LiPo batteries and safety

LiPo battery basics

Lithium polymer batteries, better known as LiPo, have a high energy density, high discharge rate and low weight, making them an excellent candidate for RC applications.

By learning the basics about LiPo batteries, you will be able to read and understand their specifications.

Battery voltage and number of cells (S)

LiPo batteries used in RC consist of cell individual connected  in series, so the battery voltage is the sum of the cell voltage. Each cell in a standard LiPo battery has a nominal voltage of  3,7V . Therefore, battery voltage is often referred to as the number of cells in the battery (aka "S").

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

For example, we call a 14.8V battery a "4-cell" or "4S" battery.

Voltage directly affects brushless motor RPMs, therefore you can use higher cell count batteries to increase quadcopter speed if the motor/ESC and other electronics accept higher voltage. But a multi-cell battery of the same capacity is heavier because it contains more cells.

A LiPo battery is designed to operate in a safe voltage range from  3V to 4.2V .

Discharge below 3V can cause irreversible loss of performance and even damage to the battery.

Overcharging above 4.2V can be dangerous and eventually cause fires.

However, it is advisable to stop discharging when it reaches 3.5V for battery health reasons. For example, for a 3S Lipo, the maximum voltage is 12.6V and you should ground when the voltage reaches 10.5V (at 3.5V per cell).

LiPo battery capacity and size

The capacity of a LiPo battery is measured in mAh (milli-amp hours)." MAh " is essentially an indication of how much current you can draw from the battery for one hour until it is empty.

For example, for a 1300 mAh Lipo, it would take an hour to fully discharge if you draw a constant 1.3A current from it. If the current draw doubles to 2.6A, the time would be halved (1.3 / 2.6 = 0.5). If you draw 39A of current non-stop, this packet would last only 2 minutes (1.3 / 39 = 1/30 of an hour).

Increased battery capacity can give you more flight time, but it will also weigh more and have a larger physical size. There is a trade-off between capacity and weight, which affects flight time and aircraft agility.

Higher capacity can also give you higher discharge current, as you will see in the next section.

C Rating (Discharge rate)

Today's Lipo batteries for quadcopters are all C-rated. Knowing the C rating and the capacity of a battery, we can calculate maximum discharge current continuous and reliable LiPo battery.

Maximum discharge current = C-Rating * Capacity

For example, a 1300mAh 50C battery has an estimated maximum continuous discharge current of 65A.

Some batteries have two C ratings: 'continuous' and 'burst'. Burst rating is only applied in a short period of time (e.g. 10 seconds).

Although the C rating could be a useful tool, it has become mainly a marketing tool and is not always accurately reported.

If the C rating is too low, the battery will have difficulty transmitting power to your motors and your quad will be underpowered. You can even damage the battery if the current consumption exceeds the safe rating.

Connectors

As a general rule, the battery connector should match the one you use on the helicopter. If you don't already own a quad, pick one and stick with it.

All Lipo batteries come with 2 sets of wires/connectors: one  balance cable  and a main cable or a  discharge cable  (except 1S batteries which have only one main cable). There are quite a few different connectors used in LiPo batteries. The main differences are shape, weight and current rating.