# Confused about motor Amps and battery Amps

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**1**Senior Member

Thread Starter

Join Date: Mar 2006

Location: Tampa,
FL

Posts: 131

**Confused about motor Amps and battery Amps**

Hello to all,

I have a new inrunner brushless motor that is rated at a maximum current of 35 amps and just yesterday placed an online order for a battery having the following specs: 3 Cell, 11.1v, 3200 mAh, 10C, 32A, Gen2 and weighs 225 g.

Today I learned from a RCU forum member that I need a battery that has an amperage rating that is greater than the amperage of the motor. My thinking, and I should have known better, was the reverse. I was thinking that a battery having an amperage higher than the motor would burn out the motor. Thus, I must order a battery that has an amperage rating higher than 35 Amps. How mucher? And so, one way of learning is to ask questions. My airplane is a trainer that, with all equipment, will weigh 3.5 lbs.

Questions:

1) With a notor having a max current of 35 amps, what should be the amperage of the battery?

2) Of the following, which battery would be best for the motor considering battery amperage, weight of airplane, flight time. etc.

a. 3 Cell, 11.1v, 2500 mAh, 20C, 50 Amp, 203g

b. 3 Cell, 11.1v, 3100 mAh, 12C, 37.2 Amps, 201g

c. 3 Cell, 11.1v, 3300 mAh, 20C, 66A, 260g

Thanks,

Ernie

'Sailplane'

An emotional high is...flying way up high into the sky!

I have a new inrunner brushless motor that is rated at a maximum current of 35 amps and just yesterday placed an online order for a battery having the following specs: 3 Cell, 11.1v, 3200 mAh, 10C, 32A, Gen2 and weighs 225 g.

Today I learned from a RCU forum member that I need a battery that has an amperage rating that is greater than the amperage of the motor. My thinking, and I should have known better, was the reverse. I was thinking that a battery having an amperage higher than the motor would burn out the motor. Thus, I must order a battery that has an amperage rating higher than 35 Amps. How mucher? And so, one way of learning is to ask questions. My airplane is a trainer that, with all equipment, will weigh 3.5 lbs.

Questions:

1) With a notor having a max current of 35 amps, what should be the amperage of the battery?

2) Of the following, which battery would be best for the motor considering battery amperage, weight of airplane, flight time. etc.

a. 3 Cell, 11.1v, 2500 mAh, 20C, 50 Amp, 203g

b. 3 Cell, 11.1v, 3100 mAh, 12C, 37.2 Amps, 201g

c. 3 Cell, 11.1v, 3300 mAh, 20C, 66A, 260g

Thanks,

Ernie

'Sailplane'

An emotional high is...flying way up high into the sky!

#

**2**Senior Member

Join Date: Mar 2002

Location: Kingston,
ON, CANADA

Posts: 4,925

**RE: Confused about motor Amps and battery Amps**

Hi Ernie:

The 35 amps on the motor means, that is the max it can handle. Prop choice controls how many amps the motor actually draws. If you were to put a prop on that draws 25 amps at max throttle it's easier on the motor and within the batts. 32 amp capability. Also you would be flying around at half throttle most of the time which would only be around 12 amps draw. Best tool to get is a Wattmeter. You plug it between the batt. and ESC and it tells you exactly how many amps, volts and watts your using. It's a tool you can't do without. Motor temp and batt temp. are a good way of telling if your pushing things to hard. If they're to hot to handle, the amps need to drop down. Hope this helps.

The 35 amps on the motor means, that is the max it can handle. Prop choice controls how many amps the motor actually draws. If you were to put a prop on that draws 25 amps at max throttle it's easier on the motor and within the batts. 32 amp capability. Also you would be flying around at half throttle most of the time which would only be around 12 amps draw. Best tool to get is a Wattmeter. You plug it between the batt. and ESC and it tells you exactly how many amps, volts and watts your using. It's a tool you can't do without. Motor temp and batt temp. are a good way of telling if your pushing things to hard. If they're to hot to handle, the amps need to drop down. Hope this helps.

#

**3**Senior Member

Thread Starter

Join Date: Mar 2006

Location: Tampa,
FL

Posts: 131

**RE: Confused about motor Amps and battery Amps**

Gord,

Many thanks for your reply. I need all the help that I can get. Also, thanks for the tip on the wattmeter. I'll get me one.

Regards,

Ernie

-Sailplane_

<< An emotional high...is flying way up high in the sky>>

Many thanks for your reply. I need all the help that I can get. Also, thanks for the tip on the wattmeter. I'll get me one.

Regards,

Ernie

-Sailplane_

<< An emotional high...is flying way up high in the sky>>

#

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My Feedback: (2)

Join Date: Jan 2003

Location: Long Island, NY

Posts: 8,567

**RE: Confused about motor Amps and battery Amps**

This note is intended to clear up a few terms and concepts around

electricity

as it applies to electric airplanes.

Think of electricity like water. Volts = pressure Amps = flow

Volts is like pounds per square inch, psi. Says nothing about how much water is flowing, just how hard it is being pushed. You can have 100 psi with zero water flow.

Amps is flow, like gallons per hour. You can have flow at low pressure and you can have flow at high pressure.

Amp hours is how much flow can be sustained for how long. It is used as a way of measuring how much electricity is in the battery. Like how many gallons of gas in your tank. It is a capacity number. Says nothing about flow or pressure, it is about capacity.

Amps and mili amps? We are just moving the decimal point around.

1 amp (short for ampere) = 1000 miliamps (mili means 1/1000)

Examples

So a 7 cell NIMH or NICD pack provides 8.4V (pressure).

The motor will draw electricity from the pack at a certain flow rate, or

amps.

If you have a have a 650 mili amp hour pack, it can deliver a flow of .650 amps (650 miliamps) for one hour. If you draw it out faster, it doesn't last as long. So your motor might pull 6.5 amps for 1/10 of an hour, or about 6 minutes.

A 1100 mah pack has double the capacity of the 650 mah pack, so it should

last "about" twice as long.

What is C in relation to batteries?

C ratings are simply a way of talking about charge and discharge rates for

batteries.

1C, = 1 time the rated mah capacity of the battery. So if you charge your

650

mah pack at 1C, you charge it a 650 miliamps, or .650 amps.

1C on a 1100 pack would be 1.1 amps.

2 C on your 1100 pack would be 2.2 amps

Motor batteries are often rated in Discharge C and charge C.

So a 1100 mah pack (1.1 amp hour) might be rated for 10C discharge, so you

can pull 11 amps ( flow ) without damaging the battery.

Then it might be rated at 2C charge rate (flow), so you charge it at 2.2

amps (2200 mah)

How did I do? Things clearing up?

If you have a 500 mah pack - any kind - and it is rated at 16C that means it

can deliver 8 amps.

If you have a 1000 mah pack - any kind - and it is rated at 8C that means it

can deliver 8 amps.

If you have a 1000 mah pack - any kind - and it is rated at 12C that means

it can deliver 12 amps

If you have a 1500 mah pack - any kind - and it is rate at 8C that means it

can deliver 12 amps

If you have a 1500 mah pack - any kind - and it is rated at 20 C that means

it can deliver 30 amps.

If you have a 3000 mah pack - any kind - and it is rated at 10 C that means

it can deliver 30 amps.

So, if you need 12 amps you can use a pack with a higher C rating or a pack

with a higher mah rating to get to needed amp deliver level.

One last point. Motor batteries vs receiver batteries

Some batteries can sustain high discharge rates. Others can not.

Those used as transmitter/receiver packs typically are made for low flow/amp

rates while those made for motor packs can sustain higher rates.

So, having a 600 mah pack does not tell you if it is a motor pack that can

put out 6 amps, or if it is a transmitter/receiver pack that would be damaged if

you tried to pull power at 6 amps. It is enough to say that they are

different.

Clearly a motor pack could be used for a transmitter/reciever job, but a

transmitter/reciever pack should not generally be used as a motor pack.

Basics:

http://www.modelaircraft.org/mag/FTGU/Part8/index.html

Lithium Batteries

http://www.*********.org/lithium_bat...eakthrough.htm

New Electric Flyer FAQs

http://www.ezonemag.com/pages/faq/a105.shtml

A series of posts on electric power system basics

http://www.wattflyer.com/forums/showthread.php?t=1933

http://www.rcgroups.com/forums/showthread.php?t=417868

electricity

as it applies to electric airplanes.

Think of electricity like water. Volts = pressure Amps = flow

Volts is like pounds per square inch, psi. Says nothing about how much water is flowing, just how hard it is being pushed. You can have 100 psi with zero water flow.

Amps is flow, like gallons per hour. You can have flow at low pressure and you can have flow at high pressure.

Amp hours is how much flow can be sustained for how long. It is used as a way of measuring how much electricity is in the battery. Like how many gallons of gas in your tank. It is a capacity number. Says nothing about flow or pressure, it is about capacity.

Amps and mili amps? We are just moving the decimal point around.

1 amp (short for ampere) = 1000 miliamps (mili means 1/1000)

Examples

So a 7 cell NIMH or NICD pack provides 8.4V (pressure).

The motor will draw electricity from the pack at a certain flow rate, or

amps.

If you have a have a 650 mili amp hour pack, it can deliver a flow of .650 amps (650 miliamps) for one hour. If you draw it out faster, it doesn't last as long. So your motor might pull 6.5 amps for 1/10 of an hour, or about 6 minutes.

A 1100 mah pack has double the capacity of the 650 mah pack, so it should

last "about" twice as long.

What is C in relation to batteries?

C ratings are simply a way of talking about charge and discharge rates for

batteries.

1C, = 1 time the rated mah capacity of the battery. So if you charge your

650

mah pack at 1C, you charge it a 650 miliamps, or .650 amps.

1C on a 1100 pack would be 1.1 amps.

2 C on your 1100 pack would be 2.2 amps

Motor batteries are often rated in Discharge C and charge C.

So a 1100 mah pack (1.1 amp hour) might be rated for 10C discharge, so you

can pull 11 amps ( flow ) without damaging the battery.

Then it might be rated at 2C charge rate (flow), so you charge it at 2.2

amps (2200 mah)

How did I do? Things clearing up?

If you have a 500 mah pack - any kind - and it is rated at 16C that means it

can deliver 8 amps.

If you have a 1000 mah pack - any kind - and it is rated at 8C that means it

can deliver 8 amps.

If you have a 1000 mah pack - any kind - and it is rated at 12C that means

it can deliver 12 amps

If you have a 1500 mah pack - any kind - and it is rate at 8C that means it

can deliver 12 amps

If you have a 1500 mah pack - any kind - and it is rated at 20 C that means

it can deliver 30 amps.

If you have a 3000 mah pack - any kind - and it is rated at 10 C that means

it can deliver 30 amps.

So, if you need 12 amps you can use a pack with a higher C rating or a pack

with a higher mah rating to get to needed amp deliver level.

One last point. Motor batteries vs receiver batteries

Some batteries can sustain high discharge rates. Others can not.

Those used as transmitter/receiver packs typically are made for low flow/amp

rates while those made for motor packs can sustain higher rates.

So, having a 600 mah pack does not tell you if it is a motor pack that can

put out 6 amps, or if it is a transmitter/receiver pack that would be damaged if

you tried to pull power at 6 amps. It is enough to say that they are

different.

Clearly a motor pack could be used for a transmitter/reciever job, but a

transmitter/reciever pack should not generally be used as a motor pack.

Basics:

http://www.modelaircraft.org/mag/FTGU/Part8/index.html

Lithium Batteries

http://www.*********.org/lithium_bat...eakthrough.htm

New Electric Flyer FAQs

http://www.ezonemag.com/pages/faq/a105.shtml

A series of posts on electric power system basics

http://www.wattflyer.com/forums/showthread.php?t=1933

http://www.rcgroups.com/forums/showthread.php?t=417868

#

**5**Junior Member

Join Date: Jun 2006

Location: New Port Richey, FL

Posts: 8

**RE: Confused about motor Amps and battery Amps**

Thank you for your time in responding to this question. This helped fill in some gaps in my understanding of battery discharge ratings too.

#

**6**Senior Member

Join Date: Jul 2005

Location: liberty,
MO

Posts: 1,079

**RE: Confused about motor Amps and battery Amps**

In response to the original question.

a) It depends on your flying style. Most batteries have TWO C ratings. A continuous rating and a max rating. If you plan on using full throttle for the full flight then the continuous C rating on the battery should be higher than the max rating on the motor. If you use full throttle for only short 5-10 second bursts then the max C rating needs to be higher than the max rating on the motor.

b) assuming that the amp ratings you provided are continuous ratings, I would go with battery B as it is more than powerful enough, it is the lightest, will provide pretty good flight times, and is likely to be the cheapest of the bunch.

Important note: as others have stated, it is not the motor nor the battery that determines how many amps are drawn, it is the propeller. You need to either get a watt meter as suggested or find a spec sheet for the motor and find out what props draw what amps. If your in luck, your motor might be listed

[link=http://brantuas.com/ezcalc/dma1.asp]here[/link] and you can play with the prop sizes.

Second note: Never EVER charge your lipo batteries over 1C

a) It depends on your flying style. Most batteries have TWO C ratings. A continuous rating and a max rating. If you plan on using full throttle for the full flight then the continuous C rating on the battery should be higher than the max rating on the motor. If you use full throttle for only short 5-10 second bursts then the max C rating needs to be higher than the max rating on the motor.

b) assuming that the amp ratings you provided are continuous ratings, I would go with battery B as it is more than powerful enough, it is the lightest, will provide pretty good flight times, and is likely to be the cheapest of the bunch.

Important note: as others have stated, it is not the motor nor the battery that determines how many amps are drawn, it is the propeller. You need to either get a watt meter as suggested or find a spec sheet for the motor and find out what props draw what amps. If your in luck, your motor might be listed

[link=http://brantuas.com/ezcalc/dma1.asp]here[/link] and you can play with the prop sizes.

Second note: Never EVER charge your lipo batteries over 1C

#

**7**
My Feedback: (10)

Join Date: Sep 2005

Location: Maybrook,
NY

Posts: 62

**RE: Confused about motor Amps and battery Amps**

Wow -- what a great reply ... I didn't even know that I was lookinh for this info until I read it!! Thanks ... now could you write the same kind of reply, but from the MOTOR end??? What do thos enumbers mean? How do I match up a motor / battery / esc to a particular airplane??? This is WAY more confusing than glow or gas.. HELP!!!

#

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My Feedback: (2)

Join Date: Jan 2003

Location: Long Island, NY

Posts: 8,567

**RE: Confused about motor Amps and battery Amps**

ORIGINAL: pingber1

Wow -- what a great reply ... I didn't even know that I was lookinh for this info until I read it!! Thanks ... now could you write the same kind of reply, but from the MOTOR end??? What do thos enumbers mean? How do I match up a motor / battery / esc to a particular airplane??? This is WAY more confusing than glow or gas.. HELP!!!

Wow -- what a great reply ... I didn't even know that I was lookinh for this info until I read it!! Thanks ... now could you write the same kind of reply, but from the MOTOR end??? What do thos enumbers mean? How do I match up a motor / battery / esc to a particular airplane??? This is WAY more confusing than glow or gas.. HELP!!!

by Ed Anderson

aeajr on the forums

This may get a little technical but I will try to keep it as simple as I

can. I will draw parallels to cars and bicycles in many places as most

people can relate to these and know at least a little about how they work.

I will use round numbers where I can and will use some high level examples.

If you are an engineer you will see that I am taking some liberties here for

the sake of simplicity. I will go through the parts of the power system,

then, toward the end, I will show you how we tie these all together to come

up with a complete power system.

POWER = WATTS

I will be using the terms Volts, Amps and Watts throughout this discussion.

Let me define them.

Volts = the pressure at which the electric energy is being delivered - like

pounds per square inch or PSI in a fuel system or water from a garden hose.

Volts is about pressure, it says nothing about flow. You will see volts

abbreviated as V.

Amps = the quantity or flow of electricity being delivered, like gallons per

minute in a fuel system or that same garden hose. Amps is about flow, it

says nothing about pressure. You will see amps abbreviated as A.

Watts = V X A. This is a measure of the energy or power being delivered.

This is how we measure the ability of that electricity to do work, in our

case the work of turning a propeller to move our airplane through the air.

Watts is about both pressure and flow. This serves the same purpose as

the horsepower rating of your car's engine. In fact 746 watts = 1

horsepower. So if you had an electric car, the strength of its motor could

be reported in either watts or horsepower. You will see watts abbreviated as

W.

If you want more depth on this, visit this thread.

http://www.wattflyer.com/forums/showthread.php?t=1933

MOTOR EFFICIENCY - Brushed vs. Brushless

Whether brushed or brushless, the motor's job is to convert electricity into

mechanical motion to turn the propeller to move air. Efficiency is how we

measure how much of the power, the watts, that our battery delivers to the

motor is actually turned into useful work and how much is wasted as heat.

A higher efficiency motor delivers more energy to the motor, and wastes

less.

A typical brushed motor, say a speed 400, is only about 40-50% efficient.

Only about half the watts delivered to the motor actually end up as useful

work turning the propeller. The rest is wasted. Motors that have a "speed"

designation, like speed 400, are brushed motors. There are other names for

brushed motors but the "speed" term is a common one. They are inexpensive

and they work. For example, you can buy a speed 400 motor and electronic

speed control, ESC, for $35. A comparable brushless motor/ESC combination

would

typically cost 3 to 4 times that much.

Brushless motors tend to be more efficient. They typically deliver 70-90%

of

that input power to the propeller, Thus you get better performance per watt

with brushless motors. Seen a different way, if you use a brushless motor,

then, for the same flying performance you will use less energy which means

you battery will last longer. Or you can use a similar size and weight

brushless motor and get much higher performance because the motor turns more

of the watts from the battery into useful work of turning the propeller.

So, as with many decisions we make, this is a cost benefit decision. Am I

willing to pay more to get more? That is up to you.

THE BATTERY IS MORE THAN JUST THE FUEL TANK

Think of the battery as the fuel tank plus the fuel pump and a supercharger

all rolled into one. It feeds/pushes energy to the motor. So you have to

look at the battery and the motor as one unit when you are sizing power

systems for electric planes. In many cases we start with the battery when

we

size our systems because the motor can't deliver the power to the prop if

the battery can't deliver the power to the motor.

The higher the voltage rating of the battery, the higher the pressure, like

a supercharger on a car engine. More pressure delivers more air/fuel

mixture to the engine which allows the engine to produce more power to turn

the wheels of the car. Higher voltage pushes more electricity into the

motor

to produce more power.

Using our electric motors, a given motor may take 10 amps ( the quantity of

electricity flowing ) at 8.4 volts ( the pressure at which the electricity

is

being delivered) to spin a certain propeller. We would say that the

battery is delivering, or that the motor is drawing 84 watts, ie: 8.4V x

10A. If you bump up the voltage to 9.6 volts, the battery can ram in more

amps delivering more energy to the motor which will produce more power to

the

propeller. In

this example, if we move from an 8.4V battery pack to a 9.6V battery pack

the motor may now take 12 amps. This will typically spin the motor faster

with any given propeller or allow it to turn a larger propeller at the same

speed.

However, if you bump up the pressure too much, you can break something.

Putting a big supercharger on an engine that is not designed for it will

break parts of the engine. Too much voltage can over power your electric

motor and damage it. So there is a balance that has to be struck.

Different motors can take different amounts of power, watts, volts X amps,

without damage. For example, a speed 400 motor might be fine taking 10 amps

at 9.6 volts or 96 watts. However a speed 280 motor will have a short life

with the same

combination of volts and amps.

If you match the right battery with the right motor, you get good

performance without damage to the motor. In many cases airplane designers

will design planes around a specific motor battery combination so that they

match the size and weight of the plane to the power system for good

performance.

PROPELLERS

Propellers are sized by diameter and pitch.

The diameter of the propeller determines the volume of air the propeller

will move, producing thrust, or pushing force. Roughly speaking the

diameter of the propeller will have the biggest impact on the size and

weight of the plane that we can fly. Larger, heavier planes will typically

fly better with larger diameter propellers.

Pitch refers to the angle of the propeller blade and refers to the distance

the propeller would move forward if there were no slippage in the air. So a

7 inch pitch propeller would move forward 7 inches per rotation, if there

were no slippage in the air. If we combine pitch with the rotational speed

of the propeller we can calculate the pitch "speed" of the propeller. So,

at 10000 revolutions per minute, that prop would move 7000 inches forward

70,000 inches per minute. If we do the math, that comes out to a little

over 66 miles per hour.

By changing the diameter and the pitch of the propeller we can have a

similar effect to changing the gears in your car or a bicycle. It will be

harder for your motor to turn a 9X7 propeller than an 8X7 propeller. And

it would be harder to turn a 9X7 propeller than a 9X6 propeller. The

larger, steeper pitched propellers will require more energy, more watts,

more horsepower, to

turn them. Therefore we need to balance the diameter and pitch with the

power or wattage of the motor/battery system. Fortunately we don't actually

have to do this as motor manufacturers will often publish suggested

propellers to use with a given motor/battery combination. We can use these

as our starting point. If we want we can try different propellers that are

near these specifications to see how they work with our airplane.

GEARBOXES

While unusual on glow or gas planes, gearboxes are common on electric

planes. Their primary function is similar to the transmission on a car. The

greater the gear ratio, the higher the numerical value, the slower the

propeller will turn but the larger the propeller we can turn. So you can

use a gearbox to help provide more thrust so you can fly larger planes with

a given motor. However you will be turning the propeller slower so the

plane will not go as fast.

With direct drive, that is when the propeller is directly attached to the

motor shaft, we are running in high gear ( no gear reduction). Like pulling

your car away from the light in high gear. Assuming the motor doesn't stall,

acceleration will be slow, but over time you will hit a high top end!

Typically direct drive propellers on a given motor will have a smaller

diameter.

With the geared motor, it would be like pulling away from the green light in

first gear - tons of low end power and lots of acceleration, but your top

speed is reduced.

So, by matching up the right gear ratios made up of the propeller and,

optionally, a gearbox we can adjust the kind of performance we can get out

of a given battery/motor combination.

NOW WE CAN START TO MATCH UP THE PIECES!

The simplest approach I have seen to figuring power systems in electrics is

input watts per pound of "all up" airplane weight. The following guidelines

were developed before brushless motors were common but it seems to hold

pretty well so we will use it regardless of what kind of motor is being

used.

50 watts per pound = Casual/scale flying

75 watts per pound = Sport flying and sport aerobatics

100 watts per pound = aggressive aerobatics and mild 3D

150 watts per pound = all out performance.

Remember that Watts = Volts X Amps. This is a power measurement.

In case you were wondering, 746 watts equals 1 horsepower, .

AN EXAMPLE!

This should be fun. Let's see where these formulas take us! We will use a

24 ounce, 1.5 pound plane as our example. If we want basic flight you will

need 50 watts per pound or about 75 watts input to your motor for this 1.5

pound plane. That is, 50 watts per pound X 1.5 pounds = 75 watts needed

for basic flying performance. If you want a little more spirited plane, we

could use 75 watts X 1.5 pounds which is about 112.5 watts.

Lets use 100 watts as the total target, just to be simple, shall we? I am

going to use a lot of round numbers here. I hope you can follow.

The Battery:

If we use an 8 cell NiMh battery pack at 9.6 V it will have to deliver 10.4

amps to hit our 100 watts input target ( 100/9.6 = 10.41amps) If my

battery pack cells are NiMh cells that are rated at 10C then I need an 8

cell pack rated at 1100 mah to be able to deliver 11 amps. Sounds about

right.

Now I select a motor that can handle 100 watts or about 10.4 amps at 9.6

Volts. From experience we know this could be a speed 400, a speed 480 or

some kind of a brushless motor.

We now need a propeller that will cause the motor to draw about 100 watts. I

don't know off the top of my head what that would be. I would go to some mfg

chart - GWS has good charts!

http://www.gwsus.com/english/product...tem/edp400.htm

I see that if I use a direct drive speed 400 with a 5X4.3 prop at 9.6V then

the motor will draw about 12.4 amps or about 119 watts. This would be a

good candidate motor/prop for the plane using a 9.6V pack that can put out

12.4 or more amps. This would be a set-up for a fast plane as that motor

will

spin that small prop very fast.

However maybe I don't want such a fast plane but one with a really good

climb and lots of low end pull to help out a new pilot who is in training.

I can also use a speed 400 with a 2.38 gearbox and run it at 9.6V spinning a

9X7 prop and run at about 12.8 amps for 120 watts. The larger prop will give

this plane a strong climb, but since the prop speed has been reduced by 2.38

times, it won't be as fast. Spinning a bigger prop gives me more thrust but

a lower top speed typically.

Back to battery packs and motors

So if I shop for a 9.6V pack to be able to handle about 15-20 amps, I should

do just fine and not over stress the batteries. In NiMh that would probably

be a 2/3 or 4/5 A pack of about 1100 -1300 mah capacity, depending on the

quality of the cells.

We view the battery and motor as a linked unit with a target power profile,

in this case about 100 watts. We use the prop and gearbox, if any, to

produce the manner in which we want to deliver that power to the air to

pull/push the plane.

If this is a pusher, I may not have clearance to spin that big prop so I

have to go for the smaller but faster prop combo.

If this is a puller, then I can choose my prop by ground clearance or some

other criteria and match a gear box to it.

See, that was easy, right? But we are not done! Oh no!

I could try to do it with a 2 cell lithium pack rated 7.4V. To get 100 watts

I now need a pack that can deliver 13.5 amps and a motor/prop combination

that will draw that much. So if I have 10 C rated lithiums, then the pack

better be at least 1350 mah. Probably use a 1500 mah pack to be safe.

Well, when I look at the chart for the geared speed 400 I see that,

regardless of prop, at 7.4V I am not going to have enough voltage (

pressure) to push

13 amps into this motor. So the 2 cell lithium won't meet my performance

goal of 100 watts+ per pound using this gear box.

If I go back to the charts and look at a different gear boxes I can't hit my

power goals using 7.4V. Maybe we go back to direct drive.

http://www.gwsus.com/english/product...tem/edp400.htm

We see that the best I can get this speed 400 to do is a total of 70 watts

at 7.2V ( close enough ) so I can't hit my power goals using a speed 400 at

this voltage. but 70 watts would be about 48 watts per pound so I could have

a flyable plane, but not an aerobatic plane using this two cell pack.

REALITY CHECK!

Now, in fact that is NOT how I would do this. I would decide on the watt

target, go to the chart, find a combo that meets my goals, then select a

battery that will meet the demand and see if my weight comes up at the

target I set. A little tuning and I come up with a workable combo.

Summary

So, in these few paragraphs have taken in a basic knowledge of how electric

power

systems are sized, the factors that are considered an how to predict the

outcome. Simple, right?

Of course there is a lot more to know and time and experience will teach

you plenty, but with this basic understanding you are better prepared to

begin playing with the power systems you put in your planes.

Here are some additional resources that may be helpful.

Good luck e-pilot!

Clear Skies and Safe Flying!

Ed Anderson

Brushed Motors

http://www.hobby-lobby.com/elecmot.htm

Brushless Motors

http://www.hobby-lobby.com/brushless-motors.htm

Battery Packs - NIMH

http://www.cheapbatterypacks.com/mai...=445976&ctype=

http://www.hobby-lobby.com/hydride.htm

Battery Packs - LiPo

http://www.cheapbatterypacks.com/mai...gid=tp&sort=PL

http://www.hobby-lobby.com/lithium-polymer.htm

Gearboxes - Speed 400 & 480 examples

http://www.hobby-lobby.com/gear400.htm

http://www.hobby-lobby.com/gear480.htm

A series of posts on electric power system basics

http://www.wattflyer.com/forums/showthread.php?t=1933

http://www.rcgroups.com/forums/showthread.php?t=417868

Maxx Products has a pretty good tip sheet on coming up

with a glow to electric power comparison. You can find it here:

http://www.maxxprod.com/mpi/tips3.html

MotoCalc

MotoCalc will tell you everything you need to know: Amps, Volts, Watts, RPM,

Thrust, Rate of Climb, and much more! It is a popular tool for predicting

the proper motor, prop, battery pack for electric planes.

http://www.motocalc.com/

#

**9**Junior Member

Join Date: Jul 2006

Location: Davis, CA

Posts: 3

**RE: Confused about motor Amps and battery Amps**

Thanks aeajr!

That guide was so helpful that I decided to sign up at RCU and even referred you!

There are still a few questions however..

This is still a bit fuzzy. So do motors require a minimum voltage to pull certain amount of amps from the battery?

As for the chart, is this the same chart you provided earlier? If so, that was a dead link Are there other charts you can refer me to?

I've read about wing loading here and there. Does that come into play when determining a power system for your plane?

Thanks!

Eric

That guide was so helpful that I decided to sign up at RCU and even referred you!

There are still a few questions however..

Well, when I look at the chart for the geared speed 400 I see that,

regardless of prop, at 7.4V I am not going to have enough voltage (

pressure) to push

13 amps into this motor. So the 2 cell lithium won't meet my performance

goal of 100 watts+ per pound using this gear box.

regardless of prop, at 7.4V I am not going to have enough voltage (

pressure) to push

13 amps into this motor. So the 2 cell lithium won't meet my performance

goal of 100 watts+ per pound using this gear box.

As for the chart, is this the same chart you provided earlier? If so, that was a dead link Are there other charts you can refer me to?

I've read about wing loading here and there. Does that come into play when determining a power system for your plane?

Thanks!

Eric

#

**10**
My Feedback: (2)

Join Date: Jan 2003

Location: Long Island, NY

Posts: 8,567

**RE: Confused about motor Amps and battery Amps**

ORIGINAL: shutterbug27mkii

Thanks aeajr!

That guide was so helpful that I decided to sign up at RCU and even referred you!

There are still a few questions however..

Well, when I look at the chart for the geared speed 400 I see that,

regardless of prop, at 7.4V I am not going to have enough voltage (

pressure) to push

13 amps into this motor. So the 2 cell lithium won't meet my performance

goal of 100 watts+ per pound using this gear box.
This is still a bit fuzzy. So do motors require a minimum voltage to pull certain amount of amps from the battery?

As for the chart, is this the same chart you provided earlier? If so, that was a dead link Are there other charts you can refer me to?

I've read about wing loading here and there. Does that come into play when determining a power system for your plane?

Thanks!

Eric

Thanks aeajr!

That guide was so helpful that I decided to sign up at RCU and even referred you!

There are still a few questions however..

regardless of prop, at 7.4V I am not going to have enough voltage (

pressure) to push

13 amps into this motor. So the 2 cell lithium won't meet my performance

goal of 100 watts+ per pound using this gear box.

As for the chart, is this the same chart you provided earlier? If so, that was a dead link Are there other charts you can refer me to?

I've read about wing loading here and there. Does that come into play when determining a power system for your plane?

Thanks!

Eric

[link=http://www.gwsus.com/english/product/powersystem/eps400c.htm]GWS motor charts[/link]

Yes, you need a minimum amount of voltage to get the goal amperage using a particular propeller, as I discussed in the posts above. Voltage is pressure and without enough pressure you can't push the amperage into the motor.

Check the charts and you will see what I mean.

As for wing loading, that has more to do with the manner in which the plane will fly and the minimum flying speed. It is a different discssion. But of course all things on the plane, whether glow or gas, electric or glider, work as a whole so they are all related. But I would not put that into this dicussion. Too many variables and too much to think about. Wing loading will have more to do with minimum stall speed and pitch speed of the prop. Wattage works outside of this as a general tool.

Work from the wattage goal and go backwards.

#

**11**Junior Member

Join Date: Jul 2006

Location: Davis, CA

Posts: 3

**RE: Confused about motor Amps and battery Amps**

Just a few more questions...

I have:

an Apache 2030 charger, max output 1200mAh

3s 2500mAh LiPo 25C, 3s 1250 LiPo ?C

Power source:

AC-to-DC adapter 12V 1000mA

RadioShack 12V 7Ah lead-acid battery

Questions:

What is the minimum amps required from the charger's power supply? (am I correct to assume I need more amps than the charger's max amp output?) Is there a way to calculate how much amps I need in my power supply?

Can my power supplies charge my LiPos?

What is a deep cycle? Is my lead-acid battery a deep cycle battery?

Thanks!

Eric

I have:

an Apache 2030 charger, max output 1200mAh

3s 2500mAh LiPo 25C, 3s 1250 LiPo ?C

Power source:

AC-to-DC adapter 12V 1000mA

RadioShack 12V 7Ah lead-acid battery

Questions:

What is the minimum amps required from the charger's power supply? (am I correct to assume I need more amps than the charger's max amp output?) Is there a way to calculate how much amps I need in my power supply?

Can my power supplies charge my LiPos?

What is a deep cycle? Is my lead-acid battery a deep cycle battery?

Thanks!

Eric

#

**12**
My Feedback: (2)

Join Date: Jan 2003

Location: Long Island, NY

Posts: 8,567

**RE: Confused about motor Amps and battery Amps**

Power source:

AC-to-DC adapter 12V 1000mA

RadioShack 12V 7Ah lead-acid battery

Questions:

What is the minimum amps required from the charger's power supply? (am I correct to assume I need more amps than the charger's max amp output?) Is there a way to calculate how much amps I need in my power supply?

READ YOUR CHARGER MANUAL

Can my power supplies charge my LiPos?

NO

What is a deep cycle? Is my lead-acid battery a deep cycle battery?

SOME LEAD/ACID ARE DEEP CYCLE AND SOME ARE NOT - WILL USUALLY SAY ON THE BATTERY

#

**13**Junior Member

Join Date: Jul 2006

Location: Davis, CA

Posts: 3

**RE: Confused about motor Amps and battery Amps**

ORIGINAL: aeajr

What is the minimum amps required from the charger's power supply? (am I correct to assume I need more amps than the charger's max amp output?) Is there a way to calculate how much amps I need in my power supply?

READ YOUR CHARGER MANUAL

What is the minimum amps required from the charger's power supply? (am I correct to assume I need more amps than the charger's max amp output?) Is there a way to calculate how much amps I need in my power supply?

READ YOUR CHARGER MANUAL

I was asking if my power supplies would provide adequate power for charging my LiPos *assuming a charger is in between the two, of course*

I've read quite a few threads about chargers reading input errors due to weak power supplies. I was just wondering how you know which power supplies would work for my charger.

Sorry about the misunderstanding,

Eric