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 should 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
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 (in series). 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.
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.
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).
The capacity is specified in Ah - Amp(ere) hours, or more commonly in our hobby, 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. Depending on the discharge rate this might be scaled up, for example with a properly rated 300mAh battery you can draw 1.2A (4 x 300mA) for 15 minutes.
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.
Maximum Discharge Rate (C Rating)
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 45-70C. 45C means you can continuously draw 45 times the rated capacity, so 13.5A or for short bursts 70 times the rated Capacity, so in our example 21A.
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 an indicator to compare different models from one 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 internal resistance of a cell decides 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.
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.
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.