LiPo Battery Safety and Procedure
- Hamza Ali
- Yuchen Lin
- Daniel Puratich
- Anthony Luo
Introduction
This document outlines how WARG safely uses LiPo Batteries.
Safety Note
LiPo batteries are perfectly safe, however it is imperative that they are treated carefully, correctly and attentively. Failure to follow these procedures can cause fires and risk catastrophic damage to the surrounding environment.
There are many reasons a LiPo battery can catch fire. However, this is usually a result of improper handling or physical damage. If you are gentle and careful with your batteries, the risk with LiPo batteries is very low.
It is important that WARG members fully understand how to interact with batteries as well as the involved risks, to ensure the maximal safety for our members and equipment.
Sources
Most of the information in this document was produced from LiPo Battery Beginners Guide - Oscar Liang and Lithium polymer battery - Wikipedia.
For more electrical fundamentals regarding batteries please see the subpages to Batteries .
- 1 Introduction
- 1.1 Safety Note
- 1.2 Sources
- 2 Battery Basics
- 2.1 Parts of a battery
- 2.2 Main Lead and Balance Lead
- 2.2.1 XT30/XT60/XT90
- 2.2.2 JST-XH
- 3 Charging
- 4 Guidelines
- 5 Operating Procedure
Battery Basics
LiPo, or Lithium polymer batteries are ubiquitous in electric hobby drones since the advantages of their low weight and increased capacity, as well as their ability to discharge at high current. This makes them a great candidate for our usage.
Parts of a battery
Here is the main list of terminologies used to describe a particular LiPo battery
| 1 | Capacity (mAh) | The capacity of a battery defines how long it is able to provide power. Milliamp hours (mAh) is a unit which describes how much current the battery can provide constantly for one hour. Generally speaking, the higher the capacity of a battery, the larger its physical size.
Over the lifetime of a battery, its actual capacity will lower. We can check when a battery “runs out” of power by checking its voltage, and measuring the total mAh outputted. |
|---|---|---|
| 2 | Voltage (V) | LiPo batteries are comprised of cells. Each cell has a nominal voltage of 3.7V. The capacity of a LiPo cell is constantly changing, matching capacity of the battery. The safe voltage range of a battery is 4.2V - 3.0V. Charging above 4.2V is dangerous and significantly increases the risk of fire. Going under 3.0V permanently impacts performance and potential damage to the battery. In order to maximize battery lifespan, and performance, the optimal range to keep the battery is from 4.2V - 3.5V. The voltage of a battery does not scale linearly to capacity. In the graph below, you can see how voltage plateaus close to 3.7V. This is the Voltage used by manufacturers on labels.  |
| 3 | Cell Count (S) | A LiPo battery can have multiple cells (S). The cells are connected in (S)eries. The nominal voltage of a battery can be calculated by adding up the voltage of its cells. 2 batteries can also be connected together in series to produce a higher voltage battery. The battery voltage has a direct impact on how fast motors spin. More voltage means more power to spin the motors, at the cost of the additional weight of the cells.
 |
| 4 | C-Rating (C) | The C-rating of LiPo refers to the maximum safe power output of a battery. Drawing more current can overheat the battery which poorly impacts the battery life, increases internal resistance, and in extreme causes can lead to fire. The formula to calculate the maximum current draw can be calculated using this formula: Max Draw = C-Rating × CapacityLarger C-rating batteries with the same cell count and capacity can have a noticeable impact on battery size and performance. |
| 5 | Internal Resistance (IR) | Unfortunately, due to the laws of physics, all electrical components have some level of internal resistance (IR), including a piece of wire. The IR of a battery is a pretty good indicator of battery life and high IR can lead to poor performance, voltage “sag” (the drop in voltage when a sudden increase in power is needed, like when increasing thrust significantly), and reduced flight times due to cells not holding charge. The following should be avoided at all costs to help minimize IR, however keep in mind that IR is going to increase slowly and inevitably:
If the IR of a battery increases significantly, or the IR of the individual cells are unbalanced, retiring the battery should be strongly considered. |
Main Lead and Balance Lead
LiPo batteries (with the exception of 1S batteries) will come with 2 sets of leads, the main lead and the balance lead. The main lead is used for powering our aircraft and avionics, and is the output of all of the internal cells placed in series. The smaller set of wires are the balance lead, the total number of wires will match the number of cells, plus one.
XT30/XT60/XT90
The most common connector for the main leads is the XT60. Larger drones can use the XT90 and smaller ones use XT30s. They have roughly the same shape, but different ratings for current and a different physical size. Most drones we fly at WARG will use an XT connector, however there are a few others that are used in some cases, but are generally uncommon to see.
JST-XH
The JST-XH connector is most commonly used for the balance lead. This is used for all batteries with more than 1 cell to individually monitor and balance the voltage of each cell. When charging, it’s important to plug in the balance connector into the charger so it can check the voltage of each cell.
The balance leads have a wire for each individual cell, and the extra wire is ground. Make sure it’s connected properly with ground on your charger. The ground wire is on the end that is typically a different color than the other wires.
If the balance leads of a battery are damaged or broken, make sure to report it. They can be repaired.
Charging
Charging LiPos is pretty easy. The extent of the work that you need to do is just set a few parameters, make sure both the main and the balance leads are connected to the charger, and start it off.
Charger Modes
Balance | Balance charges and discharges the individual cells as necessary to bring them all to the same voltage. If you notice that the individual cell voltage often varies, check the cell IR and consider retiring the battery. |
|---|---|
Storage Charge | Storage charge charges or discharges the battery to the recommended storage voltage, which is between 3.7V-3.9V. Our chargers typically use 3.8-3.85v. For every 2 weeks, a lipo loses ~ 5% capacity. |
Balance Charge | Charge your battery. The charger monitors the voltage of each individual cell to ensure they stay balanced. This is the safest and recommended way to charge LiPo batteries. |
Discharge | Drain the voltage of the battery. This is typically very slow, since it dissipates the energy in the battery into heat. |
Balance Charging
Balance charging is important since no two cells in any battery are identical. After usage in a flight, cell voltages can vary. If batteries were charged without balancing, some cells might end up under 4.2V, and other cells might end up OVER 4.2V which is dangerous and poses a fire hazard.
None of our WARG chargers support a charging mode without balance charging. If you have a charger without balance charging, DO NOT USE IT! It is not safe.
Charge Speed
You typically want to charge at 1C speed (or lower). Battery chargers typically calculate this automatically based on the capacity, however it is important you double check. Charging faster puts unnecessary strain on the battery. Some batteries are able to support faster charging, however, plan flights in advance with ample time to charge batteries for safe operation.
At 1C speeds, your battery will take roughly an hour to charge from empty to full.
A 4000mAh battery at 1C will charge at 4A
A 900mAh battery at 1C will charge at 0.9A
Charging Location
Do not charge LiPo batteries near flammable materials. Do not charge batteries unattended either. After charging, place batteries within the battery storage box. Do not leave batteries out in the bay unattended and exposed.
Operating the Battery Charger
Here is a picture of a battery charger that is similar to what WARG uses.
Once powered on, plug in the balance lead and the main lead into the same battery charger port.
Once connected, verify the battery charger mode is selected as LiPo. It will display the battery type in the top left of the display. If you need to change it, press the Mode/Esc button and use the Dec and Inc buttons to cycle through the battery types. Once hovering on the LiPo, press the Start/Enter button to select it.
To change the charger mode, press the mode button and then use the Dec and Inc buttons to cycle through them. Once selected, use the press the Start/Enter button to cycle through the cell count, capacity, and charging speed (however the charging speed will automatically be set to 1C).
To begin the operation, hold the Start/Enter button. Verify the information, and then press the Start/Enter button once more.
To stop operation, hold the Mode/Esc button until it stops.
Guidelines
General Safety Rules
Pick up LiPo battery by the body, not the wires which could be pulled off from the fragile solder joints
Don’t charge your battery immediately after using it, wait until it’s completely cool down
Never use or charge a damaged/swollen battery
Ensure the number of cells and battery type are set correctly on your charger before charging
Never over-charge, although this is normally taken care of by the charger, it would be a good idea to check cell voltages regularly
Don’t leave batteries under the sun
Always remove battery from the device it’s powering and place it in a safe area before charging
Never short the outputs of a battery
Dealing with a LiPo fire
Don’t panic, unplug all connection first if possible
Use fire extinguisher
If that’s not an option, sand is also an effective way to put out LiPo fire, cover the burning LiPo with sand
Do not breath in the smoke, just wait until the fire goes out and the battery is cool
Do NOT use water ever
LiPo Disposal
Discharge the battery completely before disposal. Ideally, use a halogen light bulb and or a hefty resistor to fully discharge the battery. (Do not use LED lights, since they generally prevent the battery from getting fully discharged and drain energy more slowly).
Swollen batteries
Swollen batteries are not safe to use or store. Discharge and dispose as soon as possible. This is natural to happen, as gas gets trapped within the cells from physical abuse. Once a LiPo is swollen, it cannot be fixed.\
Operating Procedure
When to Charge Batteries
Do not leave batteries fully charged more than 24 hours
Charge LiPo batteries the evening/night before a flight, or in the morning of a flight.
Transporting LiPos
For transporting LiPo batteries, use the LiPo safe bag. They are cheap and light-weight storage, while they might be able to slow down a LiPo fire, they are not effective for stopping or containing LiPo fire.
Make sure to store batteries back into the long term storage box after flights.
Deciding When to Land
You should land when your battery voltage reaches 3.5V to 3.6V.
You could keep flying until voltage is even lower, but it puts extra strain on the battery and might shorten lifespan faster. All cells are different in a battery, when you give the drone a burst of throttle the battery will sag and some cells might sag more than others and go below the safe limit and risk causing damage to those cells. Landing a bit early at 3.5V reduces the chance of that from happening.
Voltage also drops off significantly quicker once the battery reaches around 3.5V.
After a Flight
Keep all batteries that were transported to the flight in the LiPo safe bag. Discharge each one to storage voltage before placing back in the LiPo box.