4-2-06 Re: Time to review the plan…

Hello to Steve and All,

Steve wrote:

Books and internet research are one thing, but the experience of this list (plus my experience with 3 EVs) is priceless. I want you to have an idea what I was trying to accomplish, budget, and my performance expectations.  Also, I want to tell you what I am basing my calculations on.  Then, we can have an informed discussion.

I agree with your take about the cumulative knowledge that’s available here on the EVDL, and I like the clarity you’ve laid out.

First, what I am trying to do (design inputs)
1) Small, light weight commuter car.  Note, around here, a commuter car goes 45 – 50 MPH for 10 miles back and forth to work each way.  Why?  Smaller, lighter, etc. means more efficiency, less batteries, less weight to lug around, better handling, easier to fabricate, lower cost to build and maintain.
2) A 20 mile usable range, 365 days a year, rain or shine, hot or cold.  It gets down to the mid 20’s here and as high as the high 90’s.  Of course, for my calculations, the mid 20’s yield the worst case range.  I don’t want to kill the batteries, so I was planning on a 70% DOD max (nominal on the coldest day, 80% DOD absolute worst case on the coldest day if I have to make an extra stop on the way home).

The answer here is pretty simple, and it’s a tried and proven concept when talking about available, affordable lead acid
batteries. To get the range you’ve outlined with the 70% max depth of discharge, you need to follow this formula….when your EV is finished, the battery pack should be 1/3 the total weight of the vehicle. In other words, if the completed car weighs 3000 lbs. with the battery pack, then the pack should be 1000 lbs. To get your Festiva to fit this prescription, it would have to weigh around 1900-2000 lbs. stock,. The added electric motor, brackets, charger, and cables would pretty much equal the weight of the gas engine, exhaust system, gas tank, etc. you remove, so you’re still looking at around 1900-2000 lbs. Now you can clearly see that you need about 1000 lbs. of batteries to get that 1/3 battery weight target.

- 84 V pack – flooded lead acid – 6 V batteries (T125’s)

Perfect. 14 X ~ 75 lbs. = 1050 lbs. of batteries. This pack would be far cheaper than any 1000 lbs. of AGM type lead acid, and if you can keep average cruise currents low, the higher ahr of the flooded batteries will go a long ways towards best range per charge. However, at just 84 volts and a safe max discharge current for these flooded type 6 volters of about 500 amps, you’re never going to get the other goal you’ve outlined here:

I want GOOD ACCELERATION!  Better than the ICE version (which was very
slow).
At least as good as my Toyota Corolla with 1.8 L, 4 cylinder engine.  I’m
not asking for a drag car, but I need to keep up with the hectic traffic
here in
Atlanta.  If it can’t do that, I’m not wasting my time or money.

Pulling the 500 amps max. safe current from your 6V flooded batteries will cause the not-too-stiff 84V (nominal) pack of flooded batteries to sag down to around 65 volts. 65V X 500 amps = 32.5kw, or about 33 hp. Just 33 horsepower to move a 3000 lb. car, should make it pretty obvious this low of a voltage at 500 amps is not by any stretch of one’s imagination, going to give ‘good acceleration’. In fact, even if you doubled the amount to 66 hp, it still would only be ‘OK’ acceleration in a 1.5 ton vehicle. Maybe now, you can see why most everyone else these days, run at least 144V. At 144V and using low-sag AGM types 12V batteries, 1000 lbs. of them would only sag to maybe 125V at 1000 amps to give 125kw, or about 130 hp.

Back to your requirements of low cost though…..you ‘could’ still get by with the inexpensive 84V pack of golf car batteries and get ‘OK’ 0-40 mph acceleration, by spending a bit on the controller and dissing the old tech restrictive SCR controller…read this, get a Z1K Zilla. With this compact controller, you could dial-in 1000 motor amps and still keep the max. battery amps to 500. The controller will be able to boost the motor up to 1000 amps while its voltage goes from 0 – 40 volts as it revs up. After that, the motor amps would fall to keep the max. programmed battery amps at 500. This set up though, would protect the flooded batteries from over-current while still giving your 3000 lb. Festiva very decent low speed traffic type acceleration, probably not as quick as the 1800cc Corolla in the 0-40 sprint, but not too far behind. From 40-60 mph things would definitely be tame, but probably acceptable for your needs. This is beginning to look similar to your ‘option #4′.

To reduce costs in option #4, you could find a used EV motor, I would think for $300-$400 dollars if you looked hard enough and put out enough feelers. You could certainly find a 48V forklift traction motor for a few hundred bucks, and with simple timing adjustments, it would work very well. You could also build a basic charger with a timer shut-off for maybe $200.

My money would be on the 84V pack, the Zilla Z1K controller, a basic charger, and a used forklift or smaller EV motor. You’d end up with an affordable EV that has pretty good off-line acceleration, ‘OK’ 40-50 mph performance, and sluggish 50-65 mph performance. You also end up with an EV with a 30-45 mile range, depending on temperature and other conditions. I agree, keep the clutch.

Just my two cents worth….

See Ya……John Wayland

This entry was posted in EVDL Posts. Bookmark the permalink.

Comments are closed.