PZ

Hello,

I have been in the hobby for a few years already, but always using glow engines and I never considered electric flight because it was restricted to small planes or were lacking performance, but the technology advanced quite a lot recently and I decided to try an electric plane. I already decided on the model(GP Extra 300sp) and the motor, esp and everything else.

Now the first questions I have are regarding the batteries and their use. First of all I would like to knowapproximatelybefore I fly how much flight time would I have with my batteries. Is there a way to calculate this? To know at least an approximate number? With glow engines I simply fly a while and check how much fuel do I still have in the tank and come to an approximate number, but with lipo batteries for example, I should not let them discharge completely because I could damage them so how would I know how much can I fly? also what is the discharge limit?
My second question is in the same area, how much time would it take for my batteries to charge? I will be using 2 x 3200mah 3s batteries in the plane for a 6s config. If it takes much time then I should buy more packs so while I fly, the other packs are recharging and I do not have to wait a long time between flights? Also because I am using 2 batteries should Ipurchasea charger with multiple charging leads to speed up the operation?

Now in another topic is it true that I can not fly the airplane in full power for a long time?

Sorry for the long post but I wanted to explain myselfthroughly and thanks for theanswers.

fmw

Not exactly but you can develop an approximation. Every battery has an amp/hour rating. for instance, a 2000 mah battery will provide 2 amps or 2000 milliamps of currentfor one hour. You can use the current draw figure from your motor/prop combination to calculate the amount of time you can fly. How much throttle you use affects that, efficiency affects that, limits on the maximum discharge of the battery affects that and so on. So expect only a best case approximation by using the numbers.

Lipo batteries change chemically when they reach a point of dischargeof around 3 volts per cell. Going below that can ruin the battery and usually does. You can buy a simple voltmeter that plugs into the balance plug which will read the voltage for each cell. They are about $10.

Lipo batteries should not be charged faster than 1C or, in the case of your 3S batteries at a rate of no more than 2 amps.. It can take as long as an hour to charge a battery and balance it. I take lots of batteries to the field personally.I never take a charger.I havemore than enough batteries with me to handle the day's flying. You can do it as you like. Many people charge at the field.

No. What is true is that you can't overheat your electrical components for a long time. As long as your battery and ESC can handle the current draw of the motor and prop without overheating at full throttle, then you can fly that way until you need to replace the batteries. My own habit is to use ESC's that are at least one size bigger than the maximum current draw of the motor prop combination. I use batteries with a high C rating to prevent starving the motor of power. I can fly at any throttle setting I like for as long as the battery has sufficient charge.

Welcome to electric flight. Hope you enjoy it as much as fuel flight.

PZ

Hi fmw thanks for the answers, but somethings are still not completely clear to me.

How can I do this? I know the manufactures of the motors put some figures in their websites like the maxcontinuouscurrent or max surge current, but which one should I use? What is the difference between those two numbers? And how do I insert the prop in the calculation? I know that this figure is difficult but all I want to know is basically how long can I fly If the engine is working at 100% and the batteries are fully charged. From there I can find a more realistic figure while flying and trial-and-error.

Right now I want to understand the basic behind the manufacturer selections of motor, esp and batteries so from there I can make my own selections in the future.

How big of a number are we talking here? How much flight time in average do you get from your models?

Thanks again.

fmw

I thought I explained. You either measure the current draw with an in line wattmeter or you look things up in a table like the ones I suggested. You can also use calculators such as motocalc but I've never understood those. Here's a simple example. If your setup draws 20 amps at full throttle and you have a 2 amp/hour (2000mah) battery then the flight time will be 1 hour divided by 10 or 6 minutes at full throttlle. That's best case. You can't calclulate it until you know the current draw. You need to know that using one of the methods I outlined. Personally, I don't pay attention to the flight time. I can tell when the battery is beginning to get reduced in voltage enoughthat I need to bring it in just by the waythe planebehaves. It starts losing power from the moment you take off because the voltage decreases as the battery dishcharges. You don't notice it, however, until the voltage gets down enough to make a meaningfuldifference in the way the plane behaves. When it does, then it is time to land it and change batteries.

ESC's have a low voltage cutoff to keep you from discharging the battery too far. What it does is cut power to the motor but leaves power through the BEC to the receiver so you still have control.Since you are running 6S you will be usinga separatebattery for the receiver anyway.However, the low voltage cutoff can occur when the plane is too far away for a dead stick landing so I never let things go that far. When the plane begins tofeel sluggish, I bring her in. At that point the battery usually reads around 3 1/2 volts per cell. I replace it and take off again.

I have a tendency to overbuild the electric systems on my planes. I use motors that are a little more powerful than I need and prop them down a little. I use ESC's one size too big and I use high C rating batteries. This all adds weight and wing loading to the plane but I'm a sport flyer, not a competition pattern or 3D flyer, so it works for me. My planes run very cool and reliablyand produce typically 8-12 minute flight times. I have a 3D foamie that only gets about 6 minutes because I have a smallerbattery in it than I would usually use in order to keep weight down. You can adjust the flight time, as you know, by changing to a battery with a different amp/hour rating.

I generally carry 3 or 4 batteries for every plane I take to the field. That's all the flying I want to do.

I'll give you some parallels to a fuel plane. The battery is the fuel tank. It's size is the amp/hour rating. Higher amp/hour ratings mean larger tank with more fuel. The current is the fuel. The C rating is the fuel line. It needs to be large enough that the motor can draw enough current to do what you want it to do just like you would need a large enough fuel line in your nitro plane to keep from starving the engine. The voltage is like the fuel pressure. In fact voltage is electrical pressure. Watts are the horsepower. By ohm's law power is equal to the current (amps) times the electromotive force (volts.) This formula will handle about anything you need to calculate in the electric RC world.

fmw

I may not have answered the first question completely.  You can't use any of the maximum wattage or current figures provided by the motor manufacturers to calculate anything meaningful because there is no propeller involved in those numbers.  The propeller affects everything in the drive train just like it affects the rpms of your fuel engine.  That's why I sent you to the Scorpion web site to look at the performance charts.  They will show you how watts and amps vary with the propeller and battery connected to the motor.  To calculate what you want to calculate you need the actual measured numbers with the motor spinning a prop, not some maximum figure provided by the manufacturer.

PZ

Yes, I understood you on your first post, but all I wanted, is to know all the data before I purchase the motor and everything and avoid selecting the wrong things and only find it out while on the field. The current draw is the most difficult thing to come by since I do not know how much amps does my motor require to turn a selected prop. I see that this is difficult to do and a little trial and error at the field with a watt meter is necessary.

Thanks with the tip for the batteries although now that I think about it, with my setup of 2 batteries, I would have to bring a lot of batteries to the field!

fmw

If you buy a scorpion motor you can use their charts.  Weigh the plane and choose a combination of  prop and motor that will provide a 1:1 thrust ratio - lbs of thrust about the same as lbs of airplane flying weight.  That's a good all around place to start for sport flying.  You want  to read the current draw figure from the chart and you can choose an ESC that will handle that without undue strain..

You may be better than I am at dealing with the on line calculators like scorpion calc and motocalc.  Those allow you to plug in everything and get detailed numbers,.  I normally just choose a thrust figure and buy components to deliver that to the model.  Perhaps someone will come along who is better at dealing with the on line calculators and help you there.  My method works fine for me but my requirements are looser than those of some people and, as I said, I tend to over build a little to be conservative and easy on the equipment.

jdetray

PZ -

The formula for roughly estimating flight time is:

(Battery Capacity in Ah/Motor Current in A) x 60 = run time in minutes

Get the Battery Capacity by converting from mAh to Ah. Simply divide by 1000.

Get Motor Current by measuring your full throttle current on the ground with a wattmeter. A wattmeter is a required purchase. Get one!

Let's look at a few flight time examples.

Example 1:
1500 mAh battery
power system draws 12A

1500 mAh = 1.5 Ah

1.5/12 x 60 = 7.5 minutes


Example 2:
1500 mAh battery
power system draws 18A

1500 mAh = 1.5 Ah

1.5/18 x 60 = 5 minutes


Example 3:
2200 mAh battery
power system draws 10A

2200 mAh = 2.2 Ah

2.2/10 x 60 = 13.2 minutes


Example :
3200 mAh battery
power system draws 20A

3200 mAh = 3.2 Ah

3.2/20 x 60 = 9.6 minutes

These calculations assume you are flying at full throttle at all times, a worst case situation. So actual flight times should be a bit longer.

Regarding charging time: By definition, any fully depleted battery being charged at 1C should in theory take exactly one hour to charge. In reality, there is some variation from this because the battery may not be fully depleted and because charging is never going to be a 100% efficient process. You should figure on at least an hour for charging a pack and plan your bettery purchasing accordingly.

- Jeff

jman12

My Feedback: (13)

How many amps do I set to charge a 4s 2200 lipo?

jdetray

For a 1C charge rate, which is generally safe for all lipos, set the charger to 2.2 Amps.

A 1C charge rate is the capacity number expressed as a current value. For a 2200 mAh battery, the 1C charge rate is 2200 mA, which equals 2.2A. So set the charger to either 2200 mA or 2.2A (they are the same thing, but some chargers may display mA while others display A). A full charge will take a bit more than one hour.

You can charge at less than 2.2A, but it will take longer. Do not charge at more than 2.2A unless the lipo maker specifies that your pack can be charged at a rate greater than 1C.

- Jeff