hour. In different terms, our battery would take 900 hours to discharge to a circuit requiring 1 milli-amps. However, this is not to say that a full charge would last an hour!
“Demand Side” – e.g. a Brushless Motor
As an example, a 10V electric brushless motor in my workshop is specified to draw a current of 0.4A at idling speed, and a max continuous current of 15 A (15,000mA).
At a potential difference of 10V, this motor would consume electricity at a flow rate of 400mA when idling. So, if we use the battery in Fig 2, we’ll need at least 3 cells (printed as 3S on the battery, shown in this example) to turn this motor efficiently. Connected in series, the (3 x 3.7V) cells will have a cumulative potential of 11.1V. Our flow capacity of 900mAh is more than enough for the idling speed demand of 400mA.
However, we need a flow of 15A to be able to sustain maximum continuous, and our 900mAh LiPo battery used in our drone is by far not good enough to do that. Bear in mind that for quadcopter applications, we’ll have several of these brushless motors connected to the one battery.
As mentioned before, these batteries are a highly efficient energy storage medium. Typically, a LiPo weighing a few grams can propel a small drone for up to 15-20 minutes.
Maintenance & Storage
From the content above, it would be clear that a good charger is vital. When more than one cell is fitted, the different cells need to be kept at similar charge levels. If they become unequally charged, the residual electricity flow might contribute to an undesired rise in temperate.
For this reason the different cells will have to be charged simultaneously and equally- a process called “cell balancing”. Good chargers incorporate this function, and we have pleasure in recommending the Tenergy TB6-B Charger/ balancer available from Amazon, which is suitable for batteries with anything from 1 to 6 cells.
Your drone manual will, no doubt, prescribe “store in a cool, dry place”- more because of battery care considerations than any other reason.
Lastly, one of the benefits of Lipo’s is that their self discharge rate (discharging when not in use) is very low compared to other types of batteries, but it still happens. Therefore, if you have not flown your drone for a long period of time, it will be necessary to charge prior to taking to the skies.
A lot to read can be found on the topic of exploding batteries, especially after the Samsung recall of several million mobile devices a few years ago. The story is basically that LiPo’s heat up slightly when discharging (rather than when charging). In the Samsung case, a few of the cell connectors were found to be touching, causing excessive discharge flow, overheating and other consequences that robbed some airline CEO’s of their sleep.
The Samsung problem was successfully contained but still there are occasionally isolated occurrences where LiPo’s overheat. Statistics show that these incidents are concentrated around periods of charging or discharging- a good reason to ensure that you battery charge station is set up where there are no other combustible materials around. First Alert Smoke Detector and Carbon Monoxide Detector Alarm | Battery Operated, SCO5CN“>In addition, it is well worth installing an extra smoke detector like the one showed here, to attract your attention if something goes wrong.
Aketek Silver Large Size Lipo Battery Guard Sleeve/Bag for Charge & Storage“>LiPo bags are available on the market and are highly recommended whenever you travel with your drone battery in a car. They are not expensive and for less than $10, you can certainly prevent a really bad experience, especially with your kids in the car as well!
For obvious reasons, airlines are nervous about allowing batteries in checked-in luggage as it may take longer to detect and contain a fire. However, they do allow them in carry-on bags (although not unrestricted). Check out the FAA’s handy two-page chart.
Finally, your drone battery is not just another component of your drone, it is indeed its life blood. Look after it, and please do tell us about your experience with your Lipo in the comment section below!