Beginners Guide to LiPo Batteries for FPV Drones
The first thing you need to know about lipo batteries is that they are not like other batteries you may be familiar with. If you drain too much energy out of a lipo battery or put too much in, you will permanently damage it or potentially cause a fire, which is why it is very important to keep an eye on the energy left in your battery as you fly. That way you will know when it is time to land and keep your batteries healthy.
Storage Voltage
When you get a new lipo battery it will already come partially charged. This partial charge is called the Storage Voltage. Storage voltage is where each cell is charged to approximately 3.8 volts. (Don't worry if you are unfamiliar with battery cells, we will get more into that later). The reason for this is that a lipo battery is most stable at that voltage. Any more or less energy in the cell will cause the battery to slowly degrade and lose efficiency. This is also why it is important to charge or discharge your batteries back to storage voltage shortly after using them.
LiPo Cell Voltage
The usable voltage range for a standard lipo battery cell is 3.2v to 4.2v. Any lower than 3.2v and the battery may be permanently damaged. Any higher than 4.2v and you significantly increase the risk of a battery bursting into flames. Normally you will start with a battery cell that is at storage charge or 3.8v, place it on the charger, let the charger charge the battery to 4.2v, then go fly until the battery cell voltage drops to somewhere in the 3.5v to 3.7v range depending on how nice you want to be to your batteries and your comfort level. You might ask, why not fly until they are all the way down to 3.2v? Well, you can, but that puts additional strain on the batteries, shortens their life a little bit, and typically when you fly and give the drone a burst of throttle, the battery will sag and the voltage will drop as it tries to provide all the power it can. If this sag causes the battery cell to go below 3.2v, it will cause more damage. Also many drone batteries are created from multiple cells and you don't always know the exact voltage of each cell. If you have a battery created from 6 cells, they won't all discharge at exactly the same rate and you could end up with one cell at 3.0v and others at 3.4v. In that case you risk damaging a cell or the entire battery. So that is why the recommendation is to land when your batteries are at 3.5v per cell or greater. You may also notice that after you land and give your batteries a few moments to recover from their hard work, the voltage will recover slightly and your 3.5v battery cells may stabilize closer to 3.6v or 3.7v per cell. Some people will even stop flying at just the right time such that their batteries are at 3.8v per cell so they won't have to storage charge them when they are done. You get less flight time this way but it might be worth the trade off to you.
Battery Cells
Drone batteries come in all sorts of shapes, sizes, cell counts, connector types, and more. Everything from a very small single cell battery for a "Tiny Whoop" to a large 6 cell battery often used in drones with 5" propellers. ( Yes, they do come larger than 6 cell varieties, but if you need one of those this guide is probably not for you. ) When you add multiple cells together to create a bigger more powerful battery, the voltage of the entire battery is multiplied by the number of cells. For example a 4s battery, which contains 4 lipo cells in series, would have a maximum charge of 4.2v x 4, or 16.8v. A 6s battery would have a maximum charged voltage of 25.2v. And the more voltage you have, the faster an electric motor will spin. Which is also why it is important to pair the right rpm motor with the right battery voltage. But we will leave that for another guide. The important part here is to understand the relationship between voltage and cell count. I would also recommend having your OSD display the average cell voltage in your FPV goggles. That way it won't matter if you are flying with a 4s or 6s battery, either way it will show you the average cell voltage and you will get used to using that value to decide when to land.
You may also see a battery listed as 4s2p, or 2s2p. That is just letting the buyer know that there are some cells in series and some cells in parallel to create the voltage and capacity necessary for the overall battery. So in the case of a 4s2p it lets you know that there are 4 cells wired in series each made up of 2 cells wired in parallel for extra capacity without increasing voltage.
Any lipo battery made from more than one individual cell has the potential to have the cells become out of balance. Ideally you want all the cells in a single battery to be at the same voltage at all times. To help you achieve this, your lipo charger has a charging option called "balance charging". Some chargers will also split the charging up into "fast charging" and "balance charging" phases where they will first quickly charge up the cells to something close to fully charged, and then slowly top them off while checking the voltage of each individual cell to make sure they are all equally charged when finished. For maximum health, you should always balance charge your batteries. This is also why your 2 cell or greater LiPo battery has a second plug on it. This second plug is called the balance plug and is used by your charger to individually monitor and balance charge each cell.
It is also important to monitor the voltage of each cell in your battery from time to time. All lipo chargers that are capable of charging 2s or greater batteries should show you the voltage of each cell of the battery. If you notice that after you are done using a battery, one of the cells is significantly lower voltage than the others, you know that it may be time to retire that battery.
LiPo Battery Charging
That brings us to the section on charging a lipo battery. In order to maximize the useful lifespan of your battery you need to know a few things about charging. First of all, never leave charging batteries unattended. When a lipo is charging, the chances of a fire are greatly increased. A healthy undamaged lipo charged properly is unlikely to catch fire, but FPV drones are not exactly kind to batteries and charging a battery that was damaged in a crash or over discharged can be very dangerous.
The safest way to charge a lipo battery and the one that puts the least amount of strain on your battery is to charge at a rate of "1C" or 1 times capacity. A 1C charge rate means that the current will charge the entire battery in 1 hour ( assuming you are starting with a fully discharged battery around 3.2v ). For example, if you had a 1000mAh lipo, to charge at 1C you would set your charger for 1 Amp. If you had a 500mAh battery, you would set your charger to 0.5 Amps. If you have a 1500mAh battery, you would set the charger to 1.5 Amps, and if you have a 3500mAh battery, you would set the charger to 3.5 Amps.
Many batteries will list a maximum charging C-rating on their packaging which will be much higher than 1C, but it is still best to charge slower whenever possible.
Don’t Leave Them Fully Charged For Long
Once a lipo battery is charged, it is best to use it "soon" and then return the battery to storage voltage once done. That is because a battery not at storage voltage is constantly degrading over time and that damage is cumulative. For example a battery left at full charge for a month may have greatly increased internal resistance when used which will cause a decrease in performance and an increase in heat. There is no magic number for how long to leave a lipo fully charged. Leaving it fully charged for 1 day 10 different times is the same as leaving it fully charged for 10 days in a row. Or leaving it fully charged for 1 hour 24 times would be the same as leaving it fully charged for 1 day one time. In general most people find leaving batteries fully charged for a day or so acceptable. So if you find yourself with some fully charged batteries and no plans to use them in the next day or two, it would be best to discharge
them down to storage voltage.
Discharge Rate or C-Rating
LiPo batteries well known for being able to dish out a lot of power very quickly. This is the main reason we use them instead of other batteries like Lithium Ion. The higher the discharge rate, or C-rating of the battery, the more of their capacity they can deliver continuously without causing damage to the battery. It is highly debated how accurate the manufacturers C-ratings are for their batteries, but it is fairly safe to say you can compare C-ratings for similar batteries from the same manufacture to get an idea of which battery can deliver more power safely.
Most batteries will also list a burst C-rating. This is the amount of power the battery can provide for a quick burst, or a few seconds without causing damage and will be higher than the continuous C-rating of the battery. Having a good burst rating is great for those times when you want to quickly go full throttle. But remember that going all out for more than a few seconds may damage your battery if your motors can draw more power than the C-rating of the battery.
LiPo’s Don’t Like Extreme Temperatures
Heat and cold are the enemies of lipo batteries. Allowing lipo batteries to get hot either during use or especially during charging will damage them. And on the other side, cold temperatures will decrease the performance of a lipo battery. If you do fly in cold temperatures, keep this in mind. Try to keep your batteries warm, but not hot, before use in cold weather. Once in the air, the use of the batteries will help keep them a little warm, but you will notice that they will not perform quite as well as you are used to. Their voltage will be lower and the sag you feel when pushing them to their limits will be greater. The end result is less power and shorter flight times.
LiPo Lifespan
LiPo batteries have a limited lifespan. Eventually, after 300 or so charge cycles, you will find that most LiPo batteries have lost a lot of performance and it will be time to retire them. You will notice that you get less and less flight time and not as much "punch" as you did when the battery was new. Another indicator of a battery being ready for disposal is "puffing". Worn out or abused batteries will expand or puff up as components inside the battery turn into a gas. Finally, if your battery charger can measure the batteries internal resistance, keep an eye on those numbers. A sudden jump, or having one cell have an internal resistance that is much higher than the others is an indication that the battery should be retired.
However, how long they last and how many cycles you get out of them will greatly depend on how nice you have been to them by not over/under charging them, keeping them at storage voltage, and not letting them get too hot.
Just the Tip of the Ice Berg
There is plenty more to learn about LiPo batteries that has not been covered in this beginners guide. Luckily there are lots of other great resources out there if you search for them. So if you want to learn how to parallel charge your batteries, properly dispose of them, or learn all the details about internal resistance and how it impacts your batteries, search YouTube, check out websites like Oscar Liang’s, or ask questions in the FPVFC Discord server.
Disclaimer and Indemnification
The advice in this article are suggestions from the FPV Freedom Coalition. The source of information was gained from experience and industry best practices. LiPo batteries have safety risks including starting fires if over discharged or over charged as described in the article. The reader accepts all responsibility for charging and discharging his or her batteries and indemnifies FPV Freedom Coalition from any liability.
Last updated December 2020
