Choosing the right lipo battery and keeping it healthy
This type of power system is only possible because of modern lithium polymer battery technology. Scroll down to learn more about LiPo batteries, which one to buy, and how to get the maximum life out of them.
Understanding C-Rating
As in many facets of the RC world, it is extremely easy to misrepresent what a battery is or is not by referring to its designed "C-rating". While all batteries are not created equal, it is important to understand that expensive does not always mean better in the LiPo world. The raw material required to make a LiPo battery comes from a limited number of suppliers. It is not uncommon for two different LiPo brans to contain the same material but be labeled significantly differently. Unfortunately, it can be down right impossible to tell whether the end use if being told the truth based solely on the information presented on the sticker.
The C-rating is a critical factor in selecting the right battery for your jet. Electric Ducted Fan's demand a significant amount of current throughout the power band but in particular at full power. A battery that is labeled with a high C-rating (65c or higher), will have an easier time maintaining voltage under load. Obviously, as the battery capacity is consumed, it will continue to decrease its supply voltage until the cells can no longer supply anything.
Another factor to consider when selecting the right battery for an EDF is the battery capacity. As a rule of thumb, I recommend using nothing less than 8000mah (8ah). In theory, an 8ah LiPo battery that is rated at 65c continuous rate can supply 520amps (8x65) under load. Generally speaking, batteries that are lighter in weight but claim the same capacity and C-rating as a heavier battery will not be able to supply high currents without experiencing significant voltage sag almost immediately after power is demanded. In the EDF world, the end user pays a penalty in weight if the desire is a truly powerful battery. There is simply no getting around this fact!
"Low" Voltage Defined
A terrible aspect of the RC hobby, particularly in the electric arena, is the blatant speeding of misinformation based on nothing by anecdotal evidence and/or hearsay. Since it is so difficult to know exactly what a battery pack may be doing in realtime, people are quick to set extremely conservative limits how they use their battery packs. The best way to monitor the health of your batteries during a flight is to observe its voltage. It is also critical to understand that when the battery is under load, its voltage will be lower than what it will be at rest by approximately .2v/cell.
Lets discuss briefly some effective ways to monitor cell voltage. The most accurate way is to use a radio that incorporates telemetry and displays either individual cell voltage or overall battery voltage in real time. The next best way is to install at least one cheap LiPo low voltage alarm in the aircraft that is plugged into the balance port. The alarm voltage can be preset on these devices and will when reached, a loud beeping sound will be heard on the ground indicating that it is time to come back for a landing.
Now that we have a basic understanding of cell voltage and how we can monitor it, let us discuss what should be considered "low" voltage. All of my bench testing is done with cell voltage displayed in real time. In this case, the lowest voltage I accept under load before shutting down the fan is 3.5v/cell. When power is reduced to zero, the cell voltage will bounce back to approximately 3.7v/cell. This is easy to do because I have the pertinent information displayed right in front of me.
In flight, this becomes a little more challenging. My preferred method is to use the aforementioned low voltage alarm to indicate when it is time to return for a landing. I preset the alarm voltage to go off at 3.65v/cell. This way, as I hear the alarm going off, I know I have enough time to set up for a landing and go around if needed. Using this method has consistently allowed me to land with the batteries bouncing back to 3.7v/cell at shutdown.
Recommended Battery Brands
As discussed previously, the end user has a wide variety of brandnames to choose from when purchasing a lithium battery. What I am about to share in this section is based purely on the hundreds of fan tests in the test stand. There may be other brands out there that can outperform what I am recommending and if so, I would certainly like to learn about it.
Also, weight is a critical factor when choosing the right type of battery pack for a given aircraft. Some jets can carry heavier packs while others need lighter packs. The punch provided by a lithium battery from start to finish is proportionally related to its weight. In general, batteries that claim a certain C-rating but have the weight of a lesser rated battery, will not fare well in the long run. They may survive the first 25-30 cycles but will most likely begin ballooning badly or overheating soon thereafter. Both of these qualities should be avoided at all cost.
With that said, the best performing battery I have tested to date are the Hobbyking brand, Turnigy Graphene 65c packs. I have absued these packs to no end and even drained a few of them below their full capacity. Amazingly enough, I was able to bring them back to life even after this mistake by charging each cell individually using a constant voltage power supply. The biggest reason why I do not use these packs for flying however is that they are not available in anything greater than 6000mah.
The second best battery I have used and continue to use are Dinogy 65c and 70c packs. While these batteries don't offer the punch that is available from my Turnigy Graphene's, they do offer me the ability to have 8000mah of capacity at the same weight as the Graphene 6000mah packs. This means that I can run and use a full 6000mah of capacity while flying and still land with enough remaining in the batteries to prevent damage. Because of this, I am able to get upwards of 5.5 minutes of flight time from the thirstiest fan/motor combination I offer (18s CAT-6).
ESC Recommendations
All of my bench testing and flying is done using Fliermodel ESC's. The general rule of thumb when it comes to selecting the right ESC is to pick one that is rated for twice the maximum current you are expecting to see at maximum power with a fully loaded fan (i.e. my bench test setup). For this reason, the minimum current capability I like to use is an ESC that is rated for 400amps. I have found the Fliermodel 22s/400amp Airplane ESC to be one of their best and most efficient ESC's. It runs extremely cool and had not failed me even once. This is my go to ESC for all of my bench testing and flying.
Another brand that is of higher quality and will cost more than Fliermodel is APD. While I do not have any personal experience with this brand, others have recommended it as a good product for high power applications. Visit https://neumotors.cartloom.com/storefront/product/bldc-controller-hv-pro-apd for more information.
