Amp Draw? - RC-Monster Forums

mriccucci · **Amp Draw? -** 04.22.2008, 09:15 AM

Ok so im trying to figure out the max amp draw on my system. Current setup is a mamba max, feiago 7xl, 8ight center diff with muggy diffs, and im using some protek 4000mah batteries. I have an astro flight watt meter but im unsure how to take a reading that will show me how much is drawing as im running the truck? Suggestions?

BrianG · 04.22.2008, 09:37 AM

Without a device that takes frequent current/voltage measurements over time, it's hard to tell what the draw is. One way to approximate this is to use your runtime and battery capacity. You can try this or:

1: Take your battery capacity in Ah and multiply by 60 to get amp-minutes.

2: Divide the result in #1 by your runtime in minutes. This is the average current draw.

3: Since motors can pull 6-8x the average power in bursts, multiply the average figure in #2 by 6 and/or 8.

bdebde · 04.22.2008, 09:41 AM

Take the max reading from the watt meter and divide by your voltage.

lutach · 04.22.2008, 10:27 AM

I saw over 120A spikes when I was using my 7XL with 4S lipos. I was using a MGM 16016 controller.

mriccucci · 04.22.2008, 11:36 AM

I did a dry run with the truck in the air it it pulled like 20a, i assume that the stress of the wheels on the ground will add the amp draw to the system, my biggest concern is going over the limit of the mamba.

joeling · 04.22.2008, 11:46 AM

Hi,

High amp draw occurs while the vehicle is accelerating.

Regards,
Joe Ling

BrianG · 04.22.2008, 11:55 AM

Running the truck in the air only applies drive train drag to the motor - relatively unloaded. The 20A current you see is probably just what the motor is pulling to get the tires (which are just acting kinda like flywheels) up to speed.

The 22N03S FETs in the older versions of the MM are rated for 50A each @ 77*F, there are 6 in parallel at any one time giving you a 300A rating. Even derating to 50% of that value for ~170*F is still 150A. Of course, there is more to it than just the current rating (like power dissipation), but I would be more concerned about the PCB at that point. The Quark 125B uses the same FETs (and the same number of them), and that is rated for 125A. So, don't be too concerned with keeping it under 100A. Besides, those high current peaks are for a very short time; the average current is much much lower.

aqwut · 04.22.2008, 01:30 PM

I test my amp draw by tying the truck to a pole on cement and giving her full thrott a few times..... then I take the highest reading... either in watts or amps.. I use an AC amp meter, don't like the idea of skinny wires on the watt meter..

Lutach.. I've had over 100A spikes with my 7XL also...

Joelying.. And you are definatly correct, all of my trucks gives the highest amp rating while accelerating.. when it reaches full throttle, the amp draw drops back down...

What's_nitro? · 02:04 PM

Hey aqwut - you could always put larger wires on your wattmeter!

Front - Back - Side

Those are 10 AWG wires. It takes patience to desolder all of the 0.1 inch-spaced jumpers but it works!

aqwut · 04.22.2008, 07:36 PM

Quote:

Originally Posted by What's_nitro?

Hey aqwut - you could always put larger wires on your wattmeter!

Front - Back - Side

Those are 10 AWG wires. It takes patience to desolder all of the 0.1 inch-spaced jumpers but it works!

Good idea dude, I was thinking of doing so... but it's not able to read over 100 Amps....

I lik the idead of it with no bulky plastic cover... thanks..
it's going to be easy to desolder... I don't use soldering tips to solder.. I use wood carving tip for soldering.. fast and easy..

MTBikerTim · 04.22.2008, 08:15 PM

Tested out my 150A eagle tree last night. Massive spikes of 39 amps

. Stock xl5 with a 1800mah nicad. Still interesting though with the 289 watt peaks I can't wait to test a few more systems and different batteries.

aqwut · 04.22.2008, 08:33 PM

HOLY SH*T!! a whole 39 Amps.... and Nicads.. that's just Crazy.... LOL

What's_nitro? · 04.22.2008, 09:53 PM

Quote:

Originally Posted by aqwut

Good idea dude, I was thinking of doing so... but it's not able to read over 100 Amps....

I lik the idead of it with no bulky plastic cover... thanks..
it's going to be easy to desolder... I don't use soldering tips to solder.. I use wood carving tip for soldering.. fast and easy..

Yeah, I wish the max current was limited by the wires.....

I just didn't think 14 AWG was sufficient. Everything else I use has at least 12 AWG on it, so 10 AWG ensures that the meter isn't the weakest link.