In our previous article, we discussed the basic parameters of drone batteries, including capacity, voltage, and discharge rate (C-rating). Today, let's continue exploring three other important aspects: energy density, cycle life, and the Battery Management System (BMS).
5. Drone Battery Parameters – Energy Density
Energy density is another important parameter of drone batteries. However, many manufacturers do not include it on the battery label.
Energy density is generally divided into two types: gravimetric energy density and volumetric energy density.
Gravimetric Energy Density = Energy / Mass
Volumetric Energy Density = Energy / Volume
Unit of gravimetric energy density: Wh/kg
Unit of volumetric energy density: Wh/L
Drone batteries generally require high gravimetric energy density, while small drones place greater emphasis on volumetric energy density.
If the manufacturer does not specify these values, you can calculate them yourself based on the battery's energy, weight, and volume.
Based on current battery technology, a gravimetric energy density of 200–250 Wh/kg is considered a normal level, while 250–300 Wh/kg can be regarded as excellent. Values above 300 Wh/kg generally require solid-state or semi-solid-state battery technology.
Of course, if a battery is equipped with a relatively complex BMS, housing, or thermal management system, its overall energy density will decrease accordingly.
This is particularly true for large agricultural spraying drone batteries, where achieving more than 200 Wh/kg can already be considered a good result.
Similarly, due to the presence of the battery housing, volumetric energy density can be more difficult to estimate. A value above 500 Wh/L can generally be considered an acceptable level.
6. Drone Battery Parameters – Cycle Life
Cycle life is one of the main limitations of drone batteries. High-rate discharge has a significant impact on battery lifespan. However, drones often require high discharge rates during operation, which means some cycle life must be sacrificed to achieve the required power performance. In other words, it is difficult to maximize both power output and cycle life at the same time.
High-quality drone batteries can typically achieve 500–600 charge-discharge cycles while maintaining a capacity retention rate of ≥60%.
Capacity retention refers to the ratio of the battery's remaining capacity after cycling to its original capacity.
For FPV racing drones and other high-speed racing drones that frequently operate at high discharge rates, it is also common for battery capacity to drop below 60% after around 300 cycles.
So, what can be done to improve battery cycle life as much as possible?
First, avoid unnecessary high-rate charging and discharging.
Second, avoid operating the battery at excessively high or low temperatures. Special attention should be paid to low-temperature charging: high-rate charging at low temperatures must be avoided, as it may cause internal short circuits and potentially lead to fire.
Third, prevent overcharging and over-discharging. Always operate the battery within its specified voltage range.
7. Drone Battery Parameters – BMS (Battery Management System)
BMS stands for Battery Management System. It can be regarded as the "brain" of a battery. We will not go into the general definition here, but focus on its three main functions.
1. Battery Safety Protection
Safety protection is the BMS's primary and most important responsibility.
It continuously monitors three key parameters: voltage, current, and temperature.
When the battery faces risks such as overcharging, over-discharging, overcurrent, or overheating, the BMS responds rapidly by disconnecting the circuit or limiting the current to help prevent thermal runaway.
2. Battery Status Estimation
The BMS also estimates the battery's operating status, mainly through two indicators:
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SOC (State of Charge): Indicates the battery's remaining charge, or simply how much power is left.
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SOH (State of Health): Indicates the battery's overall health and aging condition, helping estimate its remaining useful life.
3. Performance Management and Cell Balancing
Drone battery packs consist of multiple cells connected in series and/or parallel. During operation, voltage differences between individual cells can gradually develop.
Without proper management, these voltage differences may continue to increase, causing some cells to become overcharged or over-discharged, which can shorten the overall battery lifespan.
Through cell balancing technology, the BMS helps maintain consistent voltage levels across individual cells. This ensures that cells work more evenly during charging and discharging, reducing the risk of individual cells limiting the performance and lifespan of the entire battery pack.
In actual applications, many drone battery packs do not include a BMS, especially those designed for smaller drones.
We recommend purchasing battery packs from reliable and reputable suppliers, even if they only come with a basic BMS. After all, battery safety should always be a priority.