The purpose of this article is to provide complementary explanations on LiPo battery technologies as well as battery management.
A Lipo battery has four main physical parameters:
- Discharge capacity: remaining charge of the battery.
- Voltage: Defines the maximum rotation speed of the motors.
- Discharge rate: How much capacity the aircraft draws from battery per second.
- Internal resistance: It is a physical imperfection wich causes power losses and heating. Directly linked to temperature and ageing.
The most important indicators for the battery status are discharged capacity and voltage.
When the voltage is too low, the motors will not be able to spin fast enough to sustain hovering. On Elios, this critical point is reached at 9.5v. But, as we can see on the figures presented below, the discharge capacity can not be easily linked to the battery voltage as this relation is dependent on the discharge rate, the temperature, the internal resistor, and its age. Furthermore, the capacity of the battery is decreasing with the number of cycles.
Discharge rate of the various Elios states
Effect of the discharge rate of the battery discharge at a constant temperature. The first “low battery warning” happening when the battery reaches 25% of remaining capacity. The “battery critical warning” happening when the battery reaches 10% of remaining capacity. Due to the internal resistance of the battery, we can see that the voltage drops in a correlation of the discharge rate. Please note that those figures are just illustrations and are not resulting from a measurement.
It is thus important that the pilot and the co-pilot remain aware of the battery voltage and the battery capacity at all time. Those two values are displayed in Elios cockpit below as ''ROBOT BATT''. Please note that if the battery voltage is below than 12.4v upon Elios power up, Elios consider the battery as not fully charge and is not capable of measuring the remaining charge of the battery. Hence, Cockpit will display "??%" instead of the state of charge.
Also, the low battery warning and the battery critical warning are here to warn you that you have reached the limitation of the battery in normal operation. If can be avoided, don't reach the battery critical warning.
Effect of the temperature and the ageing of the battery on its internal resistance.
Effect of the temperature on the battery discharge at same discharge rate. The temperature has a direct effect on the internal resistance. The colder a battery gets, the higher its internal resistance gets and the higher the voltage drops. One can see on the highlighted red region spike going down. Please note that those figures are just illustrations and not resulting from a measurement.
Good practice with batteries
Please follow those guidelines:
- While in operation keep batteries between 10ºC and 40ºC at all time.
- Make sure the batteries are fully charged before each flight.
- Land immediately the aircraft when the battery critical warning cues in Elios Cockpit.
- In addition to the battery percentage, pay attention at all time to the voltage displayed on Elios Cockpit – land at 10.2V. At 10V, the robot should be on the ground, disarmed.
- Do not let the aircraft idle for too long before unplugging the battery.
- If the battery’s voltage drops at any time below 8V or if the battery’s state of charge reaches 0%, it might be damaged, do not try to charge or use the battery anymore.
- For any unusual behaviour, keep the battery in a battery safe bag and the Log file aside and contact Flyability.
- Always check the battery mechanical integrity before any operation.
Low temperature operation – between 0ºC and 10ºC
- Elios should not be used in environments where the ambient temperature is below 0ºC.
- Keep your battery temperature above 10ºC with a battery warmer bag qualified by Flyability. Do not hesitate to contact Flyability if your operation requires one.
- Do not wait before flying after picking up the battery from a warm place so that it does not get colder than 10°C.
- In addition to the battery percentage, pay attention to the voltage displayed on Elios Cockpit – land at 20% or 10.2V, whichever comes first. At 10% or 10V, whichever comes first, the robot should be on the ground, disarmed. Do not wait before unplugging the battery.
- In general, fly carefully and shorten your flights.
- After a flight, let the battery cool down to room temperature before charging it. It generally takes about 30min.
- Do not charge your batteries below 15ºC or above 35ºC.
- Always place your battery in a battery safe bag while charging. Never leave it unattended.
- The charger is designed to stop charging when the battery is fully charged. But it is good practice to monitor the charging process and disconnect the battery when fully charged.
- It is strongly advised to track battery cycles. Battery should not be used beyond 40 cycles. As shown below.
- Discharge the battery to 40%–60% if it will not be used for more than 3 days. It will greatly extend their service life. The discharge can be done by using your aircraft until the battery state of charge reaches 40%–60%.
- Battery life may be reduced if not used for a long time. It is a good practice to use them at lease every 3 months
- Do not store the battery in the aircraft.