Moving up in battery cell count has been an ongoing shift in the world of FPV drones. In 2015, 4S overtook 3S as the standard. By 2018, 6S was all the rage. Fast forward to 2023, the buzz now revolves around an even higher voltage – 8S. If you’ve wondered why, you’re not alone. In this article, I’ll delve into the reasons behind this trend, discussing the benefits and drawbacks of using 8S batteries, and help you determine if building an 8S drone in 2023 is a good ideal.
When discussing 8S, we’re referring to the number of cells in a LiPo battery, which determine the battery voltage. To learn more about the fundamentals, take a look at my LiPo battery tutorial.
What Do You Need To Fly 8S?
Ready to take the leap to 8S? You can procure 8S LiPo batteries here: https://oscarliang.com/product-voun
Keep in mind, most Lipo chargers on the market only support up to 6S, so an 8S-supporting charger will be a necessary investment:
In terms of motors, you’ll need those with an appropriate KV. Generally, a range of 1200KV to 1500KV fits well for 8S, comparable to 1600-2000KV on 6S. If you have 6S motors compatible with an 8S voltage, they can still be used by using a 75% output limit in Betaflight, similar to how people fly 6S on 4S motors.
Additionally, an 8S-compatible FC/ESC stack is a necessity. As of now, the options are limited, but if you’re aware of any available stack, please let me know in the comments below.
Advantages of Higher Voltages
Using higher voltage to power our FPV drones has many advantages, which explains why 6S is more popular than 4S nowadays for 5″ FPV drones. I’ve previously explained the benefits of transitioning from 4S to 6S in this article: https://oscarliang.com/6s-mini-quad-racing-drone/.
The main take away from the above article is that when delivering a certain amount of power, a higher voltage can achieve this at a lower current. This results in less heating everywhere – your battery, the XT60 connector, the wires, the ESC, and the motors. A reduced current also mitigates battery voltage sag, this in turn boosts throttle control and responsiveness and maintains consistent performance throughout the entire flight.
Switching from 6S to 8S carries many of these same advantages, such as less voltage sag, higher efficiency thanks to reduced system losses. However, as the voltage continues to increase, the returns begin to diminish. It’s crucial, therefore, to weigh these benefits against the potential drawbacks of higher voltage, as we’ll explore next.
The Issue with Higher Voltages
As explained in my motor buyer’s guide, Motor KV represents the motor’s speed measured in RPM per volt applied. In essence, to achieve the same RPM, you need lower KV motors when using higher cell count batteries.
To make a lower KV motor, we need more turns of the wire around the stator, which can potentially lead to worse cooling performance. When switching to 8S, the rise in motor stator windings might not pose a significant cooling issue compared to 6S, and most likely to be offset by lower current and efficiency gain. However, when considering even higher voltages such as 12S or 16S, cooling could become an issue, which requires further testing to verify.
At present, 8S batteries are quite rare, and if found, tend to be far pricier than 6S batteries. They also tend to be bulkier, adding weight to the drone and increasing the likelihood of battery ejection and damage during a crash.
Additionally, very few FPV drone components (such as FC and VTX) can handle the higher voltage of 8S. If incompatible, a voltage regulator would be needed to power your electronics. Most components are designed with 6S in mind and may not have undergone enough testing for 8S compatibility, potentially increasing the risk of failure.
When using 8S, it’s crucial to monitor voltage closely. Due to performance not degrading as significantly as with lower cell count batteries even at low voltage, it’s easy to overlook and overdischarge the battery, risking a sudden drone drop without substantial warning.
Conclusion: Not Recommended in 2023
It’s tempting to build a 8S quad, but at this point in time, my personal advice would be to hold off for a bit.
Firstly, it’s likely to be a costly project. You will need to invest in new 8S batteries and compatible chargers. The options for motors and ESCs are limited. Even when available, durability can be a concern with 8S, given that most components are only tested for 6S, and comprehensive testing for 8S is yet to be conducted.
Secondly, the potential for damaging an 8S LiPo battery is higher, given the larger number of cells.
In a nutshell, 8S could potentially outshine 6S in terms of flight performance when paired with the right setup – on paper it should be more responsive and powerful. However, it does present significant challenges, such as higher costs and a lack of reliable hardware options. Also, it might not suit all flying styles. Over time, as more manufacturers delve into the development and testing of 8S-compatible components, it might become a more feasible choice. As always, your personal needs and flying style should be the guiding factors when deciding whether to switch.