Ideas for the new EVer

[Lee Hart <leeahart@earthlink.net>]

In some off-list correspondence with a new EV owner, I described some
things that new EVers tend to do wrong, and some simple indicators to
prevent damage. Without these indicators, people don't know they are
doing anything wrong (until it's too late!). They have all been
mentioned before, but maybe others would like to see it as well.

1. Don't drive until the batteries are so dead that it barely moves.

Deep discharges will seriously shorten battery life. Add some kind of
low-voltage cutout to slow down or stop the car if its batteries fall to
1.75v/cell under load. For example, a relay coil and series zener diode
chosen to drop out at 1.75v x 60 = 105v for a 120v pack.

This relay has to be manually pulled back in (by pressing a button, or
turning the key to the "start" position). By letting the batteries rest,
or just driving with a lighter foot, you can continue driving to get to
a charger.

2. Don't leave the batteries in a discharged condition.

Lead-acid batteries should not be left in a discharged condition for
very long. Hours won't matter, but days or weeks will! Charge them
promptly.
 
Also, charge them every month or so if the vehicle is in storage so they
don't gradually self-discharge and go dead. But don't just leave some
charger on it all the time; lead acid batteries don't need float voltage
or trickle chargers.

3. Don't put it on charge and forget it.

Most chargers are stupid, and err on the side of overcharging. Some will
happily charge forever and cook your batteries to death. Be sure you
have a good state-of-charge gauge like an E-meter, or at least an
accurate voltmeter and ammeter. A cheap analog voltmeter marked with
"empty-full" is not good enough!

Then make sure your charger is not over- or under-charging. Don't
blindly assume the charger "knows what it is doing." A simple overcharge
indicator is a Mallory Sonalert (electronic buzzer) with a zener diode
in series so it beeps if the batteries exceed 2.5v/cell (150v for a 120v
pack).

4. Don't forget to check the batteries.

Most batteries don't die of old age -- they are murdered! Low water
levels, loose terminals, over- or under-charging, excessive current, or
deep discharges will all kill your expensive batteries long before their
time.

For water levels in flooded batteries, here's a simple low-water-level
detector. Connect it to ONE 6v, 8v, or 12v battery. Stick a stainless
steel wire or screw in the vent cap next to the negative terminal so it
stops touching the electrolyte if the level is low. When it touches the
electrolyte, the voltage on this wire will be about 2v, which is too low
for the LEDs to light. When it doesn't touch, the resistor pulls the
voltage up to about 3v and the LEDs light. Sensing one cell is usually
enough, as water usage will be similar between cells for well maintained
batteries.

Low Electrolyte Level Detector

6v to 12v ____/\/\/\____
Battery+       4.7k     |
             resistor   |
                        |
wire in   ______________|
vent cap                |
                        |
Battery-  ____|/|__|/|__|
              |\|  |\|
       two high-brightness LEDs

Likewise, my Batt-Bridge indicator will tell you if the batteries are
out of balance from undercharging, or if there's a failing terminal.
It's just an LED, 2.4v zener (or green LED), and a matched pair of
resistors to compare the voltage of the upper and lower halves of the
battery pack (see EV list archives). The voltages should be the same; if
a red LED lights, there's a problem.

Batt-Bridge Out-of-Balance Detector

                    R1
   Pack+ _________/\/\/\_______                R1 and R2 are equal-value
                               |               precision power
resistors,
               D2 red high-    |               chosen to let about 20ma
              efficiency LED   |               flow.
              _____|/|_________|
             |     |\|         |               
Pack    _____|                _|_
Center-tap   | D3 red high-  _\_/_ D1 green LED indicates power is on
             |efficiency LED   |
             |_____|\|_________|
                   |/|         |
                               |
    Pack- _________/\/\/\______|
                     R2
5. Don't force the motor to run at low rpm.

You shift ICEs for low rpm for best efficiency, and high rpm for best
power and accelleration. Electric motors are exactly the opposite! Shift
to keep rpm high for efficiency, and low to accellerate faster.

If you shift into high gear like you would for an ICE, a series motor
will draw excessive current, and overheat.

5. Don't use the accelerator pedal to hold the car on a hill.

Don't do it! You'll fry the commutator -- it's not meant to handle full
current through one pair of commutator bars for more than a few seconds.
(It's like slipping the clutch in a car with a manual transmission to
hold it on a hill -- you'll burn out the clutch! :-)

6. Don't roll backwards with the car in a forward gear (or vice versa).

Series motors become series generators if they are turned backwards.
Most controllers are not protected against the huge currents they can
generate. Nothing happens at first, but as the motor gains speed, it
reaches a threshold where the generated voltage exceeds the forward
voltage drop of the freewheel diode in the controller, and BANG! It's as
if you slammed on the brakes. The current surge can kill the controller.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen