Overall power output calculation>>>questions

[Thunderbruiser]

I was wondering how you guys estimate the overall power output of a setup

eg a mm7700 combo w/ maxamps 2s 8000mah pack vs. mm7700/6cell 3700(nimh) or vs the same size lipo with lesss mah

i am trying to better understand this as my lipo truck will be 10xl,18/52,4s8000mah what's the approx power output i'd be looking at?

also the mm7700 is in my buddies rustler lol

[suicideneil]

Power ( watts/wattage ) = volts x amps.

Most retailers & manufacturers post the maximum amp draw that a motor is capabale of, so just take that figure and multiply it by the number of volts you plan to use. Obviously these are peak/maximium figures, and you rarely see them under normal driving (large amp spikes occur under acceleration normally)- your batteries will determine how much power the system can produce in total, so always get the best you can afford/need (higher C ratings and larger Mah capacity in other words, though 80amps continous is usually enough for the majority of applications).

[BrianG]

To determine your average power output:

1. Figure your battery capacity in Amp-minutes. Amp-minutes = Ah X 60.
2. Measure your runtime (in minutes).
3. Divide amp-minutes by runtime. This is your average current draw.
4. Multiply the average current draw by the nominal battery voltage (1.2v/cell for NiMH, 3.7v/cell for lipo, and 3.3v/cell for A123 cells).
5. Your burst power will be roughly 6x the average power.

This is your average power. However, since cars are "peaky", there will be actual spikes far greater than that and valleys smaller than that, but it at least gives you an idea.

So, let's say you have a 4s setup that gets 20 minutes of runtime on 8Ah batteries.
Amp-minutes = 8Ah X 60 = 480Am.
Average current draw is 480Am / 20min, or 24A.
Average power = 14.4v X 24A = 345.6 watts.
Burst power levels would be somewhere around 2,000w (345.6x X 6).

[Thunderbruiser]

oh ok cool thanks man

[sikeston34m]

Originally Posted by BrianG

To determine your average power output:

1. Figure your battery capacity in Amp-minutes. Amp-minutes = Ah X 60.
2. Measure your runtime (in minutes).
3. Divide amp-minutes by runtime. This is your average current draw.
4. Multiply the average current draw by the nominal battery voltage (1.2v/cell for NiMH, 3.7v/cell for lipo, and 3.3v/cell for A123 cells).
5. Your burst power will be roughly 6x the average power.

This is your average power. However, since cars are "peaky", there will be actual spikes far greater than that and valleys smaller than that, but it at least gives you an idea.

So, let's say you have a 4s setup that gets 20 minutes of runtime on 8Ah batteries.
Amp-minutes = 8Ah X 60 = 480Am.
Average current draw is 480Am / 20min, or 24A.
Average power = 14.4v X 24A = 345.6 watts.
Burst power levels would be somewhere around 2,000w (345.6x X 6).

Very Nice Brian. I like this.

Seems like there is always going to be alot of variables that are either hard to put a value on, or ones that are hard to figure into the equation.

Wouldn't efficiency have a big impact on this?

[BrianG]

The calculation outlined above is actually power drawn from the battery. Power output getting to the ground is indeed dependent also on efficiency (motor, ESC, and losses in the wire). But unless you can measure mechanical power with a dyno while measuring input power from the battery, and graphing the differences, it's all but impossible to calculate.

[aqwut]

Yeah, Brian is right... it all depends on batteries, motors & ESC.... just like a chain, "only strong as it's weakest Link"..

but since I have a few A/C Amp Testers, I tie my truck to a pole and give her full throttle on cement... I measure the peak Amp it draws and the lowest voltage that it drops to... but in the end.. it's always Volts * Amps = Watts