Capacity, energy, voltage, continuous discharge rate, peak discharge rate… Feeling a little overwhelmed by all these terms? Don’t worry! This article starts with the basics and explains each concept step by step, combining theory with real-world applications to help you gain a clear understanding of drone battery specifications.
1. Capacity vs. Energy
Battery capacity plays a major role in determining a drone’s flight time. However, we say “plays a major role” because the factor that truly determines a drone’s flight time is its energy.
So, what’s the difference between capacity and energy? Let’s look at the formula below.
Energy = Capacity × Voltage
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Energy unit: Wh (watt-hours)
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Capacity unit: Ah (amp-hours)
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Voltage unit: V (volts)
In other words, even if your battery has a high capacity, its total energy may still be relatively low if its voltage is low!
2. Voltage
Let’s continue with voltage. Voltage refers to the electric potential difference between two points in an electric field. In a battery, it is the potential difference between the positive and negative terminals.
The charge cut-off voltage is the maximum voltage a battery can reach during charging, while the discharge cut-off voltage is the minimum voltage to which it can safely discharge. Keep in mind that both overcharging and over-discharging can seriously damage a battery and pose significant safety risks.
A battery’s voltage changes continuously during operation, from the charge cut-off voltage down to the discharge cut-off voltage. So, which voltage should we use when calculating energy?
The answer is the average voltage.
Here’s a useful rule of thumb to remember:
Average voltage ≈ 3.7 V × Number of cells connected in series
How do you determine the number of cells connected in series? Most battery packs indicate this using an “S” rating. For example, 6S means six cells connected in series.
You might wonder why we need to calculate this ourselves when the battery pack already displays its voltage. The reason is that some sellers list the maximum voltage, while others calculate the voltage using an average of 3.8 V or 3.9 V per cell. That’s why it’s helpful to understand the numbers yourself rather than relying entirely on the label.
3. C-Rating (Discharge Rate)
The C-rating is another critical parameter for drone batteries because it indicates the battery’s discharge capability.
C-Rating = Discharge Current ÷ Battery Capacity
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C-rating unit: C
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Current units: A (amperes), mA (milliamperes)
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Capacity units: Ah (amp-hours), mAh (milliamp-hours)
Make sure the current and capacity units are consistent. For example, don’t use A for current and mAh for capacity without converting the units first.
Let’s look at a straightforward example. If the discharge current is 10 A and the battery capacity is 5 Ah, the discharge rate is:
10 A ÷ 5 Ah = 2C
Drone batteries often operate at discharge rates of 10C or even 20C. What does that mean in practice?
At a 20C discharge rate, a battery could theoretically be fully discharged in just 3 minutes under a constant load. This is why drones are highly sensitive to battery discharge performance. If a battery has poor high-rate discharge capability, its usable capacity can drop significantly when subjected to high discharge currents.
Imagine you’re climbing rapidly when your battery suddenly runs out of power. That’s exactly the kind of situation you want to avoid.
C-ratings can be further divided into several categories, including continuous discharge rate, burst discharge rate, and charge rate.
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Continuous discharge rate: The C-rating at which a battery can discharge continuously under specified conditions.
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Burst discharge rate: The maximum discharge rate a battery can deliver for a short period, typically no longer than 10 seconds, depending on the manufacturer’s specifications.
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Charge rate: The C-rating at which a battery can be charged, subject to the manufacturer's limits.
Important: The 10C or 20C rating printed on a battery pack often refers to its burst discharge rate, not its continuous discharge rate. Always ask the supplier to confirm the continuous discharge rating and leave an adequate safety margin based on your application.
Different drone applications have vastly different requirements for battery discharge performance. Below are several typical scenarios for reference.
| UAV Type | Typical Application | Continuous Discharge Rate | Peak Discharge Rate |
|---|---|---|---|
| Racing FPV Drone | FPV racing, freestyle flight | 60–120C | 100–200C |
| Tinywhoop / Cinewhoop | Indoor aerial photography, micro freestyle | 40–60C | 60–80C |
| Consumer Aerial Drone | DJI Mini / Air / Mavic series | 15–25C | 25–40C |
| Industrial Mapping & Survey Drone | Inspection, aerial survey, security monitoring | 10–20C | 20–30C |
| Heavy-lift / Agricultural Plant Protection Drone | Heavy payload, spraying, large frame | 15–25C | 25–40C |
| VTOL Drone | Hovering for takeoff & landing, long-distance cruise | 10–15C | 20–30C |
| Fixed-wing Drone | Long-distance cruise, long endurance | 2–5C | 5–10C |
4. Series and Parallel Connections
Now that you understand the parameters above, it’s easier to understand series and parallel connections.
Battery packs are usually labeled with an “S” and a “P” rating. S stands for the number of cells connected in series, while P stands for the number of cells connected in parallel.
For example, 6S2P means 6 cells connected in series and 2 cells connected in parallel, for a total of 12 individual cells.
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Series connections increase voltage.
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Parallel connections increase capacity and current-handling capability.
Suppose each individual cell has an average voltage of 3.7 V and a capacity of 10 Ah. The resulting battery pack would have:
- Voltage: 3.7 V × 6 = 22.2 V
- Capacity: 10 Ah × 2 = 20 Ah
One important point to remember: Series and parallel connections do not increase the C-rating of the individual cells. Under the same cell specifications and appropriate pack design, the battery pack's C-rating remains the same as that of each individual cell.
These are the most fundamental performance parameters you need to understand when choosing a drone battery. How much have you learned so far?
Next time, we’ll dive deeper into topics such as energy density, cycle life, and Battery Management Systems (BMS). Stay tuned!