Important Facts About LiPo Batteries - Handling, Charging, and Disposal of LiPos

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.

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.

Overview of Cell Count

Parallel Connection (2P etc.)

Sometimes a LiPo pack has another number followed by a “P” after the “S”, e.g. 2S2P. The “P” stands for Parallel and means that two 2S packs are connected in parallel. This is used, for example, in LiPo receiver packs to increase capacity. The voltage does not change with a parallel connection.

Capacity (xxxx mAh)

Capacity is indicated in mAh. 1000mAh means that a device with a current draw of 1A (1000mA) can be powered by the battery for one hour before it is empty. If the battery had a capacity of 2000mAh, the same device could be powered for two hours. If the device only required 500mA, a 2000mAh battery would last four hours. The goal is, of course, to fly as long as possible on one battery, but higher capacity also means a larger and heavier battery. Since size and weight are limited when flying, capacity is always chosen within a certain range.

Discharge Rate (30C, 35C etc.)

The C rating of the battery indicates how much current can be drawn from the battery at once. 1C stands for current divided by charge (Amps per Amp-hour). A battery with 2200mAh and a 30C rating therefore delivers 30(A/Ah) × 2.2(Ah) = 66A. The entire multicopter should not draw more than 66A at once. Some LiPo packs also indicate a burst rate, which is higher than the continuous C-rate, e.g. Discharge rate = 30C Continuous / 60C Burst. Burst refers to the maximum peak current that can be drawn for a few seconds.

It is very important not to use a battery with a C-rating that is too low for your copter. Doing so can damage the LiPo battery and possibly also the ESCs.

Tip: The current draw is specified for all components needed to build a multicopter. By adding up these values, you can determine the right LiPo battery.

Things to Consider When Choosing a Battery

The closer the total power of a quadcopter gets to the limit of the C-rate/discharge rate, the hotter the battery becomes. Excess heat wears out the battery faster and shortens its lifespan. Therefore, it’s worth investing in LiPos with a higher C-rate. Additionally, some manufacturers use the best-case measurements for their C-rating claims, so the stated values can sometimes be exaggerated.

A general rule: if you can’t comfortably hold the battery in your hand after flying, it got too hot. In this case, you should choose a battery with a higher C-rating. After a flight, the battery should not exceed 40°C (104°F), and ideally stay under 35°C (95°F).

Note: Internal and external heat have the same effect on the battery. For this reason, you should never leave your batteries in a hot car, for example, on a summer day.

LiPo in Use

LiPo batteries should never be discharged below 3V per cell. If this happens, the LiPo pack is considered damaged and must not be recharged. To maximize the lifespan of your LiPo battery, it's best to stay well above the 3V limit, because no matter how high the C-rate is – if the cell is discharged down to 3V, it can only provide about 1/4 of its original power. Additionally, it will heat up, which significantly reduces its lifespan.

Tip: A LiPo battery performs best when used frequently (cycles). We recommend: it's better to use fewer LiPos more often rather than owning many that are rarely used.

A valuable rule of thumb is the 80% rule. This means you should never discharge a LiPo pack below 80% of its capacity. So if you have a 1300mAh battery, you should never draw more than 1040mAh (1300mAh x 80%) from it. Note that the maximum capacity of a LiPo battery decreases over time.

Note: The charge level of a battery refers to its capacity, not the voltage, as the voltage drop is not linear.

If you don’t have a charger that can measure capacity, the best additional indicator is the resting voltage right after the flight. A LiPo cell that’s discharged to 80% has a resting voltage of approximately 3.72–3.74V. A 3S LiPo battery will therefore have about 11.2V. This can best be measured using a voltmeter or a LiPo battery monitor.

Note: The LiPo battery monitor should be checked for proper function before use to ensure the measured voltage is accurate. The LiPo checker is meant to provide a rough estimate only.

Note/Tip: If your multicopter’s performance or motor speed drops, the battery is likely already too deeply discharged and will be damaged (even after just one deep discharge!). We recommend installing a LiPo buzzer or an OSD (On-Screen Display with voltage readout) in your multicopter.

The best method to assess the condition of a LiPo battery is to measure its internal resistance. Most new LiPo cells with higher capacity and C-rating have an internal resistance of about 2–6mΩ. The older the battery, the higher the resistance and the warmer it gets under load. For smaller LiPo packs (around 100mAh), it’s not uncommon to measure an internal resistance of about 200mΩ. Internal resistance can be measured with a charger, though not all chargers have this feature.

Tip: Age harms a LiPo more than frequent use (as long as it stays within spec). So instead of stockpiling LiPos, it's better to buy fresh ones more often. This also lets you benefit from improvements in RC battery manufacturing and technology.

Charging LiPo Batteries

LiPo batteries are far more sensitive to overcharging than other battery types and can become unusable (or even catch fire). A LiPo battery with a nominal voltage of 3.7V per cell is fully charged once it reaches 4.2V. Due to the risk of overcharging, special chargers must be used that follow the IU charging method (CCCV – Constant Current / Constant Voltage). The electronics control the charge current based on the battery's state and monitor the precise end voltage. First, the battery is charged with constant current. As it nears full charge, the charger gradually reduces the current and finishes the charge with a constant voltage of 4.2V. Once the cells reach 4.2V, charging stops. Even charging to 4.21V can damage the battery! Even if the battery has an internal protection circuit, you should only use appropriate chargers.

LiPo batteries must be charged with the correct current (Amps). The charge rate is often stated on the LiPo pack. If nothing is specified, follow the golden rule: never charge lithium batteries at more than 1C. So a 2200mAh LiPo should be charged at 2.2A. However, some modern packs allow 2C or even 3C charging.

Tip: If the battery is too hot (above 40°C), let it cool down before charging.

If the battery is colder than 15°C, charge it at less than 1C (typical: 0.2C to 0.5C).

LiPo Balancing

LiPo batteries with two or more cells must be balanced, because the individual cells can discharge unevenly. This is especially common with cheaper LiPo batteries. When charging, the balancer ensures that any differences in cell voltage caused by discharging are equalized. The balancer monitors the charging voltage of each individual cell and ensures none exceeds 4.2V. Balancing is done via the balance connector, which must be plugged into the charger in addition to the main lead.

Example

A 3S LiPo battery must not be discharged below 9V (3 x 3V) or charged above 12.6V (3 x 4.2V). If you charge a LiPo battery to 12.6V without using the balancing function, and you don’t know that one of the cells has a lower voltage due to uneven discharge, you might end up with a total of 12.6V made up of, say, 4.4V, 4.4V, and 3.8V. This would destroy two cells and could lead to a LiPo fire.

The same applies during discharge. If the minimum allowed voltage is reached, one cell may already have dropped below 3V, making it permanently damaged and requiring the whole battery 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!