On Sun, May 25, 2008 at 02:32:33PM -0400, Peter wrote:
> > 2) Going to start with a battery bank consisting of Eveready GC2 Golf Cart
> > batteries from Sams. 6V, 225 Ah, $75 each. A 144V system of 24 batteries
> > will run $1650 + tax, no shipping and weigh about 1650 lbs.
> 
> The goal of the electric car is to be light, in view of the low power density of
> the batteries.

My goal is to build something that I can afford. I battle with the guys on
the DIYEV list all day long about investing tens of thousands of dollars
into lithium batteries. But it's an investment that doesn't make sense to me.

When lithium batteries are reliable, easily available, and about double the
price of lead acid, then I'll consider switching. Until then, it's lead
acid because from a cost standpoint, there's simply no comparison at this
point.

> The law of diminishing returns kicks in fast if you end up with a
> ton of batteries lugged around for no good reason. The best way is to use a
> battery as small as possible and change it often (with another freshly charged
> one). In the 70s and 80s there were trials with such changeable packs for public
> transportation (electric bus). At the time they did not catch on.

Can't store a battery bank at work, which is the only logical place to
swap. The bottom line is that I'm stuck carrying the bank around.

> Imho, try to work the problem backwards: how much do you travel between pit
> stops, what class of EV can do that now (wikipedia helps), how much it
> weighs/costs, then see how you could build a vehicle of that class or refurbish
> a used one.

All that analysis is already done. That really wasn't the purpose of this
thread, though I'm happy to talk about it if you like.

The bottom line is that if you carry enough lead around, you can get
reasonable ranges for a daily drive. That's my goal.

> Available data suggests 20-100 km range for curb weight << 1 ton.

I'm well aware. The challenge is that to carry enough lead to get a useful
range, you exceed the GVWR of the vehicle.

Trucks are designed to carry that kind of weight. 

A better tack is to improve the aerodynamics of the vehicle. Trucks are
pretty much a rolling brick with awful coefficients of drag. If you improve
the CD with sloped bed caps, mirror deletion, wheel covers, and the like,
you can greatly improve the range of the vehicle without changing the
weight. See a sample here:

http://ecomodder.com/forum/583-commercially-produced-aerodynamic-pickup-bed-cap.html

> Electronics for electric traction are not so trivial. There is inrush limiting,
> kickback handling, dynamic braking (motor tries to generate - you can't just cut
> the drive, the voltage will raise high enough to blow the insulation on the
> motor if you do that) etc etc. 'Simple' PWM requires a bridge circuit from >500W
> or so and so on.

OK. That's a start. Why kind of hardware is necessary to handle these
issues? I thought the dynamic braking and back EMF was handled by the
freewheeling diode?

> Plus the torque/rpm characteristics of usual electric motors are not suitable
> for road traction use without some suitable controllers. The easiest hack is to
> rely on the gearbox and implement a step relay that will shunt in more or fewer
> cells (tapped series stack).

No. Bad idea. The one thing that I've learned in my research is that you'll
destroy the pack using that technique.

The whole point of PWM is that the full pack voltage is applied all the
time. That means that you'll get maximum torque as limited by the
controller and the switch and max RPM generated by the PWM ratio.

All of this is known engineering. No modern PWM controller ever does
anything but apply the full pack voltage to the motor. Soft start and rmp
are controlled by the PWM ratio, and nothing else, from my research.

> This provides both drive and regenerative braking
> in a brutal way, and is the 'old' (and tried) way of doing it.

Series wound motors. Not going to do regen braking. That energy is just
going to be lost.

> Fork lift trucks
> and the like built according to these principles tended to last 30-40 years.
> 
> Imho obtain a book on electric traction control (road and rail) and read it. It
> will likely open your eyes. I never did anything larger than 50-100A traction
> (12-24V) and I think that just keeping cpu-deadly spikes and kickback under
> control will keep you busy for weeks.

They are called Isolated Gates for a reason. What makes you think those
kickbacks are going to cross back into the gate?

> Most older (pre thyristor) battery chargers are of the variac type, manual or
> automatic, newer ones are thyristor based. Charge equalization in a series
> battery is a problem. Nothing is as simple as it seems. The price tags on the
> items you quoted probably reflect the low production numbers and the man-hours
> put into fixing the problems that had to be fixed.

Thanks for the encouragement! ;-)

But I'm going to press on. Because at the end of the day my time is
currently worth less than my dollars. So if I can spend time solving these
problems instead of spending money, then I'll come out ahead in the end.

BAJ
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