Multirotors (aka drones) are fueled by LiPo battery, they are able to store and deliver large amount of power. In this guide you will learn how to read the spec of LiPo for your mini quadcopter, and how to choose and handle them safely.
- The Basics of LiPo battery
- Cell count and voltage
- Capacity and size
- C rating and discharge rate
- Internal resistance (IR)
- How to choose Lipo for quadcopter
- How to charge Lipo
- How to handle batteries
- LiPo Disposal
The Basics about LiPo Batteries for Mini Quad
Lithium polymer batteries, more commonly known as LiPo, have high energy density, high discharge rate and light weight which make them a great candidate or RC applications.
By learning the basics about LiPo batteries, you will be able to read and understand their specifications.
Battery Voltage and Cell Count (S)
LiPo batteries used in RC are made up of individual cells connected in series. Each cell has a nominal voltage of 3.7V. Therefore battery voltage is often referred to as how many cells in the battery (aka “S”).
1S = 1 cell = 3.7V
2S = 2 cells = 7.4V
3S = 3 cells = 11.1V
4S = 4 cells = 14.8V
5S = 5 cells = 18.5V
6S = 6 cells = 22.2V
For example, we call a 14.8V battery a “4-cell” or “4S” battery.
Voltage affects brushless motors RPM directly, therefore you could use higher cell count batteries to increase your quadcopter’s speed if your motor/ESC and other electroincs support higher voltage. (here is a discussion about the differences between 3S and 4S on a mini quad)
But a battery with more cells of the same capacity is heavier since it contains more cells. To make a 4S 1000mah battery, you could simply combine two 2S 1000mah, or one 3S 1000mah with an 1S 1000mah.
Nominal voltage for LiPo battery cell is 3.7V. However it’s not the voltage of the battery either when it’s fully charged or fully discharged. The number is come up by manufacturers, and It’s near the middle of safe voltage range, so I guess that kind of makes sense.
LiPo battery is designed to operate within a safe voltage range, from 3V to 4.2V. Discharging below 3V could cause irreversible performance lost and even damage to the battery. Over-charging above 4.2V could be dangerous and eventually cause fire.
However it’s advisable to stop discharging when it reaches 3.5V for battery health reasons. For example for a 3S Lipo, the max voltage is 12.6V, and you should land when the voltage reaches 10.5V (at 3.5V per cell).
LiPo Battery Capacity and Size
The capacity of a LiPo battery is measured in mAh (milli-amp hours). “mAh” is basically an indication of how much current you can draw from the battery for an hour until it’s empty.
For example, for a 1300 mAh Lipo, it would take an hour to be completely discharged if you draw a constant 1.3A current from it. If the current draw doubles at 2.6A, the duration would be halved (1.3/2.6=0.5). If you draw 39A of current non-stop, this pack would only last 2 minutes (1.3/39=1/30 of an hour).
Increasing your battery capacity might give you longer flight time, but it will also get heavier in weight and larger in physical size. There is a trade-off between capacity and weight, that affects flight time and agility of the aircraft. I wrote a guide about building a simple mathematics model to find out optimal Lipo capacity for longer flight time, which you might find interesting.
Higher capacity could also give you higher discharge current as you will see in the next section.
C Rating (Discharge Rate)
Lipo batteries for quadcopters these days all come with a C rating. By knowing the C rating and capacity of a battery, we can in theory calculate the safe, continuous max discharge current of a LiPo battery.
Maximum Discharge Current = C-Rating * Capacity
For example an 1300mAh 50C battery has an estimated continuous max discharge current of 65A.
Some batteries come with two C-ratings: “continuous” and “burst” ratings. The Burst rating is only applicable in short period of time (e.g. 10 seconds).
This article explains the significance of C rating. Although C rating could be an useful tool, it has become mostly a marketing tools nowadays.
If C rating is too low, the battery will have a hard time delivering the current to your motors, and your quad will be under powered. You could even damage the battery if current draw exceeds safety rating.
When C rating is higher than what’s required, you won’t gain much performance improvement. Instead the battery would be heavier and you will be carrying extra weight that reduces your flight time.
Rule of thumb, the battery connector should match the one you are using on your copter. If you don’t own a quad yet, choose one, and stick with it.
All Lipo batteries come with 2 sets of wires/connectors: a balance lead and a main lead or discharge lead (Except for 1S batteries which only have a main lead). There are quite a few different connectors used in LiPo batteries. The main differences are shape, weight and current rating.
1S Battery Connectors
1S connectors are tiny and have very low current rating. They are commonly used in brushed micro quadcopters.
2S-6S Battery Connectors
You will find a lot more different types of battery connectors in this category, in fact not all are listed here. But majority of them are not used that often so you don’t need to ever worry about them. For mini quad, the most popular connector is probably the XT60.
However since XT60 is only rated at 60A, and mini quad are running at higher and higher current and voltage, we might soon see a change in the popular connector used.
|XT30||Mainly 2S and 3S|
|XT60||Similar to the Above, only bigger||Mainly 3S, 4S|
|XT90||Similar to the Above but even bigger|
|EC5||Similar to the Above but bigger|
Balance lead is mainly used for balance charge to ensure all cell voltages are equal. It also allows you to monitor the voltage of each cell.
The number of wires in a balance lead starts at 3 for 2S LiPo, and for every increment in cell count, the number of wires also go up by 1.
Internal Resistance (IR)
Internal resistance exists in every battery cell, and IR could be different in each cell of the same LiPo battery. Higher internal resistance reduces the max current the LiPo pack can produce, that why as a LiPo battery gets older or overly used, you will find it losing the punch, or power.
IR isn’t shown on the label, because they can change over time. Lipo internal resistance increases with:
- crashes (physical damage)
- other abusing usage
You can measure IR with dedicated tools, some LiPo charger even comes with IR measuring features.
We covered IR in more details in the C-Rating article.
LiHV is a different type of LiPo battery, HV stands for “high voltage”. They are more energy dense than traditional LiPo battery, and allow to be charged up to 4.35V per cell. However there are mix reviews out there regarding the longevity of LiHV, as they might have decrease in performance sooner than normal LiPo’s.
In this post I compared a LiHV battery with a LiPo in terms of performance.
How to choose LiPo battery for quadcopters?
To choose the best Lipo battery for your quad, you first need to know your requirement, the preferred cell count, and what would be the max discharge current.
Once you have decided on your choice of motors and propellers size, you should be able to find data online related to that requirement. For example, I am going to use this motor with 5040×3 props, at 100% throttle it draws 36.7A.
Remember in real flights the current draw is normally smaller than that from “static thrust tests” because of the moving air. And notice that the current is significantly different between 90% throttle and 100%, and you should ask yourself how often you would fly at 100% throttle and does this matter to you. Personally I fly mostly at 40%-80% throttle range, even when I do wide open throttle punchout’s they wouldn’t last more than a few seconds.
The total max current draw for a quad of 4 motors would be 36.7 x 4 = 146.8A at 100% throttle. If you want to play safe, you could just use this figure and find the battery you want. But for me, I usually discount it by 10% and that’s 146.8*0.9 = 132.1A because of the considerations we mentioned.
There are other parts that uses power from the battery, such as the FC, RX, LED, FPV gears etc. But it is very little compared to the powerful motors so we can just ignore them from our calculation. Or add 1A to the total current draw if you want to be very precise.
Then work out the capacity and C rating needed. For example here is some general guideline I personally follow by the propeller sizes:
6 inch: 1500mah - 2200mah 5 inch: 1300mah - 1800mah 4 inch: 850mah - 130mmah 3 inch: 650mah -1000mah
Say if I was building a 5″ mini quad, and I want a relatively lighter build, I would go for 1300mAh (1.3Ah). Therefore the burst C rating requirement would be: 132.1/1.3 ~ 102 C, half of that would be the continuous discharge rate – 102/2 = 51C. However you might want higher C rating than that if you fly constantly in higher throttle range.
And of course you have to take into account what kind of flying you plan to do, and if weight or capacity is more important to you. For example “hard core racers” might want the lightest possible batteries, while for “freestylers” weight isn’t the only priority and larger batteries can be considered for longer flight time.
How to Charge LiPo
Type of charging
- Balance charge – The charger monitors the voltage of each cell, and can charge them individually while trying to keep them at the same voltage level. This is the safest and most recommended way of LiPo battery charging
- Direct charge (fast charge) – You are charging through only the main lead, and the charger isn’t monitoring the voltage of each cell. This is normally faster, but it could result in unbalanced cell voltages and the battery might not be 100% charged
- Storage charge – The charger brings each cell of the battery to their storage voltage, which is 3.80-3.85V
- Discharge – The charger attempts to drain the Lipo battery (very slowly, even slower than charging)
Why Balance Charge?
Every cell in a battery is slightly different, after the battery is discharged, you might find that the cell voltages are all different.
If we were to direct charge this unbalanced battery without monitoring voltage of each cell, chances are some cells might end up under 4.2V (not fully charged), and what would be worse, some might go OVER 4.2V. If you remember, LiPo cells shouldn’t exceed 4.2V or they will become dangerous. Remember, over-charged = dangerous!
Most decent modern Lipo chargers are programmable and allow balance charging, and they should take care of this automatically.
Choosing a LiPo charger
Charging a Lipo starts with buying a charger :) There are many factors to consider, so please check out my article about how to choose a good LiPo charger. I also covered how to determine the appropriate charging current and power in this guide.
Incorrect handle of LiPo batteries could potentially cause fire. Please take your time to read through these safety rules before handling/charging batteries.
- Pick up LiPo by their body, not the leads – wires could be pulled off from the fragile solder joints
- Charge at safe places – It’s very important to find a fire-proof location to charge your batteries. Using a Lipo-safe bag is a good option, some even build a bunker for it. I personally find using an ammo box is a cheap yet effective solution
- Don’t charge your battery immediately after using it, wait until it’s completely cool down
- It’s advisable to charge your battery at 1C or less (this is explained in the article “How to choose LiPo charger” by Oscar Liang
- Never charge your battery unattended – regularly check if the battery is getting warm or starts to swell, if so stop charging immediately
- Never use or charge a damaged battery – don’t charge if it is swollen (puffy) or has any other visible signs of damage
- Ensure the number of cells and battery type are set correctly on your charger to match the cell count in your battery
- Don’t over-charge, although this is normally taken care of by the charger, it would be a good idea to check cell voltages regularly
- Don’t leave battery under the sun
Parallel charging might not be the safest way of charging LiPo batteries, but it is probably one of the fastest for RC hobbyists. It allows you to charge multiple packs at once rather by one by one. However you do so at your own risk.
Check out my article on how to parallel charge.
Charging 1S LiPo batteries
Charging tiny 1S batteries can be slightly different than doing the bigger packs. You can perhaps charge multiple 1S batteries using a parallel board (basically just combining them as one big 1S battery). But so far I found the best and fastest solution for me was to build a cable that connects multiple 1S batteries in series, and charge them as one single 3S or 4S, or even 6S pack. (Basically I am doing series charging rather than parallel charging)
Here is my tutorial on making a 1S LiPo charging cable, explaining the advantages of doing so and the DiY steps.
How to use LiPo Batteries Safely
How to Measure Voltage of a LiPo battery
For in-flight voltage checking, here are the ways of monitoring Lipo voltage.
It’s important you have a way of checking the voltage of each cell in a LiPo to make sure they are roughly on the same level, such as using a LiPo tester. If a particular cell has much lower or higher voltage than the rest of the cells (or what we call unbalanced cell voltage), it probably indicates a problem with that cell and you need to check it carefully and balance charge it before using it.
LiPo batteries for mini quad work best between 30 to 60 degree Celsius.
Cold weather is a performance killer to Lipo batteries: lower discharge rate and effective capacity. The common descriptions of using LiPo batteries in the cold winter would be “shorter flight time”, “lack of punch” and “bad voltage sag”.
Therefore make sure you warm up the battery before flight in the winter to about 30 degree to 35 degree Celsius for optimal performance. (put them in your pocket for example :) )
More discussion about temperature vs LiPo performance.
Lipo doesn’t like it too hot either. Once they go over 60 degree celcius, they could start to swell and even catch fire.
When is time to land
One common question from beginners is: “When should I land?” I would say when your battery voltage reaches 3.5V to 3.6V. For LiPo batteries, you shouldn’t run the battery until it’s “flat”, and an acceptable level of charge should be left in the pack.
The graph below explains why. Voltage does not go down linearly with capacity used, instead it drops dramatically after about 3.5-3.6V in a LiPo cell. And if you still hadn’t landed by then, you could risk over-discharging your battery.
Over-discharging LiPo batteries cause permanent damage to the battery, and shorten battery life.
How to store Lipo batteries
If you decided not to use a LiPo battery for a long period of time (longer than a week for example), you should
- Storage charge it to 3.8-3.85V
- Store it in a LiPo safe bag
- Store it at room temperature
When a LiPo cell is at 3.8V-3.85V, it has roughly 40% to 50% charge, and this is where the most stable state is for a lipo battery. This is why whenever you receive a new battery from a shop, it comes half charged.
It’s not only unsafe to leave LiPo fully charged for a long time, it could also decrease its performance over time. If you fly every few days, then this is less a problem for you.
LiPo Safe Bags
LiPo safe bags are the must-have investment for your LiPo batteries. They are used for charging and storing your batteries. LiPo bags are designed to contain flame when fire happens with the battery.
What to do with Over-Discharge Lipo
When a LiPo battery is fully discharged, it oxidizes the cells overtime and it causes reduction in performance permanently. However, if you can catch it quick enough you can usually save the battery without too much damage done to performance.
Chargers might be unable to recognize an over-discharged battery due to voltage being too low. My advice is to discard that battery. However there is a way to rescue an over discharged LiPo, but you’ll do so at your own risk.
Travelling with LiPo
Most airlines and airports allow LiPo batteries in passengers’ hand luggage. There are a few things to bear in mind:
- Always check with your airline regarding travelling with LiPo batteries
- Do not carry LiPo batteries in your checked baggage
- Put your batteries in storage charge
- Tape the connectors of your batteries and store them in a LiPo safe bag
- Never travel with damaged batteries
Please see my guide on “How to travel with mini quad and LiPo batteries” for more detail.
What to do when LiPo is on fire?
- Don’t panic, unplug all connection first
- The most effective yet economic way of putting out LiPo fire would be using sand. Cover the burning LiPo with sand
- Just wait until the fire goes out and the battery is cool, do not breath in the smoke
- Do NOT use water ever
When to retire
LiPo batteries have a limited cycle life, every time you charge and discharge, it is 1 cycle. The battery will begin to lose punch (internal resistance increasing), and capacity. It’s said a Lipo battery could be used over 300 cycles if looked after properly (by following the rules we have mentioned above). But for me it would be a miracle I don’t damage it before that :) There isn’t really a fine line, but I personally would replace them if they become badly puffy, or unable to hold more than 80% of the specified capacity.
More discussion can be found here: http://intofpv.com/t-when-to-retire-lipo-batteries
How to disposal LiPo
Old and damaged Lipo batteries should be disposed properly. I’ve already covered the ways of disposing LiPo before in this article. One thing I need to stress is, never puncture LiPo batteries, it will cause fire!
Here are a list of common questions from hobbyists.
Are Swollen (puffed) LiPo’s dangerous?
Yes, Puffed LiPo batteries are not safe to use.
What causes LiPo batteries to go swollen?
LiPo swells because gas get trapped inside the cells. This is a natural thing to happen, physical abuse (such as damage, overheating or over-discharging) can cause the battery to generate more gas.
Can I fix puffed LiPo?
No you can’t. Once LiPo’s gone swollen it’s irreversible. Dispose it properly instead.
How to avoid Swollen batteries?
- Do not over-discharge – use some form of voltage alarm or monitoring
- Don’t overheat – avoid leaving batteries under the sun or close to heat source, don’t over-load the batteries
- Never overcharge – setup your charger properly, and keep an eye on the charger while charging
- Store your LiPo properly as we’ve already mentioned in this article
Does new battery need to be broken in?
Break-in Procedure is a controversial topic in the FPV community. Basically it’s a practice that suggest new batteries need to go through a series of slow cycles (charge and discharge) before putting into full use. I personally tried it and didn’t find any obvious difference. See this forum thread for the discussion.
Other Lipo battery technical terms
- Cut-off voltage – the voltage at which a battery is considered discharged completely; For Lipo it’s 3.0V
- Cycle life – 1 cycle is when you charge and discharge a battery. The cycle life is the total number of cycles the battery will last
- State of charge – the energy level of a battery from 0% to 100%.
- Burst C-Rating – the maximum discharge rate over a short period of time (normally within 10 seconds)
Hopefully you learned something about LiPo batteries and how to use them safely. However I make no claims that this is all you should know. Please go and do more research if you are feeling uncertain.
- Feb 2017 – Article created
- Aug 2017 – Added “how to choose lipo”