Project: The Electric Booger (budget EV conversion - '92 Mazda MX-3)

[mechman600]

Yesterday I drove home from work at midnight and took the long way so I could do a top speed run. The Electric Booger achieved a top speed of 85 km/h (indicated).

After the top speed run came the uphill. After the uphill I decided to drive around a bit more to see how the batteries performed. The result: they SUCK. BAD. My total drive was 12.0 km and for argument's sake we'll say 15.0 km if there was no top speed run and no uphill. By 3 km from home the batteries were sagging so bad that my top speed (trying not to let the voltage drop too low) was 50 km/h. Not fun.

Battery Math
I cannot find an amp-hr rating for these batteries but going by weight/size and comparing them to other batteries, they are each approx 100AH. That means I have 14,400 watt-hrs of theoretical energy. Factoring Peukert effect at 50%, that should give 7200 watt-hrs to 100% DOD. Assuming the eBooger consumes 300 watt-hr/mile (186 watt-hr/km), a 15 km trip will use 2796 watt-hrs of energy. Assuming I was down to 80% DOD last night, this means that my pack is only delivering 48% of the power I expected.

Unfortunately I do not have an amp counter so I do not know how much power the Electric Booger is actually consuming. Maybe on today's commute I will install the remote inductive amp meter and take notes.

Battery Alternatives
I can get 100AH marine batteries from Interstate for cheap (~$100 each). But I have a sneaking suspicion that they will not be much better than the batteries I already have because like mine, they are not really designed for repeated deep cycles. What I would really like is AGM (absorbed glass matt) batteries, a la Optima Yellow or Blue tops because they do not sag like flooded batteries. Unfortunately these are expensive (~$275 each) and only last 200 cycles before they start crapping out.

Lithiums would be the answer, but for a 77V 100AH pack (24 cells) with BMS (battery management system) and a proper charger we are talking $4000. Considering the eBooger is a beater to begin with, I don't really like that option. For now, at least.

Does anyone have any suggestions?

My Highly Inefficient Chargers

Yesterday I charged with my new Kill A Watt power consumption meter plugged in. My 2796 watt-hr drive home took 5070 watt-hrs out of the wall to charge. That's 55% efficient. And that's pathetic. They use 122 watts even after my batteries are fully charged and the chargers are doing nothing but making a humming noise. My round trip commute to work ended up being 627 watt-hr/mile or 54 mpge. Yikes. That's horrible, but still much better than it did with an ICE doing the job. Keep in mind these calculations are based on the assumption that the car averages 300 watt-hr/mile. I may know actual numbers after today's commute.

So right now I am quite unhappy with the way the car runs. That being said, I am only into the project for $2250, so should I complain?

[Daox]

If you can find a source for used batteries that can suit your needs I'd say go with that. If you are forced to buy new batteries I'd have to say go with lithium because they'll be cheaper in the long run. If you're currently getting away with 100Ah lead batteries you should be able to go with a smaller lithium pack because of the advantage of being able to bring them down to a lower SOC. So, your $4k pack should only really cost around $3k. But now you need a charger and BMS to go with it...

[mechman600]

Originally Posted by Daox

If you are forced to buy new batteries I'd have to say go with lithium because they'll be cheaper in the long run. If you're currently getting away with 100Ah lead batteries you should be able to go with a smaller lithium pack because of the advantage of being able to bring them down to a lower SOC. So, your $4k pack should only really cost around $3k. But now you need a charger and BMS to go with it...

I am currently running a "200AH pack" (12V pairs of 100AH batts X 6 = 12 batteries). The quotation marks indicate the crappyness of my current pack.

A 70AH X 24 cell lithium pack = 5328WH X 80%DOD/300WH-M = 14 miles. Probably all I will ever need and like you say, about $3K, including mini-BMS and a 72V 10A charger. Geez, that seems doable. But I think my average current draw is too high for 70AH cells - they are rated for 3C constant discharge current and I think the eBooger draws more than that. Tonight I will know for sure.

Performance wise, I am quite happy with 72V and 450A until the batteries start going flat, that is.

[mechman600]

I finally took a video of the eBooger in action:

http://www.youtube.com/watch?v=lCPKt1EKe-o

[Daox]

You really draw more than 210 amps continuous? That seems like quite a bit. I mean during acceleration it should be that high (and higher probably), but during cruising that seems quite. That is pushing out around 20hp...

[mechman600]

Originally Posted by Daox

You really draw more than 210 amps continuous? That seems like quite a bit. I mean during acceleration it should be that high (and higher probably), but during cruising that seems quite. That is pushing out around 20hp...

No, I was going on and off the throttle with rolling hills when the ammeter was in the picture. 60 km/h (37 mph) is 90-100 amps on flat ground = 192-212 KWH (including field consumption) which is fairly decent, no?

[Daox]

I'm at work and can't view the video right now. 70Ah cells should take 3C all day, and 450A no problem for as long as you'll need to accelerate up to your target speed. I would suggest pricing out the new CALB CA series cells (grey cells) vs CALB's older SE (blue cells) and other makers. They seem to have a significant boost in cold weather performance as well as cycle life. Worth the small price premium. I think they're going for around $1.28/Ah right now.

Normally, I think that Wh/mile is calculated from the wall, not while driving. I know my friend with a Leaf is getting around 200Wh/mile from the wall in summer. I think that is a pretty darn good number. If you had a efficient charger you shouldn't be far off from that.

[mechman600]

Originally Posted by Daox

If you had a efficient charger you shouldn't be far off from that.

That's my problem right now. My chargers are atrociously inefficient. 650 wh/mile from the wall. Still cheaper than gas, though.

[mechman600]

Lately I have been working on a PWM field current controller idea. I really want to control my sepex motor properly to get full advantage of the whole sepex thing because my tiny motor needs all the help it can get! A proper Curtis 1244 (72V, 500A) controller is $1200 + the $200 PC programming kit + shipping + tax, and that's too much.

The field current relation to armature current needs to look like this:

This means I need to measure armature current and turn my measurement into a 0-5V signal to give to a small PWM controller. I am thinking of using a 500A 75mV shunt and amplifying the 0-75mV signal to 0-5V.
Here is the schematic I have come up with:

The CA741 is an OP-AMP chip.
The circuit has two parts:
1. Minimum field current. Switched on with idle microswitch. It ensures a min of ~10A field any time the throttle is depressed. Adjustable with "Min Amp Trim".
2. Variable gain amplifier. Takes the 0-75mA created by the armature current shunt and converts it to a 0-5V output signal for the PWM field controller. Adjustable with "Amplification Trim". This 5K pot is Rb. The 220u resistor is Ra. The amplification formula is (Vout-Vin)/Vin = Rb/Ra.

I am not sure the two diodes are correct. The way I see it is the circuit with the highest voltage "wins" and that voltage is the output signal.
Any electronics gurus want to proof read my schematic and tell me if it will work?

EDIT: I see major flaws in this circuit. I have instead decided to use a 0-5VDC output amp transducer to measure current. More circuit diagrams to follow!

[dtbaker61]

good job on getting a conversion done and on the road.... too bad the hard lesson after all that work is that a super-cheap build, especially with lead, is just not very good.

to anyone else interested.... I'd suggest a minimum build of 120v of 100ah LiFePO4 cells ($5k giving 50 mile max range), 8" or 9" DC motor, and a controller capable of continuous output of at least 200amps, 400amps max. You need at least 30kW output to have reasonable acceleration around town.

You can find SOME things used in Forums or eBay, but you'll end up spending $6k in parts, $5k in batteries, and 150 hours of labor.... you should pick a donor vehicle you'll WANT to drive for 10 years.

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