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Have the MMM refurbs bought this project any closer to fruition:lol:
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A little update:
Well, due to finances, it's gonna be a while before I get this whole project done (MMM and various odds & ends). But, I decided to do a little playing around with what I do have. I made a little resistor network consisting of four 3 ohm resistors mounted on a small heatsink, which I can configure in all sorts of ways to get resistances from 0.75 ohms to 12 ohms. This was intended for future BEC tests and other small projects (power supply testing, etc). I also have an extra XL5 brushed ESC, which worked perfectly for the current levels I wanted to play with. So, I hooked up the servo tester, eagletree, and resistor network (configured for a 0.75 ohm load) to a Maxamps 2s2p 8Ah pack. So, with everything hooked up, I set the servo tester for automatic sweep. I can very the speed of the sweep, as you will see in the graph. Anyway, this is just an idea of what this project will be able to provide: http://scriptasylum.com/forumpics/ba...att_test_1.jpg As you can see, I started with a medium speed sweep, then went to slowest, then fastest, then a little playing around in manual mode. Of course, the real test won't look like this; I just thought you'd like to see the various modes available... Here is a link to the raw eagletree file if you feel like loading it in your software to take a better look: Log File |
Man, that's awesome. Is all you need the MMM? And assembling the resistors/heatsink?
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Pretty much. A friend of mine gave me a boatload of CPU heatsinks (30). I plan on getting some aluminum plate, tapping the holes to mount the resistors, and a bunch of thermal epoxy to secure the heatsinks to the underside of the plate. So, a rundown of things I need still:
- MMM - Aluminum plates. 1.8" thick X 4.75" wide X ~36" long - Thermal epoxy (probably at least a couple sets of tubes) - 120 3mmX5mm screws - A couple 3mm tap and drill sets. I imagine I'll break at least one set after drilling and tapping 120 holes. |
So you don't need the heatsink material or a shroud for it? Last I asked is if you wanted 2 fans on one end or 1 or 2 fans on each end for the shroud.
Jeff |
Sorry, I forgot about that Jeff. :oops:
I suppose I could use the HS material instead, it was just that my friend heard about my project and gave me all kinds of CPU heatsinks. Using yours would save a lot of epoxying! As far as the shroud goes, probably not (but thanks). Like I said in a previous post, I will likely mount the heatsink fins-side-down in a tunnel type of arragement, and mount the fans so they are an intake and exhaust setup. |
That's exactly how it's set up, for some reason the fins don't come out as dashed (hidden) lines. The resistors would mount on top fins down the shroud would be sitting on the table with fans where you want them. I can barely see the pic on my screen here at work, but the one at home looks OK. Access to the resistors would be on the top just like in the pic I posted earlier flat side up.
Jeff |
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Here is a side view drawing. The Blue are the resistors. I used 2 fans on each end you didn't have a CFM listed for the fans but I see a range from 12 to 60 cfm. I can do it with just 1 fan on each end or as many as you want. Don't know if you know but fans in series increase pressure, while fans in parallel increase flow (not electrically, but the movment of air).
Jeff |
Any updates lately?
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Not yet. Sorry. Funds that would be used for this project have been going elsewhere lately.
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Hi Brian,
Is this project still on hold? Last night, I put together an identical setup. Using a MM Pro, an Eagletree, and a Servo Tester. For a load, I rewired a Ceramic Heater so the heating element was fired from the MM Pro. The heater fan was left wired to regular house current. Well, the ceramic heater core does not work as a load. It's total resistence is too low it appears. I couldn't get much of an amp load to register at all. What other choices for a load are there? How about an electro magnet? Any ideas on how big it would have to be or the Ga. wire it would require? Just a thought, as larger versions are rather power hungry. |
Actually, this project is probably permanently on hold. I don't think it will have enough use to justify its cost to assemble - at least for what I originally planned. Who knows, I may revisit this later on, but let's just say it's not on my immediate "to-do" list.
That doesn't surprise me really. A 1500w heating element running on 120v means the core has a resistance of 9.6 ohms. A 4s battery will only draw 1.54A with that resistance. You could go with some dummy loads. Depending on how much current you want to draw, you have to arrange a bunch in parallel and make sure the power rating for each one can handle the heat. And the power dissipated will obviously change depending on what voltage you are testing. If you are looking to only test ~10A on 2s, that's easy enough since the power is only ~75w. Eight 4 ohm 20w resistors in parallel to get a 0.5 ohm load will get you there, and then you use the ESC to regulate that amount. Don't forget, the ESC will only reduce the current (via voltage switching) to any given load, so make up a resistor bank that will have the lowest resistance you will ever want to test. Wiring again depends on test current. For ~10A, 12-16GA is adequate. As you go up, gauge has to go up as well. This test system becomes rather complex as you start getting into the higher test currents. If you want to draw 100A @ 2s, you really need to make sure the resistors can handle the heat, there is minimal wire/contact resistance, and shedding the heat can become a problem. If you want, I still have a bunch of 3 ohm 50w resistors that I'm not using. I got over 200 of them via ebay, but it was cheaper buying this many even though I won't need near that amount rather than getting the exact amount I needed. Their 50w rating is only good if properly heatsinked. Some CPU heatsinks can work if there is a fan blowing on the fins. |
How about one or two of these for a load? 12vdc ready too, or would it be too small of load?
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Hmmm...........Thanks for the suggestion. I would like to construct a unit that would do the job in a small package though. |
Small and high power don't go well together. You need surface area to shed the heat produced and a fan to circulate the air. So, you want around 1500w eh?
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Perfect this setup, then step up to bigger if needed. I appreciate your input Brian. Thanks. :yes: |
Woah! 2500w?? You trying to heat your house? :lol: Seriously, you are gonna have a heck of a time trying to shed that kind of heat! Be prepared for a large setup and plenty of airflow. And I would do this kind of testing somewhere where an exploding lipo won't do any harm, because at those current levels, bad things can happen.
When I was in the design stage, I decided to stick to lower cell counts, up to 3s max, to keep heat down. Also keep in mind that if you design it for 100A @ 6s, then hooking a 3s pack will only draw 50A. Likewise, if a 3s pack will draw 100A on your load, then running 6s on that same load will draw 200A. So, you might need some way to create load "modules" so you can adjust the resistance as needed based on the cell count under test. 100A @ 6s will require a ~0.2 ohm load. At that current level, wire and contact resistance can make a big difference. If your wire/connectors introduce say, 0.05 ohms of resistance, that calculated 100A draw will be more like ~80A because of the added resistance. One of the roadblocks I had was coming up with an accurate way to draw a specific current. Since I was testing to confirm battery discharge ratings, I wanted to be able to draw an exact figure. But since battery voltage will fluctuate over the course of the cycle and the amount of load placed on them, I would have to keep adjusting the ESC output to make sure the current stays constant. Easy enough if testing constant discharge, but when testing bursts, the fluctuating voltage will make it darn near impossible to manually adjust it. This would need a form of feedback to tweak the ESC as needed. It just got too complicated to be called "home brew". My point is: if you are doing this for your own benefit, then this issue is no big deal as long as you are "close enough" to satisfy you. |
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A hit list of pwm settings for different voltage levels should give a ball park figure of where to start. On the issue of maintaining a constant amp draw: Sounds like a "home brew" controller is in order for that to happen. Perhaps a separate current measuring circuit that would automatically adjust pwm input to the ESC. That's gonna take some "doing". LOL One hurdle at a time though. First goal is to make it work. Then we'll iron the kinks out. |
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When you start testing bursts, it gets a little tricky. You know that the PWM signal for an ESC is 1ms (rev/brake) - 1.5ms (neutral) - 2ms (wot). So, you already only have 0.5ms to play with when going from neutral to wot (rev/brake is not used). If you "cap" the ESC throttle at say 50%, you then only have 0.25ms to play with. Now, you are really reducing the resolution of the signal so when trying to make tweaking adjustments, you may not have enough resolution to adjust the output exactly where you want. The use of an aircraft ESC may help with this since it doesn't have reverse and so it has the full 1ms of throttle range to work with. Personally, it would be easier to make load modules so that each one is X ohms, and you just parallel what you need to get into the ballpark. I hope that made sense. |
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Sounds the resolution on such a pwm controller will have to be very precise. I see your point. I think the initial goal will be to measure and graph a constant amp load. You're looking at imitating the current demands of a Monster Truck, aren't you? I've noticed with the Eagletree results, average current draw is really very low, But those spikes! My but those spikes are many times more than the average current draw. I would like be able to adjust the amp draw to a preset level. Just like some Lipo manufacturers do. Here's how the voltage per cell that's held under a 20 amp draw, a 50 amp draw and 100 amp draw for the life of the discharge cycle. Then, here's how many mah's were used during the discharge cycle. In noting differences in how Lipo cells are rated, it's not only the "C" rating number they love to "toy" with, but it's also the actual mah capacity of the cell. Both the "C" rating and the Mah rating, drastically affect the performance of the cell. One thing about the Eagletree, it's really easy to hook up a temp sensor for ambient, then another temp sensor for pack temperature. All good information to record when it comes to testing. :yes: We need a good system to use as a "measuring stick", then we can let the tests roll! :yes: |
OK, I've been searching high and low for the ideal DC Load for this, and it hit me.
A large Electrolysis Cell. One that breaks the moleculiar bond on the Hydrogen and Oxygen Atoms that form Water. I remember about a year ago when the Hydroxy Craze was at it's peak, I built and experimented with a 4S2P Plate Arrangement. The thing was REALLY amp hungry too. It was being regulated at around 10 amps on 12 volts, but it would be no problem at all to push it on up. Amp draw regulation was controlled by how much Sodium Hydroxide was added to the water. Sodium Hydroxide (Lye) was added to increase the conductivity of the water thus increasing Hydrogen and Oxygen production. The Amp Draw increased with the conductivity of the water. So we have a "Load". I know what you're thinking, but there is a safe way to do this. Good construction of the cell itself, along with a secondary bubbler that vents to the outside, plus a spark arrestor for safety's sake. :yes: To "Force" a cell like this creates alot of heat. So much in fact, it will boil the water if continued long enough. The test window would be short enough to avoid this. Afterall, how long will a 5000mah lipo last at 100 amp continuous drain? 5 minutes? Thoughts Brian? |
lol, sounds like it should work. A little complex for me, so I think I'll stick to plain old resistors. :smile:
And a 5ah pack will theoretically run 3 minutes at 100A. Of course, usable capacity will be less at that kind of continuous draw, so figure ~2.5 minutes. |
Here's an idea, from doctorbass,
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OK, on with other Load options.
Here's a rather interesting inexpensive idea. http://www.neon-john.com/EV/Battery_...ester_home.htm It's a 500amp load tester that uses a carbon pile for resistence. I would think this could be modified with additional heat sinking to handle the load. Afterall, 12 volts at 500 amps is 6000 watts. :yes: |
I was thinking the load on that video would be good to use.. That's why I posted it.
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I don't plan on spending $500 plus on a capacitor bank. Can you find me one of those for 50 or 60 bucks? lol |
Would this work? What is it?
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/QvmrH-CFfqc&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/QvmrH-CFfqc&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object> http://www.youtube.com/watch?v=0YDy6...eature=related <object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/0YDy6Y1Ucyc&hl=en_US&fs=1&"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/0YDy6Y1Ucyc&hl=en_US&fs=1&" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object> http://www.youtube.com/watch?v=0YDy6...eature=related Looks to me he is drawing a steady 350a in the second video... or maybe he can do this only because of the low voltage of 1 cell? |
He is relying on the fact that the resistance in the metal will change as it gets longer. That, and the shunt resistor he is using to measure the current are the only things limiting the current. And I would hate to feel how hot that bar and resistor are getting! 300A @ 1.9v (in first vid) is 570w, even with one LiFe cell, and that heat has to be dissipated somewhere along that relatively low surface area. Even if you wanted to do this, it would be limited to a single cell and heat would become a problem after even aseveral seconds.
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Brian, is there any reason you couldn't take 1 or 2 of these http://www.sportsimportsltd.com/12-v...ry-heater.html hooked directly up through a Datalogger? Wouldn't you think this could run on between a 2-4s?
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Yeah, on 4s, it would draw the spec'd 540w (~45A), but on 2s, it'll only draw ~270w (23A). The heating element is basically a resistor and the resistance is pretty linear.
No matter what load is being used, there is going to be a lot of heat, especially at anything over 2s lipo. It's just a matter of finding a load that can take the current, voltage, and resulting heat. |
I don't know if it is of any use but I found a thread on the above videos.
http://endless-sphere.com/forums/vie...p?f=14&t=15093 |
Yeah, he has some design challenges when trying to test at such low voltages. Might need to pump up the gate drive a bit higher to get a cleaner and "squarer" signal from the FETs. This is one reason why I thought about testing 2s or 3s packs only and using an ESC in brushed mode; the ESC is already done and can handle lots of current (nothing on the order this guy is testing though). The voltage is low enough to keep load power dissipation somewhat reasonable, yet high enough voltage to eliminate the problems this guy is having. Besides, testing very high currents at such low voltages make it imperative that the wire/contact resistance is as close to 0 as possible. A difference of even 0.001 ohms can make a substantial difference.
I also think it is important to test performance in a pack formation since that is how we use cells. It lumps contact and wire resistance together to get a system performance baseline. This guy seems to be more concerned about if the cell itself can handle the abuse without causing collateral damage. I haven't totally written off this project yet. For instance, I want to ask Patrick on his thoughts about paralleling two MMM ESCs. That will obviously increase the current capability, lower ESC resistance, and reduce the effect of copper losses. Since no back-EMF signal is present/used in brushed mode, I don't see why this won't work as long as both ESCs output the exact same signal at the same time. Ideally, it would be nice if there was a "slave port" on these ESCs so you can control the power section of each with the "brains" of the master ESC. Obviously, doing this increases the cost of the project to the point where it is no longer a relatively cheap "home brew" setup. |
How useful would a resistor bank be, that is composed of 3 relatively large resistors, each being 56 ohms?
Just wondering how many amps this would be good for, with proper cooling. |
The amperage is based on the resistance of the resistor bank and the voltage applied to it. How much the resistors can handle is based on the wattage of the resistor banks. With your example of 3 56 ohm resistors in parallel would be 18.667 ohms. at 8.4V you will have .45 amp which would be 3.78 watts. At the end of the discharge at say 6 volts you would have .32 amp and 1.92 watts. So you can see the problem with using a fixed resistance. What Brian has in mind is being able to use the MMM for and a servo tester so that he can bump up the signal and as the current starts to drop he can pump it up again to maintain the current. this would require that he have a load that is lower than the required so that at the end of the discharge he can still pull the same current.
Even at 25.2V with your 18.667 ohm load would only pull 1.35 amps, 34 watts at the start and at the end it would be pulling .96 amp, 17.35 watts. You really need a load that is much lower. A .5 ohm load is only 16.8 amps at 8.4V or 141.12 Watts, .1 ohms is 84 amps at the same voltage and 705.6 watts. At 25.2V .5 ohms is 50.4 amps= 1,270.08 watts and .1 ohms would be 252 amps which is 6,350.4 watts Some formulas you can use to determine what you can do with what you have. V= I X R, or I= V/R for figuring resistance of parallel resistors you can use an online calculator or the formula. Here is a link. http://www.1728.com/resistrs.htm Brian, I would like to know what Patrick says about brushed mode as well. Since they use regen for braking in brushless mode it would seem that doing the same for brakes in brush mode as well. I know my old brushed speed control has 2 fets across the motor terminals and just shorts the windings. I don't think you could do the same on the Castle speed controls. I was wondering if they did look at emf as a way of adjusting the braking force to bring it to a stop. Jeff |
Sikes: Like J57ltr said, 3 x 56 ohms is not nearly low enough. My design called for between 30 x 3 ohm resistors in parallel (0.1 ohm total), and 60 x 3 ohm resistors in parallel (0.05 ohm total). Each being rated for 50w and mounted on a heatsink, would be enough to dissipate the power.
J57ltr: Since the MMM is a BL ESC, there are no braking FETs. I imagine braking in brushed mode is done the same basic way as it is in BL mode; use PWM to short the phases, and in between the PWM pulses, put the inductive energy back into the battery. Although, you can disable the brakes in the ESC when used in this way, so it wouldn't matter either way. |
I was just thinking about this last night at the track.
rather than making your own, use 4ish of these and an egaletree in live mode. it seems too simple to work though:neutral: |
what if you were to hook up a large brushed motor to a MMM and run it in some thick silicone oil? the oil would create a ton of drag, which will raise the amps and the oil will act as a very good heat dissipater. to add more amps, just add more load to the motor with either thicker oil or maybe a boat propeller. lol
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