LiPo Batteries - vanderbiltrobotics/RoboticsIntelligenceDatabase GitHub Wiki
What are LiPo Batteries?
Lithium polymer batteries (commonly called LiPo) are frequently used in drones and other places where maximizing energy capacity per weight and volume is necessary. We have used LiPo batteries on our robot for this same reason. A Lead-Acid battery, a kind frequently used on cars, will weight and take up the space 2-3x of an equivalent capacity LiPo battery. Alternative battery technologies such as lithium-ion or LiFePO4 are interesting, but the wide spread use of LiPo batteries in drones and RC cars makes the price much more affordable. However, LiPo batteries can also be very dangerous if mishandled.
Safety Warnings
LiPo batteries are perfectly safe if treated properly, but it is important to know the potential risks and take measures to mitigate them. Here is an example of what happens if a LiPo battery is punctured. Note that this battery is much smaller than the capacity of batteries which we normally use.
Please read this entire guide on safety measures for LiPos. You should know this entire list, but I will list the specific measures we take.
- Puffed or damaged batteries should immediately be safely discharged and properly disposed of.
- Store batteries in one of our fire proof bags. Fully charged batteries should be placed in the individual bags.
- Use the dedicated LiPo charger in balance charge mode to ensure proper cell voltages and charging current.
- Be aware of the "Metal fire" fire extinguisher in the shop. This is the proper way to extinguish a LiPo battery fire.
- Actively prevent the puncture of the batteries. Design their enclosure to be free of sharp edges and protect the batteries from the mechanical systems.
"S" Rating: Cell Count / Voltage
All LiPo batteries will have a rating such as 2S, 4S, or 6S. This number corresponds to the number of cells in the battery. You can think of each of these cells as being its own battery, which have then been assembled in series to form one big battery. The nominal voltage of one cell is 3.7V, so the nominal voltage of a 4S battery is 14.8V.
"Ah" Rating: Capacity
The capacity of LiPo batteries is given in mAh (milli-amp hour) or Ah (amp hour). This capacity corresponds to the rough amount of current that can be drawn from a battery in an hour. For example, a 16Ah battery could sustain a 16A load for one hour, an 8A load for two hours, or a 32A load for half an hour. To calculate real energy capacity from the Ah capacity, you can multiply Ah by the nominal voltage of the battery to get Wh (watt hours).
"C" Rating: Discharge Rate
The discharge rating of a LiPo battery will often be given in units of C, such as 5C, or 15C. A C rating is a made up unit that corresponds to the number of times the battery could be fully discharged in one hour. For example, a 5C battery could be fully discharged in 60 mins / 5C = 12 minutes. This number can be combined with the Ah rating to determine the maximum output current of a battery. The formula is the maximum current (A) = (the amp-hour rating) x (the C rating). For example, a 10Ah 5C battery has a maximum output current of 50 amps. However, do not always trust this rating; it is often exaggerated, and you should use common sense with the wire gauge of the battery leads.
Charging
Always use the balance charge feature. This mode requires attaching the balance lead to the charger so that it can read and balance the charge of each individual cell. Batteries should be charged while in the fireproof bag if possible. The temperature of the battery should always be monitored while charging to make sure it doesn't get hot. Note that the charger will get hot during this process and shouldn't be touching the battery.
LiPo Cell Voltages
Unlike other battery technologies, LiPo batteries have a relatively large voltage swing from fully discharged at 3.3V to fully charged at 4.2V. With larger numbers of cells this voltage swing can become very large, and systems using these batteries need to be designed to operate under this entire range. Here is a nice chart that shows the voltage curve. Note that a LiPo battery with a voltage of 3.3V technically contains more energy, but dropping below this voltage will permanently damage the battery.
| State | Voltage |
|---|---|
| Fully Charged | 4.2V |
| Storage | 3.8V |
| "Nominal" | 3.7V |
| Discharged | 3.3V |
The nominal voltage is just the average cell voltage which is useful for calculating energy capacity.
LiPo Storage
Storing LiPo batteries fully charged or fully discharged is not good for the lifetime of a battery. A battery should be stored with a cell voltage of 3.8V to maximize battery lifetime. An improperly stored battery will slowly start to puff. A puffed LiPo battery should immediately be safely discharged and properly disposed of. However, it is also necessary to keep charged batteries on hand for use with the robot, and it is safe keep them fully charged for testing days or the days of competition. Batteries at 3.8V can still be used for small scale testing, but you must be careful to not go below 3.3V.
Over-discharge
The voltage of each individual cell should never go below 3V or you risk permanent damage to the cells. Specifically over-discharging a LiPo significantly increases the internal resistance of the battery. This resistance causes the battery to produce additional heat whenever it is being charged or discharged. If the temperature gets too hot, you risk the battery catching fire. Additionally, heat will accelerate the deterioration of the battery and cause it to puff more quickly. It may be possible to recharge and continue using an over-discharged battery, but it should be carefully monitored at all times to ensure that it does not get too hot or dangerously puff.