PLEEEAAASE help! battery and ESC needed

[remondo]

I have all the components for my latest build except ESC and battery pack, could someone please help?

I will be running two Graupner 600-Eco motors, specs:
Nominal voltage 7,2 V Operating voltage range 6 ... 9,6 V, No-load current drain 1 A Current drain at max. efficiency 7,5 A, Current drain when stalled 50 A

I want to run these from one ESC, would an Mtroniks VIPER ECO 20 turn ESC do the job? Having looked at the current draw, the ESC has a max draw of 20A, so the two motors combined will draw 15A, am i correct?

Finally can anybody recommend a battery pack, with good run times which will work on this setup?

Thanks - Remi

[green-boat]

I doubt that you will be running your motors at peak effiency, 7.5 amps. The ESC you selected will do the job with no problem. As for run time/ batteries, use the biggest battery that you can get or fit in. Run time is determined by the battery capacity and how much you motors are pulling. If your motors are pulling 2 amps and your battery has a 4 amp capacity then you should be able to run for 2 hours.

[remondo]

thanks green-boat, now I can get everything ready to be fitted

[Tachikaze]

I would have to disagree with the idea that that ESC will do the trick. The specs you provide with those motors and that ESC suggest to me that you may be running very close to the upper limit of the amp max of your ESC. I would recommend that you install a 15amp fuse between your ESC and your motors. If you blow this fuse, then your are getting too close to the rate for your ESC.

[Shaun Hendricks]

I agree with Tachikaze. It's entirely possible to stall a pair of props (underwater winch, etc.) If I have a pair of motors that have 50A stall draw, I'd design for that. I'd have a 100A ESC on that puppy in a heart beat. After all, if I hit full throttle and the boat don't move, I know I have a problem and back off. A 100A ESC could handle that pulse and survive. A 20A one would fry in a heartbeat.

I'm a cautious sort by nature so I tend to design worst case. Since 100A ESC's can be had for about $10 more than a 50A one.... I'd go that route. Just how I do things though, YMMV. :D

[remondo]

theyre like £200, i don't have that kind of money to spend on something like that.

[Shaun Hendricks]

Well, my conversions between Pounds Sterling and the 'ol Greenback are a little lacking but I don't think this is the same as you were quoting... :D

100A peak pulse, 50A normal ESC (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170026548593)

[LtDoc]

I'm afraid that using one ESC in a situation where the maximum possible current draw will exceed the ESC's rating will result in lots of smoke. If each motor stalls at 50A, then the ESC should be able to handle 100A comfortably. NOT the same as it's 'peak' current draw, which will only last for fractions of a second. If the continuous draw rating is 100A, no problems (hopefully). If you end up with a less capable than required ESC then the fuse is certainly a better thing to have to replace than the ESC - lol. Expect to replace the fuse!
- 'Doc

[Tachikaze]

I would have to disagree about the stall average. If the motor stalls at 50 Amps, that means that the engine fails to run at a 50 Amp draw, that does not neccessarily mean that the motor may not exceed that draw for a momement before it burns up.
I always install a fuse from my battery to my power direction nodule that is 10 amps below my ESC. I would much rather burn up a few fuses than a ESC.

[Tachikaze]

Remi,
I think that you are trying to press the limits of your ESC with what you want to hook up there. Do you have access to E Bay?
Check out these items: #140026649233
#120029351567
If you can give me a price limit I can give you a little more information. I use that MTronik unit you are looking at in my Two Japanese Minekaze Class combat destroyer. I am running two 14V Pittman engines on 12V for that. That ESC works well under those conditions, but I would not consider using it in the situation you are mentioning.

[Shaun Hendricks]

You can fuse for safeties sake, I prefer thermal self-resetting breakers. They aren't all that espensive and give them a minute and they are back to powering the circuit so you might be able to limp your boat home.

"Peak" is not measured in fractions of a second. Most ESC's can handle their peak draw until overheat where the PN junctions begin to break down and let out the magic smoke. You'll find that most ESC's will handle peak draw for 10-30 seconds, depending on the quality of the manufacture of the FET's. Assuming the worst, even 5 seconds is enough to see that the motors at FULL current are not powering the boat.

There is a balance that you need to achive. A full 100A draw ESC is too much for a pair of motors normally operating at 15A. That's a waste of resources. In all likelyhood, the motor winding will burn out before the ESC does at 50A per motor, and in all likelyhood, only one motor will stall out, not the other, so you aren't really going to be dumping more than 50A through the ESC- EVER. You'll back off the throttle and say 'that's funny, the boat's moving strangely' and bring her gingerly home to find that fishing line wrapped fully around one propeller. One motor will be a bit hot, but still functional. Even if both decide to jam up, you'll still let off the throttle and never dump 100A through the ESC for long enough to fry it. If you did, you'd have a good argument to return it for a new one if it failed after 5 seconds.

So, find the reasonable balance. That 1 in 100,000 chance is not worth a true 100A ESC. It is worth a $30 (US) 100A peak ESC in my opinion. If you really wanted to be safe, buy 2 ESC's at $60 (US) total and run each motor independently off a servo Y harness and a power Y harness. Personally, I don't think anything but a lead acid battery or some VERY high power battery packs could even provide 100A total! :D

[Shaun Hendricks]

Tachi-

That first one you list is identical to the one I was showing him. The seller is listing it differently but if you look at the label, it's the same one for $7 more.

The LRP is a car ESC, it is not waterproof. It would need a waterproof box and some kind of watercooling through those fins as air couldn't get to them in a waterproof box. I'd HALVE the AMP rating if going to put it in a sealed, waterproof box with no watercooling.

It's clear the "himark" ESC leaves some room for interpretation but should be able to handle Remi's needs. I have the same ESC sitting on my desk, the 'instructions' are something of a joke. I have stall tested it with my Super Cobalt 400 (which is almost impossible to stall) and it handled a 70A spike with no issues, didn't even get overly warm. I have a feeling the FET sinks are mounted to that aluminum case so it has some good air cooling from that waterproof case.

[Tachikaze]

I have been using the Duratrak Blast and the Tekin Rebel in my combat ships. We carry " water tight boxes" in the ships. Both of these have been running well. I picked up my Tekin Rebels on Ebay, one of them has been in a ship that has sunk three times. The " Water tight boxes " are not, maybe a little water resistant, I would say deeper than 3ft and they get wet. Just run some rubbing alcohol on them, set them in the sun for about 30 minutes and you are back in battle.
Running the Pittman motors and like motors, I just try to stay above the 50 Amp mark. We have one unit, I am not sure where it comes from, it has a purple casing on it, ( not Futaba), we have had a lot of problems with it running two motors. It has an internal thermal cut down, and it will shut down pretty quick under combat situations.

[Shaun Hendricks]

Tachikaze, do any of your ships pull any significant amps in combat?

I would think that they are doing scale speeds, turning gearboxes, etc. Typcially these kinds of applications aren't high current on the drive motors, but I don't know, I've never built anything of the sort. I know my car ESC's (duratrax) get VERY hot after running a pack through them and that's WITH air running over the cooling fins. In an airtight box, I can only imagine how hot they'd get, thus my safety suggestion for not using more than half the rating in actual use.

LRP does make a waterproof (resistant) ESC for cars (boats?) that is used in the monster, stadium trucks and buggies quite often.

[remondo]

Wow what a mass of information to take in, im still drunk from last night! I have been thinking of using two ESC's if the price is right, would I get away with using two mtroniks 25A ESC's?

[remondo]

looking at that one on ebay, I think I shall buy it! it looks great thanks for your help

[remondo]

Ok bought it, thanks for your help guys, later on I think I will buy another so I have an ESC for each motor

[Tachikaze]

No, we try to shoot for as low Amp draw as possible. We will often preform prolonged battles and you will need your ship in the water for an hour or so. I estimate that a no load on the two engines in most of my combat ships pulls less than 0.9 amps.

[Shaun Hendricks]

That's about what I figured, keep the drain down. There is certainly nothing wrong with overbuilding/overengineering the system if you have the bucks. If you don't, then it's all about finding the best compromise of money vs. performance vs. safety you can... therein lies the rub... :D

I'd really love to see one of these RC ship combats some time. They look like a lot of fun!

[E-Challenged]

You need to run the motors in the water and check current draw at wide open throttle. If these are Speed 600 motors they might pull a total of some 30 amps together at full throttle depending on the size and pitch of your props and gear ratios, if geared motors. Your speed control should be rated for at least 30 amps continuous. You can increase or reduce amp draw by changing the size and pitch of props and/or gear ratio. Running at 1/2 throttle does not reduce the heat that must be dissipated by the speed control and it could be damaged if it is not designed to handle the total average current flowing through the speed control. Using a fuse in the battery to speed control lead is a good idea to protect your speed control and motor if your prop gets fouled by vegetation or fishing line. An arming switch easily accessible from the outside of boat in one of the motor wires is good to disarm the motor to prevent accidental starts until you are ready to run boat.