LiPo Battery packs explained

LiPo Battery packs explained

Lithium polymer batteries, more commonly known as LiPos, are rechargeable, pouch format batteries utilising lithium-ion technology. Rather than having a hard outer shell, they have a soft “polymer” shell. This makes them lightweight, but susceptible to damage. Within the shell are a positive and negative electrode, along with a separator; a liquid electrolyte is contained within the shell and provides the conductive medium. LiPo batteries are ideal for RC applications, where we benefit from the high energy density, high discharge rate and light weight when compared to other battery types. Because of their soft shell, LiPo batteries can be manufactured in a range of different shapes and sizes, to better fit into the gear you are powering.

LiPo Battery numbers explained

Voltage in LiPo batteries may be a little different to what you are used to. Each cell has a nominal voltage of 3.7V, but is operated between 3.0V and 4.2V. Discharging a LiPo below 3.0V per cell will cause damage to the pack and can be dangerous, as can charging to higher than 4.2V unless you have a specialised HV (high voltage) pack. More on charging later. Fundamentally, LiPo batteries are not discharged until empty as this causes damage. Normally an 80% rule is applied, meaning that you only ever drain 80% (or less) of the battery capacity and other than

S: LiPo batteries are made up of individual cells (some are simply a single cell), and battery packs will typically contain a number of cells in series. The nominal voltage of the pack is the voltage of each cell added together. This is where the cell count or “S” number for the pack comes in, since packs have a number of cells in Series. Voltages of some common LiPo battery sizes are given below as examples:

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

Drone motor RPM is a function of voltage (kV) and motors produce a number of RPM per volt. This means that (subject to your motor being able to handle it) by increasing the voltage, you can increase the power produced by your drone. In drone racing, 4S is the current standard, but clearly, moving to a 5S or 6S pack instead will give you a higher top RPM and more speed. 5S LiPo batteries are now becoming more popular and 6S LiPo batteries targeted at FPV drone racing are also beginning to surface.

Capacity is most commonly measured in milliamp hours (mAh) or amp hours (Ah) which is just 1000x larger i.e. 1,000 mAh = 1 Ah. The capacity defines the current that could be discharged for an hour, until the pack is completely empty (bad idea). You could also discharge at a higher rate for a shorter duration. For the majority of FPV drones, flight times are in the region of 3-6 minutes, so clearly we are putting large loads on our batteries!

In the most basic sense, the capacity of your LiPo is essentially the size of your fuel tank. Increasing your capacity will increase your flight time, to a point. Increasing the capacity also increases the physical size and weight, meaning you need more throttle just to stay in the air. There are many different sizes of batteries, but for FPV drone racing, the most common capacities are 1,300 mAh and 1,500 mAh. These provide a good blend of having sufficient capacity without being so large that manoeuvrability becomes an issue.

In larger drones, particularly stable aerial photography platforms, much larger capacities are used. These applications are not as dependent on being manoeuvrable and so can sacrifice this for longer flight times. Battery capacities of 5,000-10,000 mAh are not uncommon.

C: The instantaneous discharge rating of a battery, it allows you to calculate how quickly the LiPo can be safely discharged. Yes, I said calculate! The C rating does not instantly tell you the current you can pull from a battery. Instead, you multiply the capacity by the C rating, to get the current (Amps) that you can safely draw continuously. We’ll look at a standard LiPo as an example:

You have a 1,300 mAh (1.3 Ah) LiPo battery with a C rating of 50

1.3 x 50 = 65A

Related: Burst rating is often given along with the continuous C rating and is typically related to the current that can be drawn for a maximum of 10 seconds. A common example is a battery with a 50-100C rating. This means that for a maximum of 10 seconds, you can discharge this example 1,300 mAh battery at 100A. The burst rating comes into play when dealing with punch outs or rapid acceleration, when you will not be going to full throttle for any long duration.

Going above the C rating of your battery is bad for your battery health and will result in a lot of heat and puffing. This is why it is important to have a suitable LiPo battery for the application. For drone racing we are generally looking at packs with a capacity of 1,300 mAh or more and a C rating of 45 or greater as a minimum. Having used packs with a lower C rating than this myself, I can confirm that overloading a LiPo can very easily cause a pack to puff. Puffing is not a problem and you should not charge or use a battery if it is bloated.

How to read LiPo Battery specs or ratings

As we have just covered what all the numbers mean, you should now be able to read the specs from the sticker on your LiPo and understand what they mean. Just in case, I’ll run through a quick example:

You have a 1,500 mAh 4S 50-100C LiPo battery. This means you get:

Voltage: 12V completely empty, 14.8V nominal, 16.8V Full

Continuous current: 1.5 x 50 = 75A

Burst current: 1.5 x 100 = 150A

LiPo Battery connectors explained

There are a large number of different LiPo Battery connectors (Connector article). For the main power connector, different types are available depending on the current that can be provided by the battery, but the rules are not always strictly followed.

The most popular connector currently used in FPV drone racing is the XT-60 connection. This provides a reasonable current rating, is easy to solder and provides good grip to plug/unplug. As you can see, the rating for the XT-60 is only 60A continuous. This may sound like a lot, but when you consider that there are now batteries capable of 95C continuous discharges being used for FPV drone racing, we may soon see a change in connector type.

In addition to the main power connector, a LiPo will normally have an additional/secondary connector known as the balance connector. This provides a connection to each individual cell in the pack and allows you to measure individual cell voltages. This is important as monitoring individual cells allows you to ensure all cells are evenly charged through use of a balance charger. More on charging later.

 

LiPo Battery selection guide

Choosing the right battery is important because as discussed above, the capacity, voltage and C rating will impact the weight and instantaneous discharge the battery is capable of. For a miniquad using standard 5″ props, the common battery sizes are 1,300mAh, 1,500mAh and 1,800mAh. Larger batteries are available with standard capacities ranging from 2,200mAh all the way up to 16,000mAh in a single pack, but these are typically for drones with larger props. These are designed for lower current draw and longer flight times and so aren’t well suited to miniquad use.

For drone racing, you need a combination of power and endurance, without adding too much weight (as this makes your drone sluggish). For quick acceleration you need high instantaneous current output, which means a higher C rated battery or a larger capacity, as these are capable of higher burst currents. Unfortunately they are also heavier. Generally, lighter is better when it comes to drones, but go too small with your battery’s capacity and although you will save some weight, you may not finish the race!

A good starting point is to select a 1,500mAh battery rated at around 50C. This provides a good balance of capacity, weight and instantaneous current and will be good for a range of races as well as freestyle. If you find yourself sagging the voltage because of long periods at high throttle, then a higher C rating or slightly increased capacity may help. If you need longer in the air because of a long race track, then an 1,800mAh or 2,200mAh battery of the same rating may be a good choice. Going above this when using 5″ props is not likely to increase real world flight time, because you will find yourself at a higher throttle position to account for the additional battery weight.

FPV Drone Battery Cells and Voltages

Battery voltage is the potential energy difference between the positive and negative terminals. A higher FPV Drone Battery voltage allows the pack to provide more power to the quadcopter without increasing the current or amp draw. A standard lithium polymer cell has a nominal (storage) voltage of 3.7V hence to increase the power that a single LiPO pack can deliver, these cells are grouped together in series (meaning the ground/negative lead from the first cell is connected to the positive lead of the next cell, forming a chain of individual cells) to increase the overall battery pack voltage. LiPO packs are commonly sold in 1S, 2S, 3S, 4S, 5S or 6S configurations where the digit followed by the ‘S’ stands for number of cells in that specific pack. The more cells that are grouped together, the more voltage the overall battery pack will have. The battery pack voltage is important as it impacts the maximum motor speed of a quadcopter. This is explained further here [insert hyperlink to the article on motors/ section on KV] but simply, more battery voltage allows the motors to spin with greater speed (RPM). For this reason, 4S LiPO’s are the most commonly used for racing quadcopters as they provide a balance between speed and weight. The following table summarizes the voltage and common applications for various LiPO cell configurations. It is important to note that the quadcopter applications listed in the following table are only typical examples from the many different battery-quadcopter combinations in existence. Exotic setups such as 5S 150mm racing quadcopters or 2S micro brushed quadcopters do exist however they are just quite uncommon.

LiPo Battery Cell Count and Voltage
LiPo Battery Voltage and Common Quadcopter Applications

FPV Drone Battery C-Rating

Battery capacity is measured in milliamp hours (mAh) which is a unit describing the current a battery can supply for a unit of time. As an example, a 1500mAh battery would be able to supply: 1500 milliamps (1.5A) of current for an hour, 3000mA (3A) of current for a total of 30 minutes, 6000 mA (6A) for 15 minutes and so on. A higher milliamp rating on a battery essentially means that it will provide more flight time per charge. When choosing a battery, a sacrifice must be made between the battery size and the weight. A larger capacity battery will provide a longer flight times however the added weight will restrict the performance of the quadcopter by increasing the craft’s momentum thereby making it respond in a more sluggish manner. In racing scenarios, the usual selected battery capacities for a 220 sized quadcopter range from 1000mAh to 1500mAh with 1300mAh packs being the most common. On average, a 1300mAh 4S pack will last for about three minutes in a racing quadcopter although flight time is entirely dependent on the manner in which the craft is flown. A professional racing pilot can easily discharge a 1300mAh 4S pack in under two minutes compared to a slower flying beginner who may experience up to five minutes of flight time with a similar battery. When flying longer or faster circuits, many professional race pilots will actually switch from a 1300mAh 4S pack to a heavier 1500mAh battery to reduce the need for battery voltage management during a race. In order to achieve increased flight times(5-8 minutes), long range quadcopter pilots will use even larger batteries up to 2200mAh as flight performance is of less regard to them than flight time.

LiPo Battery C-Rating

FPV Drone Battery Capacity

The C-Rating of a battery is a unit of measurement dictating how much current a battery can continuously supply for its given charge cycle. Simply put, the higher the C-Rating of a battery, the more current the pack can continuously supply. The C-Rating can be multiplied by a batteries capacity in order to calculate a packs theoretical maximum discharge current. Larger capacity batteries can usually supply more current as their internal electrodes have a greater surface area. For example, using the below C-Rating and milliamp conversion formulae, a 1800mAh 100C battery would be able to supply more current to a quadcopter than a 1300mAh 100C battery (180,000mA/180A maximum current versus 130,000mA/130A maximum current respectively). If a battery is forced to supply more current than dictated by its C-Rating for a significant period of time, it can damage the battery by causing the cells to puff, reduce overall longevity, cause excess heating and occasionally cause a LiPO fire. For this reason, it is important to use batteries with C-Ratings that are adequate for their application. For most 220 sized quadcopters, batteries with C-Ratings of 70C or higher are usually recommended, however, quadcopters using high KV and/or large motors may require even higher C-Rating batteries.

C-Rating Formula: Maximum Safe Current Draw (mA) = Battery Capacity (mAh) * C-Rating

Milliamp Hour Conversion Formula: 1000mA=1A (one amp is one thousandth of a milliamp)

Another use for the C-Rating formula is to calculate the compatibility between different motors, propellers, electronic speed controllers (ESC’s) and batteries. As an example, a 1300mAh 100C battery would have a safe continuous current draw of 130A (130,000mA). Dividing this number by four (as a quadcopter has four motors) it is apparent that 32.5A is the maximum current draw per motor that this battery can continuously supply. ESC’s can then be chosen accordingly to accommodate the maximum continuous current draw (in this case a 30-35A ESC would be the most suitable). As a generalisation, most batteries can temporarily exceed their rated continuous current draw safely for around ten seconds. This means that a motor-propeller combination is suitable for use with a battery if the average motor amp draw is within the calculated maximum continuous current draw per motor. It is safe to assume that for most flights, the average throttle position will be at a maximum of 75%. Using this logic, a motor-propeller combination will, on average, be suitable for use on a quadcopter if 75% of the maximum motor current draw is below the calculated maximum continuous current draw per motor. Going back to the current example, a motor-propeller combination quoted with a maximum current draw of e.g. 40A would actually be suitable for use with the 1300mAh 100C battery as the average current draw from that motor would be 30A (0.75 * 40A) which is below the calculated maximum continuous current draw per motor of 32.5A. The website: https://www.miniquadtestbench.com/ is a useful resource when performing these calculations as it provides data comparing the maximum current draw and thrust for many different motor and propeller combinations.  

LiPo Battery Capacity

LiPo Safety

The danger of LiPO batteries is something that many people underestimate but should not be overlooked. Lithium batteries store large quantities of energy in a small profile and are occasionally prone to catching fire. This is a rare scenario and is most likely to occur when charging, discharging, or if a battery becomes damaged. Regardless of the odds, a LiPO fire can result in houses burning down, self-injury or damage to gear. For this reason, it is essential for all pilots to store and ideally charge their quadcopters lithium batteries in a fire-proof LiPO bag or box. If you have already spent a few hundred dollars on FPV gear, what is a few more to potentially save a lot more in damage.

LiPo Safe Bag

FPV Drone Battery Disposal

LiPO batteries are disposable items with a limited lifespan of around 150-250 cycles (Battery University 2017) although this number is entirely dependent on the manner in which they are looked after and used. The main factors impacting the longevity of a LiPO are over-discharging and leaving packs in a charged/discharged state for long periods of time. To increase the longevity of a LiPO, it is recommended to store LiPO batteries at 3.8V per cell and to land a quadcopter at a minimum of around 3.5V per cell (although landing at 3.65V per cell is the safest option). When the LiPO is nearing the end of its life cycle, large voltage drops will be noticeable on throttle punches as the quad will fail to quickly speed up the motors. The LiPO will also lose an increasing percentage of its capacity meaning reduced flight times. To properly dispose of a LiPO, the safest option is to first discharge the pack using a battery charger and then to connect the power leads to a 12V incandescent light bulb for several hours. This will ensure that the LiPO is fully discharged to 0V. As entertaining as the fireball looks, it is not wise to dispose of a LiPO by puncturing or damaging it. Once the LiPO has been discharged completely, it is ideal to cover over the battery terminals and take the pack to your nearest battery recycling facility in order to reduce the overall environmental impact (Battery University 2017).

Who Plugged In?

On a final note, when at a race meet or flying with other pilots, it is common FPV etiquette to leave your quadcopter batteries disconnected from the craft whilst other pilots are in the air. This prevents a scenario where a pilot is blasted out of the air by your video transmitter powering up and makes the event run a lot more smoothly.


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