Paul & Sabrina's Cheap EV Conversion

[MPaulHolmes]

IF IT HITS THE ROAD THIS SUMMER!!! HAHAHA!

I guarantee that it will be "done" 1 day after I get all that crap in the mail, like the lugs and the PB-6 variable resistor (throttle). By done, I mean that it will be illegal to drive.

Today, before working at Sylvan, I chopped off the measuring things of the $2.99 volt meter from Harbor freight. I then soldered on some longer wires, and attached rings to the ends. It will be my "gas gauge" right up front. Also, I'm going to use another $2.99 volt meter as an Ammeter. It will measure voltages in the range of 0 to 0.1v, but the screen will show it as 0 to 100. 0 will mean 0 amps, and 100 will mean 400 amps. I'm using a 1.6 foot piece of the 2 gauge copper as my "shunt". What? Don't do that! Copper blah blah blah not as good as blah blah blah! haha!

It's true that copper varies its resistance as temperature changes, but we'll see what happens, yo.

[dremd]

I like your current shunt. :-)

If you buy just 1 harbor freight warranty for those chargers you will be able to have them replaced one at a time when they start failing.

[MPaulHolmes]

Dang, I didn't get the product replacement thing! Oh, that just makes me so mad. Well, I forgot to walk everyone through the piece of wood that has all the stuff on it. There's a curtis controller with a too small aluminum plate under it, next comes an awesome contactor that can run 500 amps CONTINUOUS! ya. Next comes the 2.5 Ohm 40watt resistor (four 10 Ohm, 10 watt resistors in parallel from Radio Shack), and then finally is the 300 amp fuse. Then there is a dead stick man.

[MetroMPG]

The stick man is a nice touch.

[MPaulHolmes]

It's a stick woman, with a beautiful mane of red flowing hair.

Also, I want to walk through why the shunt works, just because I thought of it and felt really proud, and found out that everyone else already knew and it made me real sad way down deep.

Resistance of 2 gauge copper: 0.1593 Ohms / 1000 feet

V = I*R

So, since the setting on the harbor freight ghetto volt meter will be set to the 0.2v range, and since you don't want to be too close to 0.2v in that setting so nothing blows up (so use 0.1v as the maximum voltage that will be measured), and because 400amp is the most current that will ever come from the batteries, we must find the R in that situation:
0.1v = 400amp * R

So, R = 0.1/400 Ohms
R = 0.00025 Ohms

So, what length of cable gives 0.00025 Ohms?

(1000 ft / 0.1593 Ohms) * 0.00025 Ohms = 1.569 ft

Now, since there are going to be lots of chunks of copper cable used to connect the batteries, just make sure one of them is about 1.6 feet long, and attach the volt meter (used as ammeter) across one of the cable pieces that is 1.6 feet long.

If you want to use other cable diameters, just look up the resistance per 1000 feet for that size cable.

On the ghetto volt meter display,

0 will mean 0 amps
100 will mean MAX amps (what it's really reading is 0.1 volts, but it shows 100 because it shows the number of millivolts! haha!)

The potential problem with this is, if the cable get warm, it will seem as if I'm using more current than I actually am.

Near room temperature, the resistance of copper increases by 0.393% per degree C, which means the error of the ghetto ammeter increases by 0.393% per degree C. So, to get an error of 10% on the ammeter (too high), the cable must be

0.00393*T = 0.1
T = 25.44 degrees Celcius HOTTER than the environment.

So, if the cable is holding at around 114 degrees F (20 Deg C + 25.44 deg C), it will be an error of only 10% too much on the ammeter. Hopefully the cable doesn't get that hot.

Dang it! That's way cooler than 145! Oh dear! I made a booboo. New plan:
I must compensate??? Maybe, once I find out how hot the cable typically will be most of the time, I can make it so the 0 to 100 scale has an error of 0 in that situation. Then, when it's cooler, I'll be drawing more amps than I think, and when it's hotter, I'll be drawing less amps, but hopefully the error will be less.

[MetroMPG]

You changed the photo! I know the difference between a stick man and a stick woman.

[MPaulHolmes]

Whatever do you mean? It has been the same picture the whole time... Are you feeling OK? Plus she's pregnant and is feeling sensitive about her weight gain.

[MPaulHolmes]

Originally Posted by dremd

I like your current shunt. :-)

Personally, I prefer my previous shunt. It's way better than my current shunt.

[bennelson]

Originally Posted by MPaulHolmes

Personally, I prefer my previous shunt. It's way better than my current shunt.

Darn it! You beat me to the pun!

At least I still had "a good lead on finding some batteries."!


Seriously though, that explanation on the homebrew ammeter was pretty good. I may have to refer back to that information later.

[MPaulHolmes]

Ya, I liked the Leed. Good thing we don't speak spanish, since everything is spelled how it sounds. Hmm...

Oh ya, I put in an extension cord that is retractable. You pull on it, and it comes right out of the gas thing. Then when you are done, give it a tug, and it rolls back in all by itself! I got it at Sears! My wife picked it out. But I had to switch the plug ends. That took a couple hours of cutting, splicing, soldering, and taping. Annoying! It's done though. I hope it works OK. I still need to attach the round thing that holds the cord to the car somehow. Maybe I'll glue it down or something.