Help! Motor draw and battery capacity

messyhead

Well-known member
Hi folks. I’m converting a balsa model to electric.

Today I got the motor for it, an Overlander Thumper 3548 900kv.

The spec sheet doesn’t tell me much about the power draw. So I need help with a couple of things.

How do I measure the draw of the motor, with the expected prop fitted? I have a multimeter, but is there some wiring harness I could make to measure the current?

Once I know what the current draw is, how can I use that to work out what battery capacity I would need? I’m going to run it on 4s, but not sure if I would need 3000mah or 4000mah to achieve around a 10 minute flight time.
 

HVB79

Member
Yes you want to measure the amp draw with the prop installed.

While you can use a multi meter to measure the amp draw of a motor it is much easier with a tool designed specifically for the task. Do a search for "RC Watt Meter".

To calculate flight time:
capacity / amps = flight time

Example #1: Assume 40 amp draw measured and a 3000mah battery
First convert milli amp hours into amp hours so 3000mah = 3ah
3 amp hours / 40 amps = flight time
0.075 hours = flight time
0.075 hours * 60 min = 4.5 min of flight time

Example #2: Assume 40 amp draw and a target 10 minute flight time
capacity / 40 amp = 0.1666 hours
capacity = 6.66ah (6660mah)
 

messyhead

Well-known member
Yes you want to measure the amp draw with the prop installed.

While you can use a multi meter to measure the amp draw of a motor it is much easier with a tool designed specifically for the task. Do a search for "RC Watt Meter".

To calculate flight time:
capacity / amps = flight time

Example #1: Assume 40 amp draw measured and a 3000mah battery
First convert milli amp hours into amp hours so 3000mah = 3ah
3 amp hours / 40 amps = flight time
0.075 hours = flight time
0.075 hours * 60 min = 4.5 min of flight time

Example #2: Assume 40 amp draw and a target 10 minute flight time
capacity / 40 amp = 0.1666 hours
capacity = 6.66ah (6660mah)

Thanks. I only have 3s Lipos just now.

If I run the motor with them to get a draw, will that matter, if I’m going to run it on 4s in flight?
 

HVB79

Member
Thanks. I only have 3s Lipos just now.

If I run the motor with them to get a draw, will that matter, if I’m going to run it on 4s in flight?

Your amp draw on 3s vs 4s will be different.
You need to buy or borrow a 4s battery to test with.
 

NickRehm

Member
You can measure current draw on 3s and calculate power draw with V*I. The motor will more or less draw similar power at the same thrust with 4s, so you can back out the expected current by setting (V*I)3s = (V*I)4s and solving for the 4s current.

Then calculate capacity needed with that current for 4s battery. It's not exact, but gets you in the right ballpark. Even simpler, take the capacity you calculate for 3s and multiply by (12.6/16.8) for 4s equivalent.
 

HVB79

Member
The motor will more or less draw similar power at the same thrust with 4s,

I don't think we know the amount of thrust generated by the 3s power system, unless we measure that somehow, so we can't hold it constant when switching to the 4s power system.

I think the question is: "How much capacity is needed to run the motor with the throttle at X% on 4s for 10 min"
not "How much capacity is needed to produce X pounds of thrust on 4s for 10 min"

If we change the battery from 3s to 4s the motor is going to spin faster, consume more power and produce more thrust at a given throttle level. For example at 100% throttle.
 

NickRehm

Member
Yes agreed there will be more power draw at the same throttle setting on 4s--and certainly more thrust.

But there will also be similar power draw at the same thrust setting. Matching the thrust setting is important if we are talking about cruising around for a desired flight time: just because you are now flying a 4s does not mean your airplane will require more thrust to fly. A 4s battery with 3/4 the capacity of a 3s will be nearly identical in weight (constant battery power density assumption). Then, at the same thrust (power) setting, it will give similar flight time since it has the same net power. I guess that's what I was trying to convey.

The only difference with the higher voltage system is that you are now technically able to pull more power at max throttle, and drain your battery that much faster. Your 'thrust ceiling,' or 'power ceiling,' has simply just increased, and so has your ability to reduce the flight time if you choose to be aggressive on the throttle. Otherwise just cruising around, at your preferred cruise throttle, you get the same flight time
 

Bricks

Master member
Don`t forget going from 3s to 4s may take a different prop to keep amp draw within limits of the motor.
 

quorneng

Master member
messyhead
It is the motors max amps that is the limit whether on 3s or 4s. The Thumper 3548 900kv has a limit of 55A. You have to select a prop to keep within this.
If the plane does not need the maximum power you can select a smaller prop, however in any case the prop will be smaller and/or with less pitch for the same amps when using a 4s compared to 3s.
Given this will require some testing a suitable Wattmeter is a vital tool. A 100A watt meter is not expensive.

If you set the prop to pull the max amps your battery must have sufficient capability to sustain 55A without damage. As an example a 3000mAh 20C battery can deliver 60A but if sustained it would be working it pretty hard and last a bit over 2 minutes!

How long a battery will last in flight depends entirely on the aerodynamic characteristics and the weight of the plane, coupled with how you fly it.
I have a plane that uses a 3000 mAh 3s that can fly for almost an hour with the motor running non stop but at much reduced throttle taking an average of 3A rather than the motors max of 38A.
 

messyhead

Well-known member
messyhead
It is the motors max amps that is the limit whether on 3s or 4s. The Thumper 3548 900kv has a limit of 55A. You have to select a prop to keep within this.
If the plane does not need the maximum power you can select a smaller prop, however in any case the prop will be smaller and/or with less pitch for the same amps when using a 4s compared to 3s.
Given this will require some testing a suitable Wattmeter is a vital tool. A 100A watt meter is not expensive.

If you set the prop to pull the max amps your battery must have sufficient capability to sustain 55A without damage. As an example a 3000mAh 20C battery can deliver 60A but if sustained it would be working it pretty hard and last a bit over 2 minutes!

How long a battery will last in flight depends entirely on the aerodynamic characteristics and the weight of the plane, coupled with how you fly it.
I have a plane that uses a 3000 mAh 3s that can fly for almost an hour with the motor running non stop but at much reduced throttle taking an average of 3A rather than the motors max of 38A.

The motor guidance says a 11xX prop. I was going to use an 11x6.

You’ve said you have a plane that uses a 3000mah 3s that can fly non stop, but not said how much it weighs. It could be a lightweight foamie.

I’m converting a balsa IC plane. I balanced it earlier, and it’s way tail heavy with a 3s 2200mah up front. So it’s already lighter than the OC46 that was there before.

I’ve got a watt meter on order, then I can figure out what capacity will be needed.
 

quorneng

Master member
messyhead
I did point out that weight is an important factor in the power required to fly.
That particular plane of mine is structurally very efficient and weighs 743 g ready to go. The battery itself constitutes 32% of that weight.
An IC airframe will be built 'heavy'. It has to be to withstand the serious vibration from a single cylinder motor. An electric motor is perfectly balanced and generates a smooth torque. indeed a brushless motor is likely to be lighter than an equivalent power IC engine but the battery required to power it is heavy.
Converting an IC airframe to electric will always be at a some degree of disadvantage, which appears either as a limited duration or greater weight, when compared to an otherwise identical plane designed to use electric power.

The success of an IC conversion to electric will depend on how well you match the motor, prop and battery to meet your expected flight performance.
 

Tench745

Master member
Hi folks. I’m converting a balsa model to electric.

Today I got the motor for it, an Overlander Thumper 3548 900kv.

The spec sheet doesn’t tell me much about the power draw. So I need help with a couple of things.

How do I measure the draw of the motor, with the expected prop fitted? I have a multimeter, but is there some wiring harness I could make to measure the current?

Once I know what the current draw is, how can I use that to work out what battery capacity I would need? I’m going to run it on 4s, but not sure if I would need 3000mah or 4000mah to achieve around a 10 minute flight time.

The website I found for the 3548/05 900KV Tornado Thumper V3 recommends an 11x8 prop for 4s. This will keep you under the 60A limit for the motor.
A quick plug into a prop calculator suggests this will draw somewhere around 40 amps at full throttle. So let's take that 40 amp draw and plug it in with your desired 10 minute flight time to get a minimum battery size.
10 minutes is .167 hours.
At full throttle on the suggested 11x8 prop you'd need to deliver 40 amps.
40A x .167hrs= 6.68 amp-hours of battery capacity. or 6680mah. You probably won't be flying full throttle all the time, so one can reasonably assume a 6500mah battery will give you what you're looking for.
 

messyhead

Well-known member
Hi folks. Thanks for the advice.

I picked up an Etronix Power Analyzer, and used it today.

I used a 3S 2200mah battery (I only have 3S batteries just now).

The motor was fitted with a Master Airscrew GF 11x7 prop

At 100% throttle, it was drawing a little over 38A

At 70% throttle, it was drawing a little over 18A

Based on experience so far, as this is a trainer model, I expect the throttle will be around the 70% mark for most of the flight to maintain a steady slow speed.

I'll go a little more conservative and say 20A at 70% throttle.

So based on the formulas you posted earlier, if I plug in my figures I get...

capacity / amps = flight time

Example #1: Assume 20 amp draw measured and a 4000mah battery
First convert milli amp hours into amp hours so 4000mah = 4ah
4 amp hours / 20 amps = flight time
0.2 hours = flight time
0.2 hours * 60 min = 12 min of flight time

Example #2: Assume 20 amp draw and a target 10 minute flight time
capacity / 20 amp = 0.1666 hours
capacity = 3.33ah (3330mah)


So a 4000mah 4S looks to be what I'll need to get around 10 minutes of flight time, with throttle between 70-100%.


Does that seem reasonable to you?
 

HVB79

Member
Your math is correct for a 3s 4000mah battery.
The 4s 4000mah battery will give you more RPM/Thrust/Speed at 70% throttle or extra flight time at a lower throttle level.

A 4s 4000mah battery is going to give you closer to 15 min of flight time so you could get away with using a 4s 3000mah and still get 10 min flight time if you really wanted to. Depending on how much nose weight you need to balance the plane the smaller battery may not save you any weight.
 

messyhead

Well-known member
Great, thanks. So that's a better result then! I do need quite a bit of noseweight, so I think I'll get a 4000mah one and take the weight/cost trade off.
 

quorneng

Master member
To be exact it is the 4000mAh that gives you the 12 minutes flight at 20A. It would be the same with a 3s or 4s at 20A.
The difference is that at 20A a 4s will be generating 33% more power (Watts = V x A) than with a 3s.

If your motor and prop draw 38A on 3s at 100% throttle it will draw nearly twice much on a 4s with the same prop. It would likely exceed both the motor's and the ESC's max amps limit. You would have to use a smaller prop to reduce the amps to keep within the motor and ESC limits.

It is the power (Watts) that determine how the plane flies.
If 20A on a 3s (11.7 x 20 = 234W) gives a useful performance then using a 4s at the same power level will only require 234/14.8 = 15.8A. The flight time will be increased although the 4000 mAh 4s battery will be 33% heavier than a 3s so you will have to use a bit more throttle compensate for the extra weight and thus loose a bit of the extra flight time.
 

messyhead

Well-known member
I ended up going for a 6000mah battery. I needed the weight for balance. I’m waiting on some connectors arriving, then I’ll check the power draw and get the right prop.
 

messyhead

Well-known member
Hi. I did some tests, and need a bit more help. I’m using an 11x7 glow prop, and a 4s 6000mah.

The battery was about 80% charged.

At 70% throttle, the current was around 27A, and at 99% it was around 58A. The power on the meter was reading about 820W at 100%.

The spec sheet for the motor shows Power 710W. What is this for?

The spec sheet also shows the minimum prop should be 11x8. What effect does the prop pitch have?

I have 12x7, 11x6, 10x7, and 10x6 glow props available. And a 13x6.5 electric prop.

Should I use a electric props? I was under the impression glow ones are fine to use too, they’re just heavier.

D3B89C83-BABC-4D78-9949-9E5197AEEA9E.jpeg
 
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messyhead

Well-known member
Hi. I did some tests, and need a bit more help. I’m using an 11x7 glow prop, and a 4s 6000mah.

The battery was about 80% charged.

At 70% throttle, the current was around 27A, and at 99% it was around 58A. The power on the meter was reading about 820W at 100%.

The spec sheet for the motor shows Power 710W. What is this for?

The spec sheet also shows the minimum prop should be 11x8. What effect does the prop pitch have?

I have 12x7, 11x6, 10x7, and 10x6 glow props available. And a 13x6.5 electric prop.

Should I use a electric props? I was under the impression glow ones are fine to use too, they’re just heavier.

I did some reading, and I’m going to try an 11x7 and 11x8 electric prop. If they’re in limits, I’ll see how the plane flies with those. I might need to go up to something like a 12x5. If my local store has some in stock, I’ll get one to try it.

I’m still not sure what the power figure is on the spec sheet.
 

Tench745

Master member
That power number is probably the max wattage the motor can be run at reliably. If you're running yours at 820W with the prop you have, it's probably over-propped and will burn up the motor eventually. How much time you spend a full throttle and how good the cooling for the motor is will effect how long "eventually" takes.