A Refresher on LiPo Batteries

by Kim Whitburn

LiPo batteries are constructed using Lithium Polymer chemistry (hence the name LiPo). They have a very high energy density compared to other types of batteries. Batteries with a higher energy density are able to store much more energy compared to other batteries of the same weight with a lower energy density, which is why LiPo batteries are so popular.

LiPo cells may be used individually or connected together in series to form the required battery. An individual LiPo cell is a battery in itself and has a nominal voltage of 3.7 volts. By connecting more cells in series, the battery manufacturer can provide various voltage packs to suit different applications. A 2 cell battery pack will provide a nominal 7.4 volts, a 3 cell 11.1 volts and so on up to 6 cells are common. The number of cells defines the voltage of the battery pack.

When you use a LiPo battery the voltage will drop as it reaches its discharged state. It is important not to let its voltage drop below 3.0 volts per cell, else its capacity may be adversely affected. You must also ensure that the maximum voltage across a LiPo cell does not exceed 4.2 volts. If the voltage of any LiPo cell in your battery pack goes beyond this range it can cause the chemicals to become unstable leading to very high internal pressures which can result in a fire, so for this reason LiPo battery chargers are designed to ensure you only charge each cell to 4.2 volts. While using your LiPo batteries most ESC’s have a cut off voltage which may reduce power to the motor when the voltage of your battery pack gets low in an attempt to avoid over-discharging cells.

Note however that most multi-rotor ESC’s have this feature disabled as a motor suddenly being cut off would result in your model crashing out of control. Warning systems are often used to provide the pilot with a warning of low battery voltage.

Batteries are rated in milliampere hours (mAh) which is a measure of how long it can provide energy for a given current draw. Put simply the bigger this number, the more capacity the battery has.

The battery discharge rate is a very important specification, also known as the battery C rating specifies how fast you can extract the energy from your battery. If your motor continuously draws more energy than your battery is rated for it may result in permanent damage to it.

To convert a 2200mAh rating to amp hours, simply divide by 1000 i.e. 2200(mAh)/1000 = 2.2Ah (amp hours). To calculate the actual maximum current you can draw from a battery in Amps you simply multiply its rated capacity by the C value so a 2200mAh (2.2Ah) battery with a C rating of 25C could safely provide a continuous current of 2.2 x 25 or 55A, and its capacity will be exhausted in 1/25th of an hour. Some batteries also specify a burst discharge rate (burst C), representing the peak discharge the battery can provide for short periods of time usually 10-30 seconds. Use the same simple formula above to calculate this peak rate.

Charging LiPo batteries.
When charging your LiPo battery do it outdoors in a suitable location to minimise property damage should something go badly wrong. Also very important to never leave a LiPo to charge totally unsupervised, keep an eye on it from time to time. Be aware that inferior B grade LiPo batteries exist and apart from being cheap they are inferior in performance and safety aspects. Use premium brand LiPo batteries from a reputable hobby supplier and the same can be said about the battery charger. The old saying, ‘you get what you pay for’ is very relevant in both cases.
• Charge your batteries outside in a fireproof location, or in a LiPo safe bag.
• Never charge your battery unattended, from time to time check to see if your battery is getting warm to the touch or starts to swell (puffy). If so stop charging immediately and seek advice before using that battery.
• Never charge a damaged battery, don’t charge if it is swollen (puffy) or has any other visible signs of physical damage.
• Ensure the batteries to be charged are around ambient temperature, let them cool if they feel hot, warm is ok.
• Ensure the number of cells and battery type are set correctly on your charger to match the cell count in your battery if using a programmable battery charger.
• If using a programmable battery charger always use the balance charge function. During the charge process the battery charger will constantly monitor the voltages of each cell and manage them to ensure they are all at the same voltage.
• To be safe, always charge your battery at no higher than 1C (it’s mAh rating) or less. Many chargers allow you to set a charging rate, and although some batteries can support higher charge rates (which speeds up charging time), it’s always best to charge at 1C or less as this means the battery will be under less internal stress, gain energy slower thus keeping it more stable and ultimately resulting in your battery having a much longer life in terms of charge/discharge cycles.
 
Storing LiPo batteries.
• LiPo cells have a voltage range for the chemicals to stay stable while being stored to ensure you get the best life from them. They typically lose approximately 1% of their charge per month through self discharge.
• You should store your battery between 3.80 - 3.84 volts per cell (40% to 50% charge). This is why when purchasing a new battery, it will only be partially charged. If you are flying on a regular basis (every few days) then this is less important as it will not have any noticeable effect if left fully charged for a few days, but if storing for a week or more you should ensure it is at around 40% to 50% charge.
• Always store your batteries in a fireproof location. You can always use an old ammo case available from disposal stores. It needs to be made from fire resistant material.
• Ensure the batteries are stored at room temperature.