C Rating? Mili Amp Hours? How many cells?
When it comes to the LiPo batteries we use in our hobby, there are a couple of specs you simply need to know about, what they mean, how to interpret them and most importantly how to compare them.
At first all those specs seem a little bit overwhelming, but when you look at them one at a time, they all make sense and are not all too complicated.
And this is what this post is all about, I will look at all the different specs and explain what they mean and how they interact with each other. I will also give you some general LiPo related tips.
Cell Count and Configuration of a LiPo Battery
A LiPo battery consists of a certain number of cells. Those cells can be in serial configuration, parallel configuration or a mixture of both. Most common for quadcopters is the series configuration, denoted by S.
1S is the most basic configuration, a single cell. A single cell has a nominal voltage of 3.7V - when fully charged 4.2V. When there are multiple Cells in series, you are multiplying the voltage of a single cell to get the total voltage of the battery. A 2S battery are two cells in series, thus having a nominal voltage of 7.4V - when fully charged 8.4V. 3S are 11.1V, 4S 14.8V and so on.
There are also HV - high voltage - batteries which can be charged to 4.35V instead 4.2V per cell. This is a relatively controversial topic and from my experience HV batteries have a shorter live span when compared to non HV batteries. I tend to use my HV batteries like normal batteries and only charge them to 4.2V. The only exception being the small 1S batteries, they are so cheap that I don’t mind replacing them more often if I can get a decent amount of flight time increase out of them.
As mentioned before there is also parallel configuration where the cells are attached in parallel. This configuration multiplies the capacity, but not the Voltage. So a 2P battery has two cells in parallel, 3.7V nominal voltage and double the capacity of a single cell. In the quadcopter hobby and especially with micros and brushless whoops the parallel configuration is not used very often.
You should make sure not to discharge your battery lower than the nominal voltage at rest (rest meaning, that the battery is not attached to anything).
Storage voltage of a LiPo Battery
You should never leave your LiPo batteries sitting around fully charged or fully discharged for a longer amount of time since they will degrade pretty quickly. Instead the battery should be stored at storage voltage of a around 3.8 - 3.85V per cell. At this voltage a LiPo battery can be stored for longer periods of time without degradation.
Read my article about how long you can keep a LiPo battery fully charged to learn more about this topic.
An overview of different battery configurations, their nominal, fully charged and recommended storage voltages:
|Battery||Nominal voltage||Fully charged||Storage Voltage|
Capacity of a LiPo battery
The capacity is specified in Ah - Amp(ere) hours, or more commonly - especially with the smaller batteries we use on micros and whoops - mAh - milli Amp hours. It basically specifies how many ampere you can draw continuously over an hour.
For example: if you have a 300mAh battery, you can draw 300mA continuously for one hour. When continuously drawing 1.2A from the same battery, the battery will only last 15 minutes before being fully depleted (300mA / 1.2A * h = 0.25h). How fast you can actually safely discharge a battery is explained in the next section.
You should never fully discharge your LiPo battery - a rule of thumb - for long battery live - is to maximally draw 80% of the rated capacity, so from a 300mAh battery you should maximally draw 240mAh. This can be easily verified by monitoring the battery when re-charging it after a flight. Usually you reach this point when you discharge your battery to 3.7V resting voltage.
Maximum Discharge Rate (C Rating) of a LiPo battery
The discharge rate is basically the limit of Amps you can draw from the battery. Usually C ratings come with a range, a continuous C rating and a burst C rating. Let us continue with the example from above, a 300mAh battery rated at 45-70C. 45C means you can continuously draw 45 times the rated capacity, so 13.5A (300mA x 45) or for short bursts 70 times the rated Capacity, so in our example 21A (300mA x 70).
The C rating is unfortunately a very loosely defined rating on which the manufacturers do not really agree on. So it is very hard to compare batteries according to their C ratings. Usually it is a good enough indicator to compare different models from the same brand, but I would not compare C ratings of different brands with each other, or at least only see it as a “ballpark” range.
The easiest way to compare C ratings of two different batteries with the same capacity is, to simply fly them, do some punch-outs and see which battery takes the longest time before voltage sag can be noticed.
Internal Resistance of a LiPo battery
The internal resistance of a cell is one of the main factors of how high the discharge rate is, the lower the internal resistance, the higher the discharge rate. Usually the internal resistance of a battery increases with its charging cycle and the battery “loses punch”, meaning after discharging and charging the battery, lets say, 20 times, your C rating will have decreased.
Usually smaller capacity cells have a higher internal resistance than larger ones. On a good quality battery, all cells will have an internal resistance that is close to each other and as low as possible. For example a 2S, 450mAh with 20mOhm and 21mOhm is something I would consider good.
A lot of chargers can measure internal resistance, but be aware that they might not be very precise. Still you can use it to compare batteries relatively to each other. The readings of the internal resistance should be taken when the battery is fully charged.
On my ISDT charger I can scroll to view the internal resistance of the cells, when monitoring them during a charge cycle, the values fluctuate quite a bit, but since I read the values at the end of the charge cycle I can compare one battery to another one and have a good relative comparison.
Batteries not lasting long
If your batteries are not lasting long anymore, it is - with a high chance - because of high internal resistance. Once the internal resistance is to high, the high current draw con not be satisfied and the voltage starts to sag quickly. Unfortunately there is nothing you can do to improve it.
I tend to keep those batteries for bench testing before I retire and recycle them.
What is voltage sag?
Voltage sag indicates how fast the battery moves from a higher voltage level to a lower one. When you start out with 4.2V and after 10 seconds of flight you are at 3.8V, that is a pretty high voltage sag. Optimally you want your battery to deplete linearly, so the time to move from 4.2V to 4.0V should be the same as from 4.0V to 3.8V. In reality this is not the case though. Most of the time you will quickly drop below 4.0V per cell.
Batteries with a high internal resistance or a low C rating usually sag more than their low internal resistance, high C rating counterparts.
Common LiPo battery Plugs
Batteries come with different plugs. If it is a 1S battery, then it only has a discharge plug. Everything with more cells in series also has a balancing plug.
The bigger the battery and the higher the discharge rate, the bigger the discharge plug should be. Plugs are rated for the amount of current they can safely transmit without melting. You can notice that the plug is at its limit when its getting hot. In this case you should definitely upgrade to the next bigger plug. With brushless whoops PH 2.0 and XT30 connectors are very popular. With 5” quads, usually XT60 connectors are used nowadays.
The balance plug is used to charge every cell at the same rate, so that the voltage always is the same across all cells of a battery. Technically you can charge without the balancing plug attached, but honestly - you should not, it can easily lead to a fire since one cell might get over discharged since only the total voltage is being monitored but not the individual ones.
Check out my article about different kinds of battery connectors to learn more about this topic and help you chose the right connector for your application.
Basically all of the above specs decide how heavy the battery will be. More cells, more weight. Higher capacity, bigger cell, more weight. Usually batteries with higher C ratings are also slightly heavier than their lower rated counter parts. Bigger plug, more weight.
You might think: Hey, I want a longer flight time, I simply take a higher capacity battery. Unfortunately it is not that easy - a higher capacity battery will also have a higher weight and a point of diminishing returns will quickly be reached. In my experience every copter has a sweet-spot when it comes to battery capacity and wight. But it also comes up a lot to personal preference.
For example: I like my builds to feel nimble and simply don’t like if a quad flies like a “school bus”. If I feel it is too heavy, I will go with a lighter battery. For 1S, 65mm whoops I like to go with 300mAh batteries. On 2S builds, like 85mm whoops or 2.5” micros and toothpicks I like to go with a 450mAh, 2S battery.
- 29.9.2018: Created
- 4.6.2021: Updated wording, added section about sag and storage voltage
Chris is a Vienna based software developer. In his spare time he enjoys reviewing tech gear, ripping quads of all sizes and making stuff.
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