PapaHomer

Hello everyone. What is the smallest battery anybody has tried in an Eflite apprentice? I have the bnf version and they reccomend a 3200mah. I have a turnigy 3200mah and 5000mah. 5000 lasts about 25 minutes of casual flying but needs more speed on take off and landing. My daughter has been coming with me lately and she wants to start flying and. I'm curious about the smallest 3 cell that would work in the apprentice so it can take off and land at the slowest speed possible. Thank you

jester_s1

You probably won't save any weight going below the 3200 since you may wind up tail heavy with a lighter battery. I'd not mess with it. Slowest possible speed is not a good goal for a trainer plane since too floaty is way worse than too fast. Also, you'll be cutting flight time which hurts the learning process. 8-10 minutes is about right for a training flight.

JCavadas

Hello,

The battery pack minimum capacity is directly related to its discharge rating. You have to find out your motor max consumption first, and then select a battery.
Example: if your motor consumes 35A at full power (full charge battery), then a battery that takes that load would suffice (Ex 3500 mAh battery with 10C disch rate = 35 A)
Different brand and model batteries normally have different maximum discharge rates, known as "C rates", clearly printed on the outside of the pack.

For 35A:
a 3500mAh battery with 10C discharge rate (3500x10 = 35000mAh = 35A, the same the motor pulls)
a 2000mAh battery with 20C (2000x20 =40)
etc...would be enough.

I recommend going for over rated discharge packs, packs that would give you at least 20 % more discharge rate capacity than you motor consumption. Keeps your batteries cooler, safer and increases their service life.

Fly safe, be safe

jester_s1

It's been demonstrated many times that C ratings mean nothing. They can be useful for comparing batteries within a brand, but there are not established standards for performance and C ratings. One brand's 25C pack will outperform another's 45C pack. If you want to get into the actual quality of packs and their performance, the only thing that actually matters is internal resistance. There are ways of checking IR, which is a handy way to not only see which batteries are actually the best but also see how well they age.

What's actually important for the OP's situation though is flight time. If his plane draws an average of 30 amps during the flight, a 3000 mah battery will be dead flat is just under 10 minutes while a 3500 will allow a small margin of error. If a pilot is ok with shorter flight times, he can go down to a smaller battery. EDF pilots routinely accept 3 minute flight times because they want crazy amounts of speed but have to keep the weight down. IR numbers are going to really matter to them. To an Apprentice pilot, not so much.

JCavadas

I was not referring to battery performance, just wanted to suggest to be careful when choosing packs to be sure that at least the A the motor pull will not be too much for the battery, possibly overheating and maybe damaging the battery.
I fully agree with what you said.

jester_s1

That's never a concern. The battery will simply drop voltage and not perform adequately before actual damage is done. Most battery damage comes from not using a storage voltage or from improper charging. That said, the vast majority of batteries die from old age.

JCavadas

I just went on to more info as this was confusing me and I now understand the problem I had was probably from discharging the battery too much, and not from pulling too much A.

Thank you for the information Jester s1
Apologizes to Papahomer for giving incorrect information, I wanted to help with what I had experienced, but I interpreted what happened to me in a wrong way.

speedracerntrixie

My Feedback: (29)

Pulling too much current will in fact damage a Lipo, I have a few 5S 5000 mah 25C batteries here that are slightly puffed due to running too much prop load. The batteries were never run below 30% capacity, always charged at 1C and storage charged after a flying session. After dropping down from a 20.5 X 13.5 prop to a 20X12 the batteries are much happier.

As that relates to the OP's question, your key to longer flight times could be with the prop. The stock Eflight prop is not the best. An APC E prop of the same size would be more efficient, dropping down one inch in pitch with an APC prop would give very close to the same performance but with less current draw. You can either get longer flight times or reduce battery size a little to lighten up the airplane. Also keep in mind that the electric setups are most efficient the closer you are to full throttle. Flying around at half throttle uses more then half current so dropping down in prop pitch and using a higher throttle setting increases efficiency even more.


I won't get too much into C ratings other then to say that I fly my 3.5 meter sailplane with a motor/prop combo that pulls 40 amps. The battery used is a Pulse 860mah 75C. I get 3 launches from ground to 150 meters on one charge.


On landing speeds, most trainer type airplanes are set up rather nose heavy. If you need to trim the elevator for a specific throttle position and the airplane climbs with more power and dives will less, odds are you are too nose heavy. Bringing the CG back some will lower landing speed and require less elevator to flare. A couple things beginners could benefit from.

jester_s1

Agreed on the nose heavy comment. I can't tell you how many times I've seen people advise new modelers to balance their planes "a little nose down just to be safe" as if having the CG at 26% instead of 25% of MAC will cause the plane to crumple itself into a ball and catch fire mid flight. I'd say a lot of new pilot woes could be avoided by having an experienced modeler set the CG and throws on the model.

speedracerntrixie

My Feedback: (29)

Yep, that was me back in the day flying nose heavy airplanes. Lots of bad landings. All too often when I try to share what I have learned over the years on my way to becoming an IMAC unlimited and masters class pattern pilot to guys struggling with a poor flying airplane I usually get pushback. For some reason people resist the fact that some of the flight trimming techniques used for aerobatic airplanes are useful on all airplanes. To say that most R/C guys get set in their ways is an understatement. My favorite is when a guy stated " I want my model P-51 to have the same flight charictaristics of a full scale P-51 " although he then admitted he had never flown a full scale P-51 LOL.

jester_s1

Meaning if he pulls a little too hard on the elevator it'll snap and go into an unrecoverable death spiral? That's relaxing.

PapaHomer

Wow... I got a lot more information than I asked for. I thought flying a slightly nose heavy plane was the way to go for beginners, thats what I read and saw on youtube consistently atleast. I was just curious about a smaller lighter battery to fly a bit slower even if its only for 5ish minutes. With half throttle on a turnigy 3200mah 30c I can actually get 17-19 minutes with each cell landing with 3.8 volts. This is why I reached out, thank you for the info. So it sounds like brushless motors are like boats, you want to run at the higher rpm range. Over prop and you'll lug the engine and hurt something, under prop it and you'll break something.

jester_s1

You won't break anything by underpropping. You just may not get the power that you need.
The great misunderstanding about electric motors of all types is that they don't actually have a standard power output. IC engines do. So if you underprop an IC engine you may break something because it'll turn too many RPM. But electric is different. An electric motor will turn the RPM that the volts applied to it says it should turn. So if you have a 1000kv motor and put 10 volts on it, you'll get 10,000 rpm. You can have a big prop, little prop, or no prop, and it'll still turn 10,000 rpm. What will change with load is the amps it pulls. Electrics pull the amps they need to turn the RPM that the voltage requires they turn. You don't damage anything until you go over the amp limit.
That property of electrics gives them a really neat quirk- you can make considerably changes in the flight and power characteristics of a model with nothing more than a prop change. Is your model overpowered with short flight times? Go down an inch in the prop diameter to tame it down a bit. Barely pulling through loops? Put a bigger prop on. Electric is much more flexible than IC power because with IC you have to prop the plane based on the horsepower and torque output of the engine.