I think I've got it... but a little help?

[n0clevername]

I've been hovering around various forums for a few weeks now trying to figure out the whole brushless/lipo thing. I think I've got it, but I still have a few unanswered questions and concepts I'd like confirmed (or refuted).

1) I'm a little hazy on the relationship between mah/c level of a lipo, amperage of the ESC and kv of the motor. Is the mah of the pack limited by the amperage of the ESC? It's my understanding that (for example) a 2C 1800mah lipo will discharge fully in 1/2 of an hour. Does this mean the lipo discharges at 2x1800 (3600ma, (3.6amps))? Or does this simply mean it discharges 1800ma in 1/2 hour? Assuming the former is accurate, does that mean a 35a ESC can handle (again, for example) 20C 1500mah (1500*20=30,000ma=30amps) or conversely a 10C 3000mah (3000*10=30,000ma=30amps). With regards to these (fictional) lipos, what would be the difference in runtime/acceleration/rpm? It seems to stand to reason that by doubling the mah and halving the discharge rate you end up with the same current and therefore the same runtime/rpm.

2) How does the kv of the motor equate to the amperage of the ESC? Is a motor's kv rating nothing more than the measure of the motors rpm? It's my understanding that the peak rpm (without load) can be measured by multiplying the C*mah*kv=rpm. Is there an appropriate conversion between kv and BL turns? Without purchasing an ESC/motor combo, how should you match the amps on the esc to the kv of the motor?

3) Charging a lipo is supposedly the most dangerous part of their operation, and it has only been recommended to charge a lipo at 1C, regardless of manufaturers specifications. Does this imply that all lipos should take only one hour to charge? If running a 20C 4000mah pack, it should be charged for 1 hour at 4amps where as a 1C 1800mah is to be charged 1 hour at 1.8amps? I understand it's more important not to charge beyond 4.2v/cell, but is it assumed this should take no more than an hour?

I apologize in advance if any of these are stupid questions, and thank you profusely for your responses. This is my first post here and I hope I've found an appropriate forum. I've been looking at the Hobbywing EZRun 35a 9t for my DF-03 as the price is right for my first brushless system. I've heard good things about these systems and look forward to turning my buggy into a jet! (but hopefully not a cruise missile...)

Thanks again! :wave:

[kufman]

1) The C rating of the lipo tells you how much current it can provide. If it is a 20C 5000mAh pack, it should be able to do 20*5A = 100A. The ESC will not limit the current, but it will allow current to flow at what ever rate the motor wants it. The motor is really the thing that determines how much current the system needs to provide. Unfortunately there is no good way to determine how much current a motor will draw. If the motor draws more than the battery or ESC can handle, you will usually kill either the battery or esc but the motor will be just fine.

2) There really isn't a formula to determine the current that a motor will draw. Most ESC companies have motor recomendations for their ESC's. Good esc's like the RS-Pro and GTB can handle just about any 540 size motor. I believe novak says the GTB is good down to 3.5 turn motors and Tekin says 4.5 without a fan or something like that.

3) Lipos would charge in 1 hour at 1C if they only had a constant current portion of the charge. You can get a battery to about 95% in 1 hour at 1C. The last 5% or so is done in a constant voltage mode where the charger puts out a constant voltage the battery draws what ever current is wants. This portion of the charge will be at a current of less than 1C so it will be slower. Due to this, the overall charge time from empty to full can be over 1 hour.

I don't have any experience with Hobbywing products but a 35A controller sounds low to me.

[n0clevername]

Thanks, Kufman, it's all comming together. I've been running/upgrading brushed systems most of my life, but the whole brushless thing makes me glad I payed attention in high school algebra...

What do you run that 35A sounds low? Like I said, since it's my first brushless setup, I want to go cheap. I'm really just looking to beat out a df-03 with a Trinity Speedgem 19t, too much more seems a little like overkill for me. Do you have any suggestions?

and a little clarification;
Since the motor determines the current, is it safe to use the 20C 5000mAh (100A) pack on a 35A ESC, provided the motor is within the recommended range? Would this affect peak output of the motor, give longer runtime or just fry the ESC?

And as for the batteries I made up earlier ("20C 1500mAh (1500*20=30A) or conversely a 10C 3000mAh (3000*10=30A)") would they be identicle in terms of performance or would the second battery run slower, longer? When mAh remains constant, but C increases, runtime decreases, but max motor output increases? Whereas when C is constant with mAh increasing, the only difference would be runtime?

...my head hurts...:confused:

Thanks again for the help!

[JEAM]

I agree with kufman about a 35A ESC being a bit low.
Imagine the motor has to propel some paddletyres at the beach on a hot summer day and the car gets stuck, I promise you your motor will pull a lot more than 35Amps,

Most numbers provided by manufacturers are measured under ideal circumstances...

[n0clevername]

Thanks, Jeam, but like I said, it's only going in a 1/10 buggy. I'm not even convinced the diffs are sealed well enough to keep out grit, so paddles at the beach are reserved for my modded Savage 25ss (wheelies on pavement, rooster tails on sand!)

It's my understanding that Hobbywing manufactures many of the "higher quality" units which are then rebranded. The setup I'm looking at is listed as 35A constant with 190A burst current. I've also understood many companies list their ESC's at the burst rate, making them appear to handle more than they can.

As mentioned, it's my first BL setup so I'm looking to go faster than my 19t brushed, but I don't need to go freeway speeds. Are there any other recommendations for an inexpensive setup that wont shred my gears?

What's everyone running that 35A is way too low? This (http://www.hobbywing.com/english/ArticleShow.asp?ArticleID=411) is the ESC and This (http://www.hobbywing.com/english/ArticleShow.asp?ArticleID=408) is the motor I'm looking at. They're sold as a combo on ebay, so I assume the motor matches the ESC flawlessly (plus a programming card, bonus!).

[drsmooth]

I have two of the Hobbywing escs & so far, they have performed great. I got one of the Xerun V2 120 amp escs for my Evader 10th scale truck & was so impressed with it, I bought one of the Ezrun V2 25 amp escs for my 16th scale Tamtech, replacing a perfectly good Castle Micro Sidewinder. They're both easy to program at the track with the program box, smooth throttle response from stop to wide open, no cogging what so ever! I run a Novak 13.5 sensored motor in the Evader, and a Reedy 6100 kv sensorless motor in the Tamtech. I'm sold on them!:D

[kufman]

According to their specs (on those websites you listed), you could only use the 13t version of that motor with that controller. I don't know how much more it costs, but you would be better off with this controller

http://www.hobbywing.com/english/ArticleShow.asp?ArticleID=419

Since the motor determines the current, is it safe to use the 20C 5000mAh (100A) pack on a 35A ESC, provided the motor is within the recommended range? Would this affect peak output of the motor, give longer runtime or just fry the ESC?

Yes this is the best way to do it. You always want your battery to be the highest rated piece of the system. Failure to do so can be expensive.

What's everyone running that 35A is way too low? This is the ESC and This is the motor I'm looking at. They're sold as a combo on ebay, so I assume the motor matches the ESC flawlessly (plus a programming card, bonus!).

The next most robust thing you want in your car is the ESC. If the motor gets hot you can get away with it a couple times. If the ESC or battery get really hot they may not give you another chance.

Honestly, 35A is a really low rating even for ROAR spec motors and the like. The difference between road running and running on the grass could be enough to kill this controller even with their recommended motor. This is just my opinion and experience, take from it what you will.

[n0clevername]

DrSmooth, thanks for the input! How often do you run your Tamtech and Evader, and whats the local track like (packed, loose, huge straights?). I'm also curious as to what packs you're running in them.

Kufman, thanks again. That's actually another ESC I've been looking at, and for what I'm doing, it sounds like that might be the route I go. (After rereading the specs on the 35A, I noticed you're right, for off road use, 13t would be recommended.)
What are your personal setups and vehicles? I've found it's good to learn from people who know what they're doing.

[drsmooth]

I run both on asphalt road course tracks. I use a 5000 2s 25c lipo from Kershaw Designs in the Evader, and in the Tamtech i run either a 1300 2s 25c lipo from FMA Direct or a Rhino 1750 2s 25c lipo from Hobby City.:D