Today we're going to be testing whether or not a higher C battery really is worth it. Some people say you get more top speed, more thrust, more pull out from a higher C battery so we're going to run some numbers, put these batteries head-to-head and settle this once and for all.
What we have here are two 1000mah 3 cell batteries. One is rated at 65 C and the other is rated at 35 C. The motor we're gonna be testing with is a hobbyking fun fighter replacement motor We're gonna run this on a 5.25x6.25 prop and it should only pull about 20 amps with this setup. So you gotta ask yourself, if I have a battery that's 1000mah and rated at 35 C that's good for 35 amps, right? What's the point of having is 65 C battery which can deliver 65 amps when realistically the motor only needs 20? Shouldn’t the 35 C battery be more than enough?
It’s a fair question and that’s why we're going to put these batteries head-to-head and see if there really is any truth to this.
During the test we are going use a watt meter to tell us how many watts and amps we’re pulling. I've also got a little battery voltage meter that is going to tell us how much each cell is sagging under load. There is an RPM sensor setup on my radio that will show just how fast the prop is spinning. RPM is important because I'm not going to test this on a plane, we're going to be doing this here on the bench. RPM tells us how many revolutions per minute this motors going to take with this prop.
Knowing how fast the motor is spinning with each battery will to tell us how fast that translates to in the air. And if one battery gives you a faster spinning prop than the other then you know you're getting more power. Which translates to more speed, more thrust, more responsiveness. So let's run each these up and take a look at the results.
35 C 65 C
VOLTS 3.32 3.93
WATTS 157 213
AMPS 14 17
RPM 15200 17300
MPH 89 102
Ok guys, so straight out the gate we're going to say the 65 C battery definitely perform better but lets look at why that is. One thing that was apparent, is that under load the cells held a higher voltage compared to the 35 C battery. We held around 3.93v on the 65 C and only 3.32v on the 35 C.
That's a pretty big difference.
But you might ask yourself, ‘so what Nic, what does a .6 of volt really matter?’ The voltage numbers that we see are actually gonna set the stage for the rest of the numbers in our test. Your battery is only able to deliver 4.2 volt on a fully charged cell. But as soon as you apply load the cells are going to drop to try and keep up with the demand. So every bit that the cells drop directly reduces the number of volts coming out of the battery and into the motor. And here is the kicker, the volts going into the motor impact the RPM bottom line. The lower the volts, the slower the motor is going to spin. This is how kv operates, it's going to be whatever your voltage is times the kv of the motor. So if voltage goes down, your kv also goes down, RPM, thrust, speed, responsiveness all go down.
Other than direct RPM we are going to look at the real world impact of RPM on speed. On one full rotation of the motor whatever your prop pitch is will be how far the motor would travel, in inches, in that one rotation, assuming perfect efficiency. So if I say the motor is spinning at 17300 RPM we know then based on the pitch of the prop how far it is traveling in inches. We can then divide that and give you a number of miles per hour.
The formula for that is RPM (17300) X the pitch of your prop (6.25) / 1056.
So it plays out like this:
- The 65 C battery held 17 amps and about 213 watts. That gave us 17300 RPM.
- The 35 C battery did not perform as well and only held 14 amps and about 157 watts. That gave us 15200 RPM, and that's being generous.
If you watch the test closely you’ll see that the RPM was dropping very quickly on the 35 C. The 65 C battery held that 17300 RPM range pretty well. It was starting to slowly go down but it was pretty solid. The 35 C battery was dropping off very fast and rapidly went down at the end. So you're losing speed the longer you're holding the throttle and also throughout the pack. The next time you throttle up you're gonna get even less RPM. The 35 C translated to about 89 miles per hour, which still isn't so bad overall. But when you compare it to the 102 of the 65 C that's a 13% reduction in speed.
The follow-up question here would be is it really worth the cost, weight, and size increase? Does the extra weight impact flight to the point that a lighter battery would actually perform better?
The 35 C battery weighs 85 grams and the 65 C battery comes in at 114 grams. So a difference of about 29 grams. Putting this into other terms, for 29 grams additional weight you're getting 13% additional top speed and 50 watts more power. Your only real penalty here is 29 grams of weight, which are on a plane that is going to hold this battery you’d be looking at you maybe 20-25 inch wingspan. A plane that size is not going to matter one bit carrying the extra 29 grams. Considering how much additional power you're getting this seems like a no brainer.
The last thing to consider is cost.
The batteries definitely do not cost the same amount. Ballpack of $6 vs $15, so there is some additional overhead. For me the extra cost and weight is not that big of a deal for like additional power using the same setup. So to answer the question, ‘does a higher C battery really perform better?’ The answer is yes. Even if your motor only needs 20 amps, higher C batteries are going to give you a lot closer to full performance, be more responsive, and give you higher top speed.
Thanks guys for tuning in, we'll see you next time.