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Important Facts About LiPo Batteries

LiPo batteries (short for Lithium Polymer) have taken the RC world by storm for a good reason. There are even three good reasons why LiPo batteries are advantageous over conventional battery types like NiCad or NiMH when used in multicopters:

  • They are lighter

  • They have more capacity in a smaller format

  • They allow for a high discharge current to drive even the most demanding DC motors.

Because of these benefits, LiPo batteries have become popular among many RC model drivers, but RC flight pilots owe it to the LiPo that the sport became so popular in the first place. However, the perfect power storage does not yet exist, and LiPo batteries also have their downsides:

  • They are not long-lasting. Typically 150-200 charge cycles, with proper care and handling possibly more.

  • Due to the volatile electrolytes in the LiPo cells, there is a risk of ignition if mishandled.

  • LiPo batteries require more intensive care than other battery types if one wants to keep them for a long time. Proper charging, discharging, and storage directly influence the battery's lifespan. If misused, the battery must be disposed of prematurely.

LiPo and Values

LiPo Labeling - Structure

Number of Cells (2S / 3S, etc.)

LiPo batteries have a nominal voltage of 3.7V per cell. The advantage over NiCad or NiMH batteries, which have a cell voltage of 1.2V, is that smaller quadcopters (e.g., Hubsan X4) can be powered with a single LiPo cell. For larger flying devices, LiPo packs are used, where two or more cells are connected in series. This allows for a higher voltage demand to be met.

The number of cells connected in series is indicated on the packaging. In RC modeling, batteries ranging from 2S-10S are commonly used. The "S" stands for Series. So, 2S batteries have two cells at 3.7V each, which means 2 x 3.7V = 7.4V.

Example (Click to open)

A 3S LiPo battery should not be charged/discharged below 9V (3 x 3V) or above 12.6V (3 x 4.2V). If you charge a LiPo battery up to 12.6V without the balance function and you don't know that because of differences during discharging one of the cell voltages is lower, then it would result in a total voltage of 12.6V consisting of 4.4V, 4.4V, 3.8V for example. This would destroy two cells, leading to a potential LiPo fire.

The same can happen during discharging. Once the minimum allowed voltage is reached, in this example, one cell would already be below 3V, thus destroying it, and the entire LiPo battery would need to be discarded.

LiPo Storage

Storage has a significant impact on the lifespan of a LiPo battery. As previously mentioned, the cell is considered damaged as soon as its voltage discharges below 3V. However, these 3V refer to the voltage under load. If the battery is NOT connected to a consumer, it's called the open-circuit voltage. 3V under load roughly equates to 3.5V open-circuit voltage (No current flows, so no voltage drops across the internal resistance of the voltage source). If a LiPo battery is stored with a cell voltage close to 3.5V, you risk damaging the cells as every battery self-discharges over time. Although self-discharge for LiPo batteries is slower than other battery types, it's still a concern over long periods. If stored for weeks or months at 3.5V, there's a risk it may drop below 3.5V and become irreversibly damaged due to cell oxidation.

Tip: Although the self-discharge rate of 1% per month is extremely low, it is highly dependent on temperature. It's advised to recharge LiPo batteries every 3 to 4 months to prevent deep discharging.

LiPo batteries should be stored in a charged state, but not fully charged! Fully charged LiPos age faster as the electrodes oxidize. The rate at which a LiPo battery ages depends on the storage charge and storage temperature. A fully charged LiPo can easily be stored for up to four days at room temperature without damage.

For optimal battery life, it is recommended to store the LiPo battery at a charge state between 50%-80% of its capacity in as cold a room as possible. Cold slows down the chemical reaction.

Tip: To conserve the LiPo battery even during short storage, charge it to about 4.1V/cell. Before use, fully charge it to 4.2V/cell within a few minutes.

Note: A LiPo battery that's 50% charged has a cell voltage of approximately 3.85V.

LiPo Disposal

Even correctly stored and maintained LiPo batteries age with use and over time. Once the batteries are depleted or show signs of swelling or damaged cells, they must be properly disposed of. In line with legal regulations, it is recommended to dispose of them at the local battery disposal facility.

Before disposal, they must be fully discharged! Some chargers have a discharge function to drain the battery as much as possible. Then, connect a suitable light bulb to the battery to completely discharge the LiPo battery.

Tip: For a 3S battery, it's best to use a 12V LED bulb.

The battery should only be disposed of once it is completely drained and poses no further risk. Before disposal, cut and insulate each cable separately.

Important: Never throw LiPo batteries in household trash!

Safety/Dangers

Incorrect handling of batteries, especially LiPos, can lead to fire and explosions. Therefore, here are the most important points to note:

  • Keep LiPos and other batteries away from children.
  • LiPos, batteries, and accumulators contain toxic substances that can cause burns on the skin.
  • Always ensure the correct polarity (positive and negative).
  • Never deeply discharge the cells. After use, charge to storage voltage and store safely.
  • Never short circuit.
  • Inspect LiPo batteries for damage, especially after a crash - there may be a risk of fire and explosion.
  • Protect from moisture and heat.
  • Only charge intact and undamaged LiPo batteries. Swollen and damaged LiPos must be properly disposed of.
  • Only charge with suitable chargers with integrated balancers.
  • Never exceed the charging and discharging currents.
  • When charging and storing, keep away from flammable items and place the LiPo battery on a fireproof surface.
  • Never leave unattended while charging.

Even if LiPo batteries appear perfectly intact from the outside, they always pose a certain fire risk. Invisible cell damages can suddenly lead to dangerous short circuits, which are often the cause of house fires. Therefore, LiPo batteries must always be handled and stored with extreme caution. Once a LiPo battery catches fire, there's no saving it. It's distressing to watch an expensive LiPo battery burn, but it's even worse if the fire spreads to your home or nearby stored items. Please always adhere to the safety rules mentioned above.

Important in case of a fire:

  • The smoke produced during a LiPo fire is toxic and often causes much more damage than the fire itself!
  • Do not extinguish a LiPo fire with water! Extinguishing with water can release large amounts of HF gas (hydrogen fluoride). This is because LiPF6 (lithium hexafluorophosphate) is often used as a salt for the polymer/electrolyte. This salt reacts with water to produce HF. However, it can also form when burned if there's humidity in the air.
  • Use CO2 fire extinguishers or sand to smother the flame. Due to the moisture, avoid using foam extinguishers!