Will a 1/3 horse power sump pump motor burn up at half power?
 

I am building my own battle bot and I'm working on running it off of two 12V batteries. It will be driven by two heavy sump pump motors. I will only be able to obtain a possible half power off of the batteries and I need to know if that will ruin the engines. I have been told it will by a freind but I dont beleive her. So I need some help. I dont want to waste/ruin the motors after all!

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

OK, There's not quite enough info here but I'll give it a try.

By "only be able to obtain a possible half power off of the batteries" do you mean that the pump is 24V and you are running it at 12V? If so, then your friend is absolutely correct. The motor will try and do the same amount of work with only 1/2 the voltage thus requireing twice (roughly) the current. This will heat the motor more.

In addition, most sump pumps are designed to work submerged in water. They use the water to cool the motor, thus will need extra cooling if it is run using air to cool it.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

What are your loading conditions? Do you have a model number for the motor so that we might be able to better understand? Are you using the batteries that we used for competition or are these some other chemistry (NiCd, Li-ion)? are these batteries in series, parallel, or individually connected to the motors?

Why are you only able to obtain half power? How are you gearing this drive train? wheel size? wheel traction? In general, depending on the loading condition, the power your motor demands is not a static number

Will it burn up? Well that depends a lot on the motors thermal constants (casing, efficiency, etc) and it's ability to handle a stall current among other things

More information is required to give you a good answer.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

when I posted this I knew nothing about the topic I was asking about. so here arwe the facts. I have a 115 A/C volt motor, I'm using a 12 D/C volt battery to power it. the motor is 5.7 amps, 60 hz, 1725 RPM, and it is non submersible. air cooled and isnt ment to be wet. I have a model #: xqj48517028j p and I ndont know what the p means. it also says it is thermally protected, single phase. what is single phase? and it is 60 cycle? what is that. fla= 5.8 sf= 1.0 and I have no clue what any of that means. help me out, thanks.:confused:

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

My guess is that you won't be able to run it directly off the 12 bolt battery. It sounds like it uses the 110 volts out of your wall which is single phase. I.E. there is only one sinusoidal wave coming out of your standard wall outlet. (opposed to 3 sinusoids in 3 phased 220 volts). 60 cycle refers to the 60 hz which is the frequency of the sinusoid in your wall.

The thermal protection most likely means that the motor will stop running if it gets to hot to prevent it from damaging itself.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

well what are sinusoids. and it is a wall plug type motor. so that means like even if I wanted to I couldnt convert it? and what about like wheelchair motors cause I got those. and I mean what if I used like a fan motor from a like normal around the house use post fan or whatever? would i not be able to use that with a battery. :confused:

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Sinusoid are sine waves. You would need an inverter to go from the DC of the battery to AC but most inverters do not produce a true sine wave. A regular house fan is mostly likely an AC motor; all mine are. You could maybe use a fan motor off your PC which is a DC motor.

The wheel chair motors are mostly likely DC so they could be usable.

Wikipedia has good info on AC and DC

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Let's put it this way: The motor runs on 115 volts. A 12V battery provides about a tenth of that.

So whatever the motor says it'll put out in terms of power, you can expect the actual power output to be no more than about a tenth of what it says if the voltage is the driving factor. (Current--amperage--is a different factor, and can have an effect as well.)

What you have is a household-voltage, low amperage motor. You won't trip a lot of breakers with it, but you won't be using its full potential.

Not ideal.

The exception would be if it has internal voltage regulators to take 115V down to 12V--and then I'd say to remove those for this use, as 12V is your max.

I'd go with the wheelchair motors instead--those are much more likely to be 12V or something in that range.

The fan motor would depend on what type of fan and the ratings on the voltage and amperage were.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

alright thanks. ya see I'm trying to get a powerful motor that will be hard to stop while it is receiving power. that way it will be able to power my robot through all opposition. (this robot is for like a hobby, not the competitions) and I'm thinking that I have some older wheelchair motors, and other stuff. but they arent very fast and they dont have much power. I'd probably need either a gear box or some good belts to allow diffrent speed and torgue. I kinda think I know what i'm talking about. I mean I'm just getting on a team so I dont have alot of robotics experience. ::ouch::

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Go FIRST Retro and use some 12 Volt Drill Motors. They'll come with gearboxes with high low speeds too! Judging by your team number your team might still have some.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

I think that you may want to talk to your local physics teacher. At the least, he can help you through some of the concepts and the math behind them.

I'll give a brief rundown of some of them.

Voltage: difference in electrical potential energy between two points.
Amperage or current: how fast that electrical energy can move.

Power (or energy): force*distance.
Work: power/time.
Torque: force * distance, usually referenced with regard to rotational motion. You'll want to know this eventually.

Gearboxes multiply and divide torque and rotational speed, which translates via wheels to linear power and linear speed.

BTW, you may want to look up some of John V-Neun's whitepapers. I think that those will help you with some of the math that is behind the design of a drivetrain, and quite possibly help you optimize the drive for the motors you have.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Please do not take this the wrong way, but coming from someone who has built homemade/ non-first robots, I can see that you may want to do a lot more studying on basic AC and DC electrical circuits and the wide variety of electrical motors before you get too deep into this project. At your level of experience, I would stay away from AC motors all together and stick with dc. AC is possible, but take a lot of know how to work on a battery powered robot. I took a digital circuits and industrial controls class that really helped me understand some of the questions mentioned here. I don't want to discourage you at all, just know what you are getting into before you spend too much time or money on something that won't work.
My $0.02

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

alright thanks guys. and I probably will stick to the DC stuff because I dont really have the money to work with converters and inverters and such. I'm probably going to be on here alot. and I'm also getting other info from other sites. I'm learning terms and what does what. i'm also studying basic electric diagrams. then there are the people who may be helping me from school so, i hope it will come together. and I'm pretty good with physics (without the classes), electrical is my main shortcoming. but yeah thanks. I will charish this time that i have spent on here forever! lol

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

power and energy are not the same thing.

you have that backwards.

power = work/time

torque translates to linear force, not linear power.


~

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

They aren't, but for some reason known to nobody, they share the same units. Joules in the SI, BTU and its equivalents in the Eng. system. (Eng. == either English or Engineering, which for functional purposes are indistinguishable.) This makes the calculations very similar when all's said and done.

150 kJ << without my telling you, is this power or energy?

150 kN-m << That's power, but to get that I had to expand the unit. Also note that that is a torque unit as well.

Energy has a number of ways to get to the kJ unit, none of them simple. Something about energy/mass * mass in a number of weird combinations makes it a little hard to translate out of handwriting.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Power is measured in Watts. Energy is measured in Joules.

Work and Energy have the same units. Power and Energy do not. Think about it, to find power Power you take the derivative of work with respect to time. This will divide the units of work by time. :)

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Well thats energy because power would be j/s.

And I don't think its torque because you're not taking the cross product

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Sure you are! Provided the force vector and radius vector are perpendicular, we can write the cross product as the following determinant:

i j k
0 r 0
0 0 F

Which simplifies to t=rF.

There might be some trig figuring out the force perpendicular to the radius, but it's definitely a torque. :)

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Darn it, that's what I get for trying to do thermodynamics and dynamics-type unit-crunching after 11PM when I've got a headache. At least I'm not trying my homework in said subjects at this time...

As for torque, the only time it wouldn't share units with power would be if the force generating it was directed directly away from the axis of rotation, along the radius vector. Then torque goes to zero. Or if the force is acting at the axis of rotation, directed away from it (or towards it).

As long as you can get a component of force that is perpendicular to the radius, at a distance greater than zero, you can get the torque. Force *distance, where the distance is specifically defined as the radius between the force and the axis of rotation.

A statics course uses the same type of math to calculate the moment on a point in a beam.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Al,
It not a simple matter of voltage in this case. A motor turns by using magnetic fields. In DC motors, the fields are generated via permanent magnets in the case and the current through the windings. In AC motors, there is generally no magnets and no brushes. The magnetic fields are generated by the AC current flowing through the motor windings and the induced currents that flow in other structures in the motor. Connecting this sump motor to a DC battery would cause it to heat up over time but it would never move.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Torque is measured using units such as oz-in and in-lbs, in refers to the radius of the shaft, wheel, or gear (etc.) in or lbs refers to the tangential force. The lack of a slash or the word per tells you there is no division involved. torque is the product obtained by multiplying the force tangent to the radius times the radius.
For example, all of these will yield the same torque ratings:

• 1 lb of force tangent to a 6 in radius wheel
• 2 lbs of force tangent to a 3 in radius wheel
• 8 oz of force tangent to a 1 foot radius wheel
• 1 oz of force tangent to an 8 foot radius wheel
• 6 lbs of force tangent to a 1 in radius wheel
• 48 lbs of force tangent to a .25 in diameter (.125 in radius) shaft
• etc

For all of these the torque is 6 in-lbs, or 96 oz-in. If you want more torque and you have large wheels, get smaller wheels. Wheelchair motors should be relatively strong. Most un-geared DC motors don't even provide more than 60 oz-in (3.75 in-lbs) of torque. CIMs provide alot at 343.27 oz-in (~21.45 in-lbs).

Also check out my response on this thread if you still want more powerful motors.
http://www.chiefdelphi.com/forums/sh...threadid=84877

torque converter

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

torque and power never have the same units. ever.

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Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

It is energy (or work). NOT power.


No, it is not.


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Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Correct, provided you're in one of the countries that still uses the English system of measurement for things like this. If you're not, N-m is the metric equivalent (and you'll need to convert it if you'd like it in English). [insert metric-English discussion here]

Ether, I'd like to invite you to read the whole thread before you respond. If you had, you'd have found that I'd already been corrected, and in a much more constructive manner than just denial.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

My apologies if my replies to the questions you directed to me were too concise. I will be happy to explain, if you wish, and in whatever detail is necessary, the difference between energy and power, and between torque and power, and between force and power. These are important concepts for robotics and it would be a shame to misinform students who are just learning these concepts.


~

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

I'm a college student, so I know them. I just got confused. If you tell me that you've never gotten confused when it's late at night/early in the morning, I'll probably figure that you're lying.;)

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

I get confused sometimes, and I make mistakes too. But I don't lie. And when I am wrong and someone corrects me, I eat humble pie. I don't flame them.

~

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

It was not my intention to flame you.

However, when you compare two separate one-line statements that effectively say "you're wrong" and nothing else, to the responses that iCurtis and Coffeeism gave, which type of response is more constructive and still answers the question? (And, which one came earlier enough that I had time to read it, reply, and get some sleep?)

It gets kind of annoying when a question is answered thoroughly and then, as the discussion moves on, someone else answers it again without supplying any additional information, with enough time to read the answers already given.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

For those that are interested, 1075's powered forklift-cart is built atop the chassis of a power wheelchair, it used 24V motors, but we're running them at 12V on a 2004 IFI system, and all works fine. I suspect we may be shortening the life of the motors by running them on 12V, however, they're designed for long service-lives and our cart doesnt see a whole lot of use outside of on-season FRC competitions. We generally take our hand-cart to the offseasons held in smaller venues.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Phil,
I don't think you are shortening the life of those motors. You are just running them at half speed.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Eric, I don't want this to degenerate into a pissing match, but this characterization of what occurred cannot be allowed to stand. I was the first to respond to correct the errors in your post11. In post14, I explained that a) power and energy are not the same thing, and b) power = work/time, and c) torque translates to linear force, not linear power.

You then responded to my post with your post15, repeating the error (claiming that power and energy share the same units), and issued a challenge directed at me asking "150 kJ << without my telling you, is this power or energy?"

I then politely responded to your direct challenge, explaining that Kj is energy (or work), not power.

The only criticism I will accept is that my answers were perhaps too concise.


~

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

It's not the response to post #11 that I have a problem with. It's your responding to post #15, 7 hours after the issue had been addressed with an explanation and the correction accepted, with absolutely no explanation. That's what annoyed me.

Your answers were so concise that they may as well have been "You're wrong." with no further explanation. I would suggest not using that format the next time an issue of this sort comes up unless you happen to like making not-friends. Even a comment about "power uses unit X and energy has unit Y" would be better than "It's X not Y" or "No it's not." Something like what Coffeeism said: "Well thats energy because power would be j/s." is much more helpful and constructive.

Again, if you're going to just say no, you might want to explain why you're saying no.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

You forgot to mention that the 7 hours was during the middle of the night, and that I responded the following morning, and that I was responding to a challenge in your post that was directed specifically at me.

There was further explanation in my response. I explained that Kj is a measure of energy or work, not power. And I explained in the prior post that power is work/time, so I can be forgiven for not repeating that.

And I suggest that if in the future you feel compelled to scold someone, especially someone you don't even know, as a matter of simple courtesy you first take them aside and do it privately and see if the issue can be resolved amicably.

~

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

This conversation has edged past where it needs to be. I am guessing that the original poster still wants information, but is being dissuaded from trying by this behavior.

Take it outside guys.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Eric, the OP changed motors, and posted another thread to that effect elsewhere, asking about powering a 12V motor with a standard 9V battery. (The response given there was that you really want a 12V battery.)

Outside is where it's already going.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Enough is enough boys. Time to cool off and get perspective of where you are. This thread will be closed for about 24 hours.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

As I promised, the thread is again open. We will be watching.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

I feel that aside from debates on work and power we should look more at what changing the voltage on a DC motor will actually do, and more importantly the monstrous difference between and AC and DC motor.

If I run a normal 12v DC motor at only 6v the amount of power that can be produced by the motor isn't cut in half but instead on fourth. The reason for this is that Current is determined by the voltage and resistance. The amount of electrical power being used by the motor (not relevant to the amount of mechanical power it is currently producing) is directly related to Current and Voltage. The specific equations for both of these is:

I = V/R (I = current, V = voltage, R = resistance)

P = I*V (P = power)

P = V^2/R <- can be derived from previous two equations

Since the resistance of the motor at any given rpm can't change (physical property of the motor) the power varies directly with the square of the voltage. By changing the voltage at which you run a DC motor you can't hurt it. The way in which you will hurt a DC motor will be to overheat it, this result of over-volting it and pushing too much current through the wire. Basically, for you cart that is running at half voltage, if you aren't using really special DC motors you won't hurt them by running them at half voltage.

The topic of AC motors is quite a bit more complex and as a result I believe that most people here were missing the elephant in the room when it came to using a sump pump. The problem here is that it is single phase. A single phase AC motor will run go from having plenty of torque to none in a 60hz cycle. This result because the sinusoidal wave will hit 0 volts as it goes from positive to negative. If you open up most AC motors in your house you will find that to compensate for this two things are done.

1. The motor is much larger than the application requires so that it can get whatever system it was designed for started. It will also have a fly wheel or some other form of high inertial mass to keep the motor turning when it isn't producing torque.

2. The motor actually has a second set of coils that are 90 degrees (electrically) out from the main power coils, and a capacitor that changes it into a 2 phase motor. This extra circuit is turned on when the motor is starting up and is turned off by a governor when the motor gets up to speed.

The reason that I highlight this weakness of single phase AC motors it because a standard robot drive-train will be driving with a lot of direction changes. The motor would be constantly asked for large amount of continuous output torque which a single phase AC motor is not capable of. This is why high power AC motors are always 3 phase. Since there are three sinusoidal waves there will always be a power coil. In fact, 3 phase AC motor will have a much flatter output torque curve than most DC motors since these motors are usually bushed and have a certain amount of cogging torque (take an un-powered CIM and turn it with your hands to feel this).

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Alex,
AC motors come in many types and flavors. The capacitor in AC motors doesn't always phase shift the current in a second winding, for instance. The fact that some motors run on three phase power is due to the need for all motors to have rotating magnetic fields. Single phase motor play some interesting games to achieve rotating fields. DC motors perform the same function using the brush assy and commutators for multiple windings or in the case of brushless DC motors, produce multi phase AC current for what is essentially an AC motor. What is confusing is that some AC (synchronous) motors react such that max torque is present at their design RPM. These types cannot be made to run at different speeds. If you load them to the point that the shaft rotation is vastly reduced there is no torque and the motor will stop. (Current continues to flow in this case) Some AC motors using a starter circuit, will not start without current flowing in the startup winding. If you spin them with power applied, they will run and produce torque at their design RPM. These motors often contain a centrifugal switch that keeps current flowing until the motor reaches an RPM that opens the circuit. If the switch or capacitor fails, the motor will hum and buzz and get hot but will not start.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

[EDIT] This post is not responding to any particular post in this thread, just providing some information possibly relevant to the discussion.

The question whether or not running a brushed DC motor (like a CIM) with a supply voltage lower than that for which it was designed is harmful to the motor depends on what exactly you are trying to accomplish.

If your intent is to get the same output power from the motor at 9volts as at 12volts then yes, this will cause more motor heating and may damage the motor over the long haul or shorten its life.

The reason for this is that the motor is less power-efficient (output power divided by input power) at lower supply voltage. With a 12volt supply, you can get a specified output power from the motor with less input power (less current), and thus less motor heating.

For example, see attached graph. The output power is the area of the speed-torque rectangle.

At 12V, the rectangle is tall and skinny, so the motor speed is high and the torque is low. Because the torque is low, so is the current, and therefore so is the I^2R heating*. And because the speed is high, the gearbox is designed to gear down the speed to increase the torque applied to the load to the desired value.

To get the same speed and torque output from the gearbox to the load with a 9V supply voltage to the motor, you need the same output power from the motor**. To get the same output power from the motor the rectangle is short and wide. Because the torque is high, so is the current, and therefore so is the I^2R heating. And because the speed is low, the gearbox must gear up the speed (or gear it down less than with 12V) to increase the speed (and decrease the torque) to the same values applied to the load to the desired value.



* there are also some heating effects due to eddy currents in the magnetic circuit which are speed-dependent, which I have ignored in this analysis

**ignoring gearbox efficiency, which may change with different gear ratio. factoring in changes of gearbox efficiency changes the numbers a bit but does not change the overall conclusion.



~

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

some great info! and thanks for stoping the argument! another question I have is (since I have decided to go the route of a previous roboteer i knew and use 4 lawn mower starters for my main drive and steering.) I will be using 2- 12v batteries which should be sufficient. I was wondering how I would accomplish speed change? the motors should do it quite easily. and then for backing up and quick direction change. how would they function? they will be connected using chains to the slightly bigger wheels. I do want about 8 mph in speed so info?...

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

For motor control (speed and direction), you'll need a speed controller of some form. IFI's Victor speed controllers and the Jaguar speed controllers used in FRC are rather expensive, though; if you went that route, you'd want to borrow some from the nearest FRC team. There are a couple of homebuilt versions out there, but you'll REALLY want to know what you're doing if you go that route. There are R/C speed controllers out there, but you'll need to check the ratings on those before you use them--most R/C controllers are designed for less than 12V (or 24V, if you've hooked up your batteries in series).

BTW, how are your batteries hooked up: in series or in parallel?

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

And, Weird Al/Tony: Google those terms if you're unfamiliar with them before you answer.


Another answer on speed and direction control: Strange as it may sound, considering the discussions about low voltages and their effects upon motors, but changing the speed of a DC Motor is as simple as changing the voltage feeding it.

FRC uses Victor and Jaguar speed controllers. These are so-called "H-Bridge controllers" (google that, too) but I fear they may be a bit out of your desired price range. What they do is take a signal from the computer or something that's controlling the robot, and change it to a variable voltage. They even reverse the voltage polarity so the motor can run backwards. And (most importantly) they can do this while supplying 50 or more Amperes of current - that's a lot.

So, let's take your lawnmower starters. Under a bit of load - say, moving a 150 pound robot up a slight hill, on grass, at 8 MPH (just under 12 feet per second, which is easier to measure) - I'd guess that the motors would draw about 13 Amps each.

If you want really good speed control, then an H Bridge is almost your only option. But if 2 or 3 speeds (also in reverse) can be considered OK - maybe for the first prototype? - then there are cheaper ways.

One example is a "resistor pack". You get some heavy wire (that can handle 15 Amps easily) and make some low value resistors - say 1/2 and 1 Ohms - to drop some voltage. As I mentioned before, lower voltage means slower speeds, so if you used a relay* to put one, the other, or both relays in series with the motor, you'd get different speeds (slower for more Ohms). This is exactly how some older cars changed the speed of the heater blower - the driver moves a switch that puts resistors in series with the blower motor, slowing the fan down. Matter of fact, if you can get to a car junkyard and find a resistor pack, that would make a dandy speed controller piece.

Anyway, the point is, you don't have to spend much if you look carefully and get stuff that can be used for things cheaply. The hardest part of all this is having the experience to recognize when something in your hands can be used to make something else happen.

So, my suggestion to you is to learn a bit more before spending any real money. Put together what you have, get pieces working first (experiment with one DC motor, one battery, and some heavy wire) and start to get that experience in how things work. After playing around for a while, you'll be able to put little things together and make a robot.

*Make sure the relay can handle the Amps! Car junkyards are a great source of DC stuff - relays, motors, switches and more... Oh, and stuff like that is cheap, and it can be cheaper if you tell them you're a kid who is getting this stuff to learn about robotics. Most junkyard owners will share your enthusiasm, and may just let you take stuff for free. (Maybe they'll want to help!)

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

i'm probably going to use parrallel because then I can have one speed controller per motor for each of the four wheels and it will have more movement options and abilitys

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

Series/parallel shouldn't make a difference in terms of how many motors you can control at a time. FRC robots can easily run 4+ motors at a time, and the cRIO, off a single 12V battery, all the time. It does make a difference in terms of voltage supplied and how long it can be supplied.

If you have two 12V batteries in series, you get 24V. If you have those same two 12V batteries in parallel, you get 12V with (as I recall) more runtime.

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

More runtime, higher ampacity, lower output resistance (voltage droops less under load), and higher chance of releasing magic smoke when first plugged in (as the batteries equalize).

Re: Will a 1/3 horse power sump pump motor burn up at half power?
 

I can certainly attest to there being less voltage drop under heavy load.

Our forklift cart runs 2x deepcycle marine batteries (70AH each) and only drops 100-200mV under full load of its drivetrain, moving ~1000lbs.