ElectroFlightRC - Basics of Electric Flight

LiPo Batteries

Table of Contents

Litium Polymer Batteries - LiPo’s

Basics of LiPo Batteries

Litium Polymer Batteries - LiPo’s

As of the writing of this document, LiPo batteries are the most popular and suitable type of batteries for electric flight. This document and the ElectroFlightRC.com website assumes the use of LiPo batteries.

So let's get started...

Here are two links with lots of information related to LiPo batteries:

Understanding LiPo Batteries:

This is a website related to RC Heli’s, but it has very good info about LiPos.

LiPo Basics by TJinTech:

This is a website maintained by Chris (TJinTech). He has managed to put together a site with extensive and detailed information about LiPo’s and many other electric powered RC subjects. It has a bias toward Heli’s but we won’t hold that against him ;o)

Basics of LiPo Batteries

Cells:

LiPo’s are composed of one or more cells. These cells each have a voltage of 4.2V at rest when fully charged and 3.7V under load. Cells are usually connected in series which means that the total voltage of the battery is a factor of 3.7V (under load). For example, a 3 cell LiPo has a total voltage of 11.1V (3 x 3.7V). Battery voltage is usually stated as voltage under load (3.7V). Sometimes, the manufacturer (or retailer) will not give the number of cells (see Configuration), but will provide the battery voltage. In this case, divide the total stated battery voltage by 3.7V and you will know the number of cells. For example, 1 cell = 3.7V, 2 cells = 7.4V, 3 cell = 11.1V, 4 cells = 14.8V, 5 cells = 18.5V and 6 cells = 22.2V.

Capacity:

This is measured in milli-amp-hour (see links above for an explanation of milli-amp-hour). Typically this will range from 500mAh for small batteries (tiny actually) to 5000mAh for large batteries. Although, you can now start finding batteries with greater than 5000mAh capacities.

Configuration:

You will usually see a battery referred to as a 3s or 3s1p. This means that the battery has 3 cells in series (from the 3s) and only one of these 3s “packs” in parallel. So the total cells = 3. The number before the “s” represents the number of cells in a “pack” and the number before the “p” represents the number of “packs” in parallel. So a 3s2p would have 2 packs in parallel where each pack is 3s (3 cells in series). In this example, if each cell had a capacity of 2000mAh, the complete battery (3s2p) would have a total voltage of 11.1V and capacity of 4000mAh. However in this case, the details of the battery would be given as 3s2p 4000mAh. Typically, individual batteries are always 1p. The only 2p packs I’ve seen so far are transmitter and receiver packs - which have much lower C ratings.

Discharge C Rating:

The discharge C rating tells you how many milli-Amps (mA) you can draw from the battery. 1C is calculated to be 1 x mAh capacity of the battery but converted to Amps only (remove the hour component). So a battery with a 4000mAh capacity has a 1C = 4000mA. The cell count (or voltage) of the battery has no impact on the 1C value. So the discharge C rating of a battery multiplied by the 1C value of the battery tells you the maximum Amps that can be drawn from the battery. Usually the manufacturer gives 2 ratings related to C - one for constant current and one for peak current which can only be sustained for a limited amount of time - typically 10 to 15 seconds.

Example: A battery with the following specs - 3s1p 4000mAh 20-30C has a total voltage of 11.1V (as described above) and a total capacity of 4000mAh and a 1C value of 4000mA. This battery can sustain a constant current draw of 20 x 1C (4000mA) = 80,000mA or 80A and a peak current draw of 30 x 1C (4000mA) = 120,000mA or 120A.

One thing to understand about the C rating is that at 1C draw, a battery can sustain an Amp draw of it’s 1C value for a full hour before it is completely depleted. However, in the case of LiPo’s, you should never deplete a battery more than 80% of it’s capacity - see the 80% rule. At a 2C draw, a battery will last about ½ of one hour. At a 3C draw, it will last about ⅓ of one hour - and so on. So with the example 3s1p 4000mAh 20-30C battery, at a constant 20C draw, the battery will output 80Amps for at most (60 minutes / 20C) = 3 minutes. At a 40A draw equivalent to 10C (40,000mA draw / 4000mAh = 10C) the battery would last at most (60 minutes / 10C) = 6 minutes.

Discharge C ratings range from 10C to 45C and some manufacturers are starting to produce up to 65C batteries. However, know that the higher the discharge C rating, the heavier the battery (usually) and the more expensive the battery.

Charging C Rating:

This rating works the same way as the discharge C rating, but applies to charging the battery. Typical charging C rates range from 1C to 5C and some manufacturers are starting to produce batteries with charging rates as high as 10C. Again, these batteries with higher charging C rates tend to be more expensive and heavier.

In the past, it wasn't recommended to charge a battery at more than 1C. However, with new batteries coming out with new chemistries and higher rated charging rates (i.e. 5C to 10C), it is now possible to charge at higher C's. However, I would still be conservative when charging. My batteries that are rated for 5C charge rates, I charge at 3C at most. I have a couple rated at 8C and 10C, but I don't charge those higher than 4C. There are basically 3 reasons for this. First, I don't want to over stress the battery and significantly reduce it's lifespan. Second, I don't have a charger and power supply (and generator) that can generate enough Watts to charge at 10C. Third, in my experience, once you reach 3C to 4C charging rates, there is only a marginal time savings by going to 5C or higher. The reason is that the longest time spent charging the battery is at the tail end of the charging cycle when the battery is being balanced and the Amps are low anyway. So, yes, the initial part of the charging cycle goes faster, but the tail end still takes the same amount of time. With my FMA Direct Powerlab 8 charger, I can charge a battery in about 12-13 minutes at 4C if starting at about 20% charge or so. That would probably only drop to 10-12 minutes at 5C. So not a big time saver.

Internal Resistance:

Like all other electrical components, batteries have resistance which is referred to as IR. The lower the IR of a battery the more current it can release which in turn means lower voltage drop under load. The IR of a battery can be used to determine the quality and/or the health of a battery. You will notice the IR of a battery will increase with higher cycles and/or with more abuse such as over discharging the battery.

Minimum Voltage:

You should never discharge to less than 3V per cell under load. However, it is difficult to know the voltage of the cells under load unless you have real-time telemetry that measures either total voltage of your battery pack or the voltage of the individual cells. For this reason, a general rule of thumb has been developed to help the prevention of battery damage. It is called the 80% rule - see below.

The 80% rule:

In short, this rule means that you should not discharge your battery more than 80% of its rated capacity. The battery’s voltage has no bearing on this rule, only the capacity. For example, you should never draw more than 4000mAh from a 5000mAh battery (regardless of the cell count).

How do you know how many mAh you have drawn? Simple, you need a good charger that will tell you how many mAh it has put back in the battery when it charges it. When you first fly a plane, set a timer for a short flight - 5 minutes lets say. Land the plane and recharge the battery. Lets say we are using a 5000mAh battery and the charger says it put back 2800mAh, that’s only 56%. You still have some room. Take the plane up for another flight with a freshly charged battery - 7 minute timer this time. Land and recharge the battery again. If the charge says it put back 3400mAh, that’s 68%. Keep doing this until you reach about 80% and that should tell you roughly how many minutes you can fly for 80%. Make note of how much wind there was during your test because higher winds will require more power and shorten your flight. Take a minute or two off your final time so that you have a margin of error in case you have to abort a landing or two. Cut your flight short if the wind is much higher then when you did your testing. It is also a good idea to check for excessive heat in the motor, ESC and battery when testing to make sure you aren’t potentially causing damage to those components.