Quadcopter Drone Battery Basics
Almost without exception radio-controlled aircraft such as planes, helicopters, and multirotors make use of lithium polymer (LiPo) batteries to power their motors and onboard electronics.
LiPo batteries are especially suitable because their high energy-to-weight ratio. For many years electric flying machines were not particularly practical and were outperformed by engines that use high-energy, dense fossil fuels.
Now these batteries pack enough watts per pound to give us practical flight times without the mechanical complexity and dirty maintenance work associated with internal combustion engines.
These high-tech batteries have to be treated with respect, though. Although highly-refined and responsible for powering everything from smartphones to electric cars, LiPo batteries can go wrong; when they do, things can get really ugly.
Let’s get some of the jargon out of the way when it comes to LiPo batteries. There are a few specifications you need to understand when assessing a battery, especially if you are buying an aftermarket one.
First, there’s the number of cells. A cell is actually what we commonly refer to as a battery. Technically, though, a battery is a collection of cells. So a single cell is not really a battery, but we still call them that. A regular AAA battery that you’d put into a TV remote is actually one cell with a single positive and negative terminal.
Lithium batteries have variable cell counts. Small drones and smartphones usually use a single cell, denoted as “1S”. As you’ve probably figured out, your “2S” would be a two-cell battery, and so on. Although lithium cells wired in parallel do exist, almost all batteries are wired in series.
The total voltage of a battery is determined by multiplying the nominal voltage of each cell by the number of cells; a 2S battery with 3.7V cells will have a rated voltage of 7.4V.
You will also see a rating in “mAh” or milliampere per hour. A battery with a 1000mAh rating can provide 1000 milliampere for 1 hour before being depleted. Basically this means that, everything else being equal, a battery with twice the mAh rating will provide double the flight time.
Next up is the discharge rating, measured in the unit “C”. The discharge rate is basically the maximum power the battery can discharge without getting damaged. A battery with a large capacity and low discharge rate is useless if it can’t release enough energy to power everything. A 1C battery with a capacity of 2200mAh can only discharge at 2.2 amps. At 2C it would be 4.4amps, and so on. This doesn’t mean it will output that much, but that whatever it is connected to can safely access that much amperage. You can also calculate how long it will take to fully discharge the battery. A 1C battery will take an hour at its rated amperage. A 10C will take 6 minutes and a 20C will take three. Simple eh?
Remember that this is for a continuous maximum draw on the battery, while the power needs of the craft in flight will vary depending on what you are doing. Usually a battery is chosen that, at a minimum, can continuously supply the amperage required by running at full throttle. This way there’s little chance the battery will be damaged and the craft can perform at the maximum potential of its motors.
As a general rule, always go for the highest discharge rate you can afford – it can’t hurt the craft and will only improve performance.
Now, LiPo batteries can in rare cases decide to, umm, explode or catch fire. You’ve probably heard of some people having their smartphone batteries explode or those new-fangled hoverboards burning down people’s houses. That’s the ugly side of LiPo technology.
These issued almost always happen while the battery is charging or discharging. It can also crop up if a cell is punctured or otherwise suffers a strong physical blow. So if you have a crash, approach the battery with caution. Especially if it is swollen, making a whistling noise, or is smoking.
While you can’t do much about a flameout or explosion that happens thanks to a crash or other sort of accident, you can take some precautions when charging your batteries.
First of all, choose a place where a flameout will do the least damage – away from things that will burn. Charging on a tiled or cement floor away from any sort of paper or upholstery is a good idea. You’ll also want to invest in a LiPo containment bag. This is a special bag that keeps fire and, to some extent explosions, contained inside. These bag are very cheap and you charge the batteries inside them.
Also make sure that there’s no way the batteries will be exposed to water or heat, like direct sunlight, in the place where they will charge. Don’t let them charge unattended. Skipping out for some fried chicken only to come back to the fire department hosing down your house is not a fun way to spend a day.
Keep the battery out of reach of fire and children.
Keeping Up Appearances
LiPo batteries need maintenance too. In the off season when you aren’t flying you’ll need to prep your batteries. LiPo batteries degrade over time and lose their charge capacity. They don’t suffer “memory” effect like some other battery technology, so you don’t have to periodically discharge them. In fact, you should always store them with about a 70% charge and then take them out every few weeks to top them up to that point again. Don’t let them run dry and don’t charge them all the way while in storage. Lipo batteries should not be discharged below 3V per cell. Your RC craft generally won’t allow this, and if the battery dips below 2.5 it’s considered too dangerous to recharge. The lower the voltage of the LiPo the higher its resistance to charging, and the more likely it will catch fire when you try to charge it again.
While many small drones and RTF (ready to fly) packages come with their own chargers, it’s a good idea to invest in a proper LiPo balance charger. It’s called a balance charger because it ensures that the voltages of all cells are the same in the battery, although it will charge a single cell just as happily.
Not only will this one device charge basically all of your batteries, it can diagnose problems with a battery and show you a lot of information such as cell voltages. In some cases you can even rehabilitate LiPos that have been stored incorrectly by charging them very, very slowly – although this is really only for people who know exactly what they are doing.
LiPo batteries are currently at the heart of all RC flying craft and will probably remain so for the foreseeable future. Most people will never have a problem with them, but now you know what to watch out for.
Always treat your LiPos with respect, the sort of respect you would give to a hand grenade, basically.