Home-brew battery measuring station - Page 8 - RC-Monster Forums

Metallover · 11:33 AM

I was thinking the load on that video would be good to use.. That's why I posted it.

sikeston34m · 11:37 AM

Quote:

Originally Posted by Metallover

I was thinking the load on that video would be good to use.. That's why I posted it. :shock:

That would be excellant to use, but they are rather expensive.

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

Metallover · 01.20.2010, 04:38 PM

Would this work? What is it?

http://www.youtube.com/watch?v=0YDy6...eature=related

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?

BrianG · 01.20.2010, 04:59 PM

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.

01.21.2010, 07:16 PM

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?

BrianG · 01.21.2010, 07:24 PM

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.

Metallover · 01.23.2010, 09:01 PM

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

BrianG · 01.23.2010, 11:39 PM

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.

sikeston34m · 01.24.2010, 05:54 AM

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.

J57ltr · 03:52 PM

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

BrianG · 01.24.2010, 08:46 PM

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.

whitrzac · 01.25.2010, 09:14 AM

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

scarletboa · 01.27.2010, 01:50 AM

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