RC Lipo Battery Guide: Explanation, Safety, and Care

Lipo Battery

Without LiPo batteries, quadcopters and drones probably wouldn’t exist.

Why?

Because no other battery provides as much energy while delivering high amounts of current in so small of a package.  This means: longer flight times, better maneuverability, and faster flying.  In other words, more fun.

However, there is a lot of confusion about what terms mean, how to properly charge lithium polymer batteries, and how to be safe when using batteries.

In this lipo battery guide, you’ll get:

  • An in-depth explanation of battery specs – Nominal and cut-off voltage, capacity, C-rating, internal resistance and more
  • How to increase the life of your lipo battery
  • What to look for in a lipo battery charger
  • How to use a lipo battery charger
  • Why balance charging is important
  • The difference between different types of battery connectors
  • Proper lipo battery safety and care
  • A glossary of common lipo battery terms

So, let’s start with battery specifications.

Battery Voltage

The first spec that most people look at when picking out a battery is the voltage. As mentioned in my article on motor Kv constant, the speed of your motors is proportional to the voltage you supply to them so higher voltage batteries are capable of turning motors faster than lower voltage batteries. The voltage of your battery will determine the type of ESC’s and motors that you will need to use.

Lipo battery voltage is specified by the number of cells in series. Each cell has a nominal voltage 3.7 V. Different battery companies mark their lipos in different ways but most people tend to refer to their batteries as 1S, 2S, 3S, etc.

The following voltage chart shows each battery designation and the corresponding voltage.

1S = 1 cell in series x 3.7 V = 3.7 V
2S = 2 cells in series x 3.7 V = 7.4 V
3S = 3 cells in series x 3.7 V = 11.1 V
4S = 4 cells in series x 3.7 V = 14.8 V
5S = 5 cells in series x 3.7 V = 18.5 V
6S = 6 cells in series x 3.7 V = 22.2 V

You may also see lipos that use a letter “P” to designate voltage. For example, 2S1P or 2S2P. This is not as common with quadcopter batteries but you may see it with LiPo’s meant for other types of RC vehicles.

“P” stands for the number of cells in parallel. 2S1P means “2 cells in series and 1 cell in parallel.” If a battery doesn’t have a “P” then it is assumed to be “1P.” So 2S1P and 2S are the same thing.

3S2P means “3 cells in series and 2 cells in parallel.” This battery would have a total of 6 cells with 2 parallel groups of cells with 3 cells in series in each of those groups.

Maximum Voltage and Cut-off Voltage

Consider the 2 battery voltage curves in Figure 1. An “ideal” battery and an “actual” battery are shown. An ideal battery would be capable of providing constant voltage for the entire time it is discharging until it is completely discharged.

lipo battery discharge curve

Figure 1 – LiPo Discharge Curve

Actual batteries don’t behave this way. Instead they start at a higher voltage and then their voltage will slowly decrease as the battery is discharged.

So while an ideal battery might have a voltage of 3.7 V during the entire time it is discharging, an actual battery will start at the maximum voltage of 4.2 V. As it discharges, keep decreasing.

How far will it decrease? If you let it, it will fully discharge down to zero volts.

However:

Battery manufacturers usually recommend you don’t discharge your battery below a certain minimum voltage. This minimum voltage is called the cut-off voltage. Many manufacturers recommend the cut-off voltage for lipos to be 3.0 Volts.

Practically speaking, this means you should set your voltage alarm to something higher than 3.0 V so that you can safely land your quadcopter before it drops below 3.0 V.  Many people set their alarms to 3.3 V.

Battery Capacity

Battery capacity is measured in Amp-hrs or milliamp-hrs. It gives you an indication of the total energy that a battery can store. In general, the higher the capacity, the longer your battery will last.

Using the gas tank analogy, a high capacity battery has a big gas tank that allows it to store a lot of gas.

The best way to understand the capacity of lipos is to consider how it is measured. The general procedure is to take a battery and figure out what constant current causes the lipo to drop below the battery cut-off voltage in exactly 1 hour.

The capacity is then just that current times 1 hour.

So a 3000 mAh capacity battery will drop from full voltage to the cut-off voltage in 1 hour if you discharge the battery at 3000 mA (3 Amps). 3 Amps x 1 hour = 3 Amp-hrs = 3000 mAh.

There is one important thing you need to note …

The capacity of a battery decreases as the current draw increases. This means that if your capacity is 3000 mAh for a constant 3 Amp current draw, then your capacity will be less than 3000 mAh if you draw a constant 6 Amps or a constant 30 Amps.

I’ve seen a lot of explanations of capacity that don’t tell you this. You will often see it written that if your battery’s capacity is 3000 mAh, then that means you can draw 3 Amps for 1 hour, 6 Amps for 30 minutes, or 30 Amps for 6 minutes. In general this is not the case.

In order to tell how current affects capacity, you need to look at a lipo battery discharge curves. Most good battery datasheets will show different discharge curves for different constant current draws. For example, see Figure 2.

lipo battery discharge curve datasheet

Figure 2 – Discharge curve from datasheet

This figure shows the discharge curve of a 1S, 1700 mAh, 35C battery (more on what “35C” means later). Each line represents a different constant current (42.5 A, 47.6 A, 51.0 A, etc.). This figure shows us that for a given cut off voltage the discharge capacity decreases as the current draw goes up.

In this example, for a cut off voltage of 3.0 V, the capacity at 1.7 A (1C) is 1700 mAh and I would estimate the capacity at 42.5 A (25 C) to be about 1630 mAh and the capacity at 68.0 A (40 C) to be about 1460 mAh.

So the 2 important things to remember about capacity are:

  • Capacity tells you the amount of current that the battery is capable of delivering for 1 hour
  • If you discharge your battery at higher currents than that, your capacity goes down.

(Note that Amp-hrs is not a measure of energy. Energy is measured in Watt-hrs. If you assume that the battery is discharging at constant voltage, you could calculate energy from that. However, as we learned in the previous section, batteries do not discharge at constant voltage.)

Lipo Battery C Rating

The C rating is a useful rating for batteries that lets you compare batteries of different capacities. In our discussion above about capacity, I talked about discharging the battery in 1 hour during a constant current discharge.

This current – the current required to discharge a battery in 1 hour – is defined as a rate of 1C. 2C would be 2 times that current. A 0.2C rate would be 1/5th of that current.

So in our 3000 mAh battery example, the 1C rate for the battery would be a discharge current of 3 Amps.

3000 mAh = 3 Amp-hrs => 3 Amp-hrs/ 1 hr = 3 Amps.

Batteries are given a maximum continuous current rating and a maximum burst current rating. The continuous current rating tells you how the maximum safe current your battery can discharge for long periods of time. The burst rating tells you the maximum current your battery can discharge for short periods of time (10 seconds).

Both of these ratings given as C ratings. For example, if you have a 1300 mah capacity battery and it is rated for 30 C continuous, then the maximum continuous current is 1.3 * 30 = 39 Amps. The same battery might have a 40 C burst rating, meaning the battery can safely output 1.3 * 40 = 52 Amps for short periods of time.

How Accurate are C Ratings?

The general consensus is that many manufacturers do not provide very accurate C ratings on their batteries. This makes it difficult to compare one battery to another.

Internal Resistance – ESR

Internal resistance, or equivalent series resistance (ESR), tells you about how much the battery resists current flow. This resistance comes from both the ions in the battery but also the electrical resistance of the metallic parts in the battery.

A battery can be modeled as an ideal voltage source in series with its ESR. When you do this, you can see that you want a battery with a low ESR. The lower the better.

LiPo Battery Life Span

As you use your battery, charging and discharging it, the capacity of it will slowly drop. So if your battery is 1300 mAh out of the box, it might drop to 75% of that (about 1000 mAh) after 200 charge/discharge cycles. A LiPo may last 300 to 500 cycles, depending on how it is cared for.

What should you do to maximize battery life?

There are a number of things you can do.

First, using the proper charge voltage is necessary. Never charge your battery past 4.2 V per cell.

Second, don’t over-discharge your battery.

Third, never store your battery fully charged. The recommend storage voltage is 3.8 V to 3.85 V. Most chargers will allow you to set it to a storage charge.

Fourth, use balance charging.

Do all those things and your battery will last for more charge/discharge cycles.

Some people say you should store your LiPo’s in the fridge (NOT the freezer) in order to improve the battery life. I don’t personally do this but some people swear by it.

Lipo Battery Charging

LiPo batteries are able to be recharged. You need to buy a charger that is specifically made for LiPo’s. The reason for this is that LiPo’s require a specific method to charge them (more on that below).

If you charge a LiPo without a charger made for them, then you could either:

  • Not fully charge your battery
  • Shorten the life of your battery
  • Most importantly, damage your battery and cause a safety issue. As you’ll see below, LiPo’s can cause fires if they aren’t treated properly

How to Charge a LiPo Battery

Most modern LiPo chargers use something called balance charging. If you remember from earlier, every battery is composed of cells connected together. Balance charging is just a way of monitoring the voltage on each of these cells and charging them individually.

Why?

Because every cell in a battery is slightly different. When you finish flying your quad and take the battery out, each cell in a battery will have a slightly different voltage and a slightly different capacity remaining. So the best way to recharge these batteries is to charge each cell individually. Chargers that have balance charging take care of this automatically.

Most LiPo chargers today are programmable and allow you to choose the type of battery, the voltage, current, and a number of safety measures.

How to Choose a Lipo Charger

If you don’t already have a charger, you need to make sure you pick one out that meets your needs. Here are the most important specs to pay attention to when picking out a charger:

  • Voltage – You’ll see a lot of chargers that are capable of 6S or 8S, which is more that enough for most quadcopter batteries
  • Current – Chargers will be specified by a maximum current. As a general rule, you should charge your batteries a current of 1C. So, for a 2200 mAh capacity battery, the charge rate should be 2.2 Amps and for a 1700 mAh capacity battery, the charge rate should be 1.7 Amps. Some batteries will specify that it is safe to charge at a higher current than 1C. You should choose your charger so that its maximum current is higher than the current you need to charge your batteries.
  • Power – Chargers will also be specified by a maximum power. Power is Voltage x Current. So a 3S, 1300 mAh battery charged at 1C will require at least 3 cells x 4.2 V x 1.3 A = 16.4 Watts to charge. I generally like to add about 20 % to my requirements to make sure I have a safety factor. In this case, I’d want a charger that was capable of 20 W or more.

How to Use a LiPo battery charger

Every charger is different so anything I say here will be general. It’s best to read the manual if you have any specific questions about how to use your charger.

  1. Do NOT charge a battery that is puffed up, bloated, swelled, punctured or damaged in any way. Look for cuts or nicks in the lead wires and do not use if you find any.
  2. Find a safe spot, away from anything flammable.
  3. Connect your charger to the battery’s main power connector. See the Power Connectors section below.
  4. Connect your battery’s balance leads to the balance port on the charger.
  5. Place your battery in a fireproof LiPo bag. This is the bag that I use. It’s cheap and it’s good insurance just in case something does go wrong.
  6. Plug in your charger. Depending on your charger, you could plug it into a wall for AC power or to another battery for 12V power.
  7. Set your battery type
  8. Some chargers will have you choose between balance charging and non-balance charging.
  9. Set your current – Remember, 1C is recommended unless the battery manufacturer says otherwise. Charging with too high a current is also dangerous.
  10. Set your voltage – For LiPo’s, this should be no higher than 4.2 V / cell. Anything higher than that is dangerous. You may find that your battery lasts longer if you only charge to 4.1 V / cell. But you will get less capacity in your battery, so there is a tradeoff here between battery life and flight performance.
  11. Start charging
  12. Do NOT leave your charger unattended while it is charging.

I recommend buying a good charger. Do not get a cheap one. A good charger will reduce the chances of fire and increase the life of your batteries. The SkyRC iMax B6AC v2 is a popular, good quality charger.

So, what does a LiPo charger actually do?

The best chargers use a method called constant current-constant voltage (CC-CV) charging.

Constant voltage charging just applies a set voltage (say, 4.2 V / cell) to the battery and keeps charging until the battery reaches the desired voltage.

The problem with constant voltage charging is that when you first apply voltage to the battery, there is a large difference between the applied voltage and the battery voltage. This causes a large current to flow, which can be harmful to the battery or dangerous.

Constant current charging fixes this by setting the current to a certain value (say, 1C) and keeps charging until the battery reaches the desired voltage. This fixes the high current issue at the beginning of charging that the constant voltage method faces.

However, constant current charging faces the opposite problem – too high of voltage near the end of a charge. In order to keep a constant current flowing to the battery, the supply voltage has to keep increasing as the battery voltage keeps increasing. Applying too high of a voltage to a battery can be harmful to the battery or cause a fire.

The solution to these two problems is the CC-CV charging method that I mentioned above. Good chargers will begin the charge by applying constant current. This gets rid of the problem of too high of current at the beginning. Then once the maximum safe voltage for the battery is reached, the charger switches over to constant voltage charging until the battery is fully charged. This gets rid of the problem of too high of voltage at the end of the charge cycle.

Lipo Battery Connectors

There are generally two types of connectors on a LiPo battery: the main power connector and the balance connector. There are a variety of different types of connectors that vary in size, current rating, and ease of use. Some connectors are proprietary and are only used on 1 brand of battery.

Battery’s always come with female connectors. Why? For safety reasons. If a battery had two male pins sticking out, there is a chance they could get bent and short together.

I recommend that you use whatever connectors come with your battery. Unless you know what you are doing and have a good reason to do so, do not try to change the connector on your battery.

Power Connectors

XT60 (XT30, XT90)

XT60 battery connectors

Male and Female XT60 Connectors

The “XT” style of connectors is very popular for batteries.

XT60 is used often with 250 frame quadcopters (2S, 3S, 4S). The maximum continuous current rating for XT60 is 60 Amps. You can get the same style but one size smaller (XT30) or one size larger (XT90), which have current ratings of 30 Amps and 90 Amps, respectively. These are very easy to solder with their curved tabs.

Deans Ultra battery connector

Deans Ultra Connector

Deans Ultra Connector

Deans Ultra, sometimes just called “Deans connectors” or “T” connectors, are probably not as popular as they used to be. They are rated to 50 Amps continuous current. They are also difficult to solder because their tabs are flat.

EC3 (or EC5)

EC3 battery connector

EC3 Connector

The EC style of connectors are somewhat popular as well. The 2 most popular sizes are the EC3 and EC5. The EC3 is rated to 60 Amps continuous current and the EC5 is rated to 120 Amps continuous current.

JST-RCY

There are a lot of different types of JST connectors. The JST-RCY is a red connector that is sometimes used on smaller batteries. It rated up to 3 Amps continuous current. Some very small quads will have a battery that uses a JST-GH connector.

Connector Current Rating Wire Size Weight
XT60 60 Amps 12 AWG – 18 AWG 6 g
XT30 30 Amps 20 AWG – 26 AWG 2 g
XT90 90 Amps 6 AWG – 10 AWG 14 g
Deans Ultra 50 Amps 12 AWG – 18 AWG 4.5 g
EC3 60 Amps 12 AWG – 16 AWG NA
EC5 120 Amps 8 AWG – 10 AWG NA
JST-RCY 3 Amps 22 AWG – 28 AWG NA

Balance Connectors

Balance connectors come with different number of pins, depending on the number of cells in your battery. JST-XH are probably the most common type of balance connector but Thunderpower TP) and Hyperion (Polyquest) also have their own type of balance connectors.

JST-XH

JST-XH Balance Connectors

JST-XH

By far the most popular balance connector is the JST-XH. There are actually a number of different connectors with different number of pins, depending on the number of cells your battery has. There is a standard method of wiring JST-XH balance connectors.

TP (Thunderpower) and Polyquest (Hyperion)

These are proprietary connectors that are only found on Thunderpower and Hyperion batteries.

Accessories

There are a number of connector accessories that I find handy.

  • AB Clips – Balance connectors (JST-XH) are very small and hard to get a hold onto.  This inevitably means that when you disconnect your balance connector from a charger, you will pull on the wires instead of the connector housing.  Over time, you will pull the wires right out of the housing.  AB clips are a little piece of plastic that wrap around the balance connector housing and give you a bigger thing to hold onto so you don’t ruin your balance connector.  They are cheap and they will save you a headache down the road.
  • Breakout cables – If you use bullet connectors for your ESC power, then you can get XT60 to bullet connector breakout cables. Very handy.
  • Connector adapters – If you use all XT60 connectors but get one battery that has an EC3 connector, you can get an adapter to.  You can get almost any combination of adapter.

LiPo Battery Safety and Care

Do a google search for “lipo fire” and you’ll find all types of carnage. I’ve tried to stress the importance of safety throughout this article. The main points of safety are:

  • Don’t over-charge
  • Don’t over-discharge
  • Never use or charge a damaged battery (punctured, puffed, cracked, etc.)  Read my article about puffed lipos.
  • When charging, use a LiPo fire-safe bag
  • Don’t charge your battery immediately after using it.  Wait at least 15 minutes for it to cool down.
  • Similarly, don’t fly immediately after charging your battery.  Wait at least 15 minutes for it to cool down.

The nice thing about this safety advice is that a lot of these are the things you want to do anyways in order to get good battery life.

Glossary: LiPo Battery Terminology Quick Reference

Here are some common terms that you may see as you learn about LiPo batteries. These are quick definitions and will be covered in more depth throughout this article.

  • LiPo battery – a particular type of battery chemistry that is popularly used in quadcopters and other RC hobbies. Lipo batteries are capable of providing high current for relatively long periods of time
  • Primary/Secondary cell – You might see battery cells referred to as primary cells or secondary cells. Primary cells are cells that can’t be recharged very easily and are often discarded after they are discharged. Secondary cells are cells that are more easily recharged. Lipo batteries that are used for quadcopters and other RC hobbies are made from secondary cells.
  • Cell – all batteries are made of 1 or cells. A cell is just the name given to a device that converts chemical energy into electrical energy.
  • Capacity – Measured in A-h (amp-hours) or mAh (milliamp-hours), capacity indicates the amount of current that the battery can deliver for 1 hour.
  • C rating – a rating that tells you how quickly a battery is discharged relative to its maximum current
  • Series/Parallel – Batteries with multiple cells can be connected in series or in parallel (or both). Placing cells in series or parallel allows battery manufacturers to increase either voltage or current, respectively.
  • DoD – Depth of discharge – This is a measure of how much of the energy in a battery is used.
  • Open circuit voltage – the voltage of the battery when measured with no load
  • Internal resistance/Equivalent series resistance/ESR – measured in milliohms, ESR tells you how much the battery internally resists current flow 
  • Nominal voltage – The voltage that a battery will “typically” operate at. This is the voltage you will find on battery labels. The nominal voltage of 1 cell lipo batteries is 3.7 V.
  • Cut-off voltage – the voltage at which a battery is considered discharged
  • Energy – Measured in Wh (Watt-hours), the energy is the total amount of work that a battery can do.
  • 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 – A number from 0% to 100%. It tells you how much energy you have left in the battery before it is discharged.
  • Energy/Power Density – energy and power density tell you how much energy or power you get out of a certain volume of material
  • Maximum Continuous Discharge Current – the maximum current that a battery can be discharged at continuously without causing damage to the battery
  • Charge voltage – the voltage at which a battery should be charged

Conclusion

Well, there’s my brain-dump on LiPo batteries.  I hope it is a useful guide to the world of lipo batteries.  There’s a lot more that could be said so if I missed something or you have any questions, let me know in the comments.

5 Comments

  1. José Alexis Villarreal February 19, 2017 at 8:58 pm

    Excellent description about lipos batteries. Thanks

  2. Very good piece, if only I had read this last year when I started flying helicopters keeping my 6s lipos fully charged ,resulting in 3 of them puffing!

  3. This was very useful and very helpful for me to understand WHY I kept puffing my batteries and destroying so many 6C’s for my T-Rex 700’s. Turns out not only was I OVERCHARGING but I was also not caring for them correctly. Since following some of your guidelines, no more puff-ups and batteries being destroyed. The key thing for me was to watch the high-end charges, this was killing my batteries as my charger was not accurately noting the charges being held so this left me with highly unstable and overcharged lipos. Thankfully no issues ever happened as i am very careful and watched them while they charged.

  4. Eric,
    Thank you kindly for taking the time to write this in depth article! It has answered so many questions all in one place. I am new to lithium polymers having just ordered a couple 2S 30C 4000mAh LiPo batteries for my robotics projects and will be sure to follow your advice on charging, storing, and selecting the charger (unfortunately I must return the charger that is being shipped to me as it’s not high enough power).

    I’m curious if you know what the wires are tied to in the balancing connectors? I would guess that in these 2S LiPos the 3 wires are connected to the point between the series cells and at each end. Like this: wire1-cell1-wire2-cell2-wire3 where cell1 and cell2 are in series. I assume this gives the charger ability to monitor the voltage potential across each cell, but I don’t understand what the charger can do to “balance” the 2 cells, unless it’s passing a small current through these balancing wires into the cell that needs more energy.

    I wonder why a third cable doesn’t come out of these batteries with a simple thermistor tied to them so that the charger can monitor the temperature of each cell and shut down charging if it gets too hot. However, I’m sure that there is a very fine line before the “thermal runaway” happens in these cells during charging. I’d like to learn more about this if you have any suggestions on good reads.

    On another note, I recall from my old chemistry courses that batteries have a “self-discharging” characteristic that is dependent on both humidity and temperature. The refrigerator is good for batteries as the air is more dry than on a shelf in your room and humidity allows the battery to dissipate into the air. The fridge is also at the lower end of the comfortable cool temperature range for storing batteries. The lower temperature reduces the ability for the holes (electrons) to migrate from one side to the other, as opposed to the higher temperature.

    Cheers,
    Tim

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