Home-brew battery measuring station

[BrianG]

Originally Posted by J57ltr

Brian I have a 1F cap and some heatsink material I can donate to the cause. The second pic has 50W resistors on it so it should be perfect size for your setup. Also if you need I can make a fan shroud for it as well.

Jeff

Wow, I must have missed this post somehow! Didn't mean to ignore you.

Anyway; yeah, that would be awesome! Those heatsinks look like the PERFECT size. I intended to make the resistors lined up like you have in the pic, just a 46 more.

I got six 80mm fans for this project ($2 each at allelectronics.com), but was thinking I might put the fins side down, build a little tunnel, and put two fans (one at each end of the sink) and cool that way. This would allow an easy way to replace resistors if I blow some. Plus, it would look nice.

I also finished the power supply for the servo tester. I got a 12.6v/1A transformer from the same place. The supply actually has two outputs; one regulated for 8vDC for the servo tester (needs 6v-9v), and the other output is unregulated and puts out around 14vDC at ~750mA for the fans; perfect!

I was thinking a little though; To make this test repeatable, I'm gonna have some problems:

1) Constant current. The current through the resistors depends on voltage. I can program the ESC to output X amount of voltage, but as the battery depletes, the voltage will fall, and so will the current. So, testing a certain discharge rate (constant current) will be tough unless I constantly tweak the servo tester accordingly.

2) Pulse discharge profile. The servo tester I got has an automatic function which will sweep from min throttle to max throttle repeatedly. I can vary the speed of the sweep from slow to fast, but the profile is not "peaky" like we see in our cars. It starts at min throttle, ramps linearly up to max, sits there for a fraction of a second, ramps down linearly to the min, sits there for a fraction of a second, and the process repeats.

I wonder if I could see if Patrick will sell me one of those new ICE controllers he is using along with the custom software he is using?

[J57ltr]

That's exactly how I have done my heatsinks and it works great I just used 1 at the end that was a 4 5/8" square fan thermister controlled (ended up just shorting it out because it just wouldn't run fast enough for the application). Mainly a fan at 1 end because it's easier to bend. I have a ACAD pic I drew up but I can't remember how to capture a screen shot(edit: nevermind). I have it setup with 2 fans on 1 end.

As far as constant current you must add in the more parrallel resistance in order to keep the current constant. You could use a window comparator to turn on a fet or relay to switch in more resistors. I have a line on some .001 or .01 ohm resistors that are like 100W or 75W same style but bigger.

Jeff

[pinkpanda3310]

Have the MMM refurbs bought this project any closer to fruition

battery tests would be awesome.is there anyway to use a fluke meter?not sure how many amps they can measure.what about using automotive battery testers for the amp draw?some good ones use carbon piles.our high school had one that was adjustable.just some thoughts,i do appreciate the fact that this all is being done on a slim budget.

also,since electrical minded folk might read this,how hard would it be to build a non adjustable output driver for a brushless motor?like to power a motor on a dyno,or a starter box?

[J57ltr]

Flukes are only rated for 10A for most of them and 20 on a few models. You could use a shunt and measure higher loads, but then you have to convert milivolts to amps via a chart from the specs of the shunt

Using the Datalogger he is using will give recordable shots of exactly what's happening.

To build an non-adjustable drive for a brushless motor.

Simple version:
Use a servo tester an ESC, motor, power supply/battery. Connect them together and set the motor RPM you want. Remove the tester and measure (in Ohms) the pot for adjusting the "servo" between the middle terminal and one of the outer terminals, then the same for the middle and other terminal.

Write down those numbers and find resistors that match those 2 numbers. Remove the pot. There will be 3 holes where the leads of the pot were connected. Solder the 2 resistors into the 3 holes left by the pot and use the numbers written down earlier to determine which resistor goes on which side. You will have a lead from each resistor in the middle hole and the other lead goes into the outer hole. You will also need to have a Dual Pole Dual Toggle (DBDT) switch to be able to switch in 2 resistors of the same value half the resistance of the pot. (if the pot is a 5K you will need 2 2.5K resistors.) That way you have a neutral position and a place so the ESC can arm.

More complicated version should be handled in another thread:

Jeff

for some reason, i thought my buddies fluke measured higher?oh well.
i was looking to replace the esc with a simple max rpm driver for a brushless motor.something simple and cheap.possible other uses might be for a starter box or a dyno,or better yet,to use a brushless motor to run a comm cutter!
HA! i digress though ,it should be in another thread...

[BrianG]

Yeah, my Fluke is fused for 10A. Gotta figure these meters are for general purpose measuring, and not for currents in the 100's of amps. You can use clamp style meters, but that's just more cost. And using any meter would require the tester to constantly jot down voltage a current measurements (so would need two meters), and that would not be close to accurate. The Eagletree measures voltage and current using a hall-effect sensor (does not drop any voltage that a shunt would) to measure up to 150A, and can record as fast as 10 samples per second (100ms). For ~$70, the ET device is hard to beat for what it does.

And as far as a non-adjustable BL motor driver, I can think of a simpler way: Use an ESC like J57 said, but use a failsafe device as the throttle driver. They generally have an adjustment pot, but it's meant to be set once.

[BrianG]

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:



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

[Metallover]

Man, that's awesome. Is all you need the MMM? And assembling the resistors/heatsink?

[BrianG]

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.

[J57ltr]

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

[BrianG]

Sorry, I forgot about that Jeff.

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.

[J57ltr]

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

[Metallover]

Any updates lately?

[BrianG]

Not yet. Sorry. Funds that would be used for this project have been going elsewhere lately.