>> Selecting a LiPo battery for your model seems like a relatively easy thing to do, right? An internet search for RC LiPo batteries, however, will overwhelm you with endless brand names, costs, sizes, and, of course, C ratings. How do you make sense of all of this mess? I have a few guidelines that I use when searching for LiPos. I hope that this information helps make picking out your next battery a little easier.
Most of the park flyer jet models, such as 50mm and 64mm EDF (electric ducted-fan) jets, typically fly on a 3S or 4S LiPo battery pack. Knowing the cell count is essential, but what about the mAh capacity, such as 1,500, 1,800, or 2,200 mAh? Most models these days come with a factory recommendation for the mAh to give you a starting point, but what if you make an upgrade? What if it is a scratch-built project? I like to take a wattmeter reading then work out a LiPo capacity based on the amp reading and expected flight time.
These LiPo batteries are all four-cell 2,200 mAh capacity with similar discharge ratings, but their physical sizes (width and length) vary from brand to brand.
A larger-capacity pack might not fit into your model or could add too much extra weight. Here is a size comparison between a three-cell 1,300 mAh and an 1,800 mAh battery pack.
For my flying style, I typically use 3 minutes for the expected flight time. A 3-minute flight equates to a 20C discharge rate (60 ÷ 3 = 20). You can then calculate the battery capacity based on a wattmeter reading. If, for example, the wattmeter reading is 30 amps, you need a battery capacity of 30 ÷ 20 = 1.5 amp hours, or 1,500 mAh. If you want a longer 4-minute flight at full throttle, that is a 15C discharge rate (60 ÷ 4 = 15) and you would need a capacity of 30 ÷ 15 = 2.0 amp hours, or 2,000 mAh.
This calculation is assuming that the full 30-amp draw for the entire flight matches closer to my flying style because I have a bit of a lead foot when it comes to the throttle. Some of the pilots in my club are not as heavy on the throttle as I am and they like to cruise around at partial throttle, so they will get longer flight times because of the lower amp draw. Whatever your flying style is, just remember to set a flight timer so that you do not run out of battery power when you are about to land.
For park flyer jets, I have found that sizing the battery for a 3-minute flight (20C discharge) is the best compromise of performance and flight time. Going from a 1,500 mAh LiPo to a 2,000 mAh LiPo to gain an extra minute of flight time can add too much extra weight and make the model’s performance suffer. The higher weight means higher stall and landing speeds, and if the model is moderately powered, it might make it more difficult to fly and it might feel “heavy” in the air. This is why I have settled on the 3-minute flight time for my park flyer jets.
Now that you know what capacity of battery to get, what about the C rating? My answer is to always go with more C rating than what is used in the capacity calculation. Because I am using a 20C discharge rate, I look for LiPos in the 30C to 40C range.
Why not a higher C rating? I have found that some brands of LiPos get quite large and heavy as you start to push over a 40C discharge rate. This is something that people might not realize when buying a LiPo battery online. The pack size and weight need to be carefully compared with other brands to see if it is larger than “normal” or heavier than “normal.”
As an example, I have some 55C-rated 1,500 mAh LiPos that weigh exactly the same as a 30C 1,800 mAh pack of a different brand. When I fly both packs in the same model, as expected, I get more flight time with the 1,800 mAh pack, so pack size and weight are important things to look at when buying LiPos.
On some of my larger EDF models, the weight or capacity difference can be even greater. I have some 6S 4,000 mAh packs that weigh the same as a 6S 5,000 mAh pack. That is 1,000 mAh more capacity for the same weight, which is significant for flight time. The same applies to ultramicro and micro EDF models, where a few grams can make a big difference in flight performance.
The weight difference in these two four-cell 2,200 mAh packs is 27 grams.
What about brands? LiPo battery brands can be a bit like, “What is better: a Ford or a Chevy?” and can lead to a lot of debate. I am not necessarily brand loyal nor do I buy into a brand. I see what is available in the size that I want then compare it with other LiPos from several companies. I pay attention to the pack sizes, weights, C ratings, and of course, the cost.
There are a few places that sell inexpensive LiPos and I have bought some of them to try. They tend to be larger, heavier, or a combination of both when compared with other brands. On the opposite end of the spectrum, there are also quite expensive LiPos on the market, so a comparison has to be made to decide whether the extra cost is worth the lighter weight or higher C rating that those brands offer.
I tend to buy “middle-of-the-road” battery packs from a variety of suppliers, depending on what is in stock or on sale. Overall, I have been happy with the middle-of-the-road packs because they seem to be similar in size, weight, and performance. They also tend to last me several flying seasons, which saves me money in the long run.
Using a wattmeter to measure the amp draw is the best way to get an idea of the battery capacity that you will need.
See you around the flying patch!
By Don Slusarczyk | [email protected]
