How to calculate motor current draw? - RC-Monster Forums

suicideneil · **How to calculate motor current draw? -** 04.13.2007, 02:50 PM

So, I was looking at various new batteries to replace my exploded one (the other is for sale on feebay as I need a matching pair), and I noticed that alot of packs had a discharge rate of 30-50mah. This I was dimmly aware of, but my question is - How do I find out how much current my motor is trying to draw from the batteries?

I personally run the HvMaxx 6.5 on 14.4v, the motor has a rating of 375watts.

BrianG · 04.13.2007, 02:54 PM

Motor draw has many factors (vehicle weight, gearing, terrain, drive style, battery voltage, number of motor turns, etc). The best and most accurate way is to measure worst case on something like an Eagletree device.

suicideneil · 02:59 PM

Yeah, I kinda figured that would effect it. Whats an Eagletree device exactly/ what does it do?
EDIT: google only yielded pictures of birds in trees......

BrianG · 04.13.2007, 03:00 PM

You plug it in between your batteries and the ESC and it logs all kinds of stuff. Then, you hook it to your PC and can generate graphs which can tell you a LOT. Here is a link: http://www.eagletreesystems.com/MicroPower/micro.htm

squeeforever · 04:35 PM

All you really need to do is make a guess, preferably higher than what you think it will actually draw, and then x1.5~x2.0. That should make give you plenty of headroom. Say a 10XL for example. I would guess a 45-50 (keep in mind, it probably would draw more like 30 amps, but like I said, guess a little more so you have some headroom) amp draw on average if your running hard or racing, so I would say 75 amps continuous should be plenty for that motor. But thats just me...

BrianG · 04.13.2007, 05:10 PM

Another thing you could do is time your run on a given set of fully charged batteries. Re-charge the batts and see how much mAh is put back in. Then, calculate the average amp draw based on that mAh and the time you ran. I'd multiply by ~10 to get the peak draw since that seems to be a decent approximation from the various graphs I've seen. What do you think Squee?

suicideneil · 04.13.2007, 05:47 PM

Hmm, that eagletree thingy is rather nifty, and not that expensive either. On the otherhand it seems a little too hardcore for my needs. I'll just get the hightest rated NIMh I can find for a sensible price- 48mah max draw seems to be it, but I will keep looking- I sent the faulty pack off yesterday so hopefully I will hear something about that in the next few days. Cheers for the help.

squeeforever · 04.13.2007, 07:43 PM

Quote:

Originally Posted by BrianG

Another thing you could do is time your run on a given set of fully charged batteries. Re-charge the batts and see how much mAh is put back in. Then, calculate the average amp draw based on that mAh and the time you ran. I'd multiply by ~10 to get the peak draw since that seems to be a decent approximation from the various graphs I've seen. What do you think Squee?

Wow...Brian, the electronics expert is asking my advise on something like this?!? Kinda shocking really...I'm no expert, but I think what you suggested will work as well...

BrianG · 04.13.2007, 08:17 PM

LOL. I'm sure you have more experience in the hobby than I. :)

squeeforever · 04.13.2007, 08:48 PM

Well, I dunno about that...:o I would say your one of the most skilled members around here...

glassdoctor · 04.13.2007, 09:22 PM

average amp draw won't tell you much.... it will only be around 20amps... give or take.

You need to get 'good" cells... but jsut as important, you need to get a pack with good construction. No tabbed stick packs.... get hand soldered packs, either stick or side by side. There are many good places to get prebuilt packs.

The Fusion packs are really good.... and I think Promatch will build them for you as weel as Ballistic, etc.... Maxamps does too, and they claim their house brand 4400s are as good as the IBs and GPs