[ausev] A/C in an EV - volt meter

[Christopher Robison]

On Tue, 2007-06-19 at 07:54 -0500, Claire Sorenson wrote:
> Chris, when I was at Rob's open garage, you mentioned something about the
> needing to watch the volt meter to make sure it stays within a certain range
> or the batteries are being harmed (I may not be repeating that quite right)?
> I was going to buy a meter, but I peaked under my dash cover and low and
> behold the meter works.  It was only the guage that indicates the percent
> battery charge that was diconnected and a new one installed.  So, could you
> tell me again what I should be looking for?
> 

At a minimum, any EV should have two gauges, one showing pack voltage
and one showing battery current.

The critical voltage is 1.75 volts per cell. A lead-acid cell is
nominally 2 volts, so the number of cells will be half your nominal pack
voltage -- in your case, 48 cells.  48 x 1.75 gives you 84 volts as your
low-end limit.  As your pack discharges, you'll find that you have less
power available without going below the limit, so you may have to be
lighter on the throttle. This problem seems worse in lower voltage
packs.

When at any point (accelerating, cruising, letting them sit) you allow
the voltage to go lower than 1.75 volts per cell, you're shortening the
life of your batteries.  It won't be a catastrophic failure or anything
sudden; you're just abusing the batteries and if you do it regularly
you'll be replacing them more often than you'd otherwise have to. This
can get expensive.   :o) 

By the way, this is assuming that your battery pack is balanced. If it's
not balanced, some batteries and their cells will be lower than others
to begin with, and you may be above 84 volts but still having some cells
below 1.75.  To prevent this requires occasional "equalization charges"
which bring all the batteries to the same voltage, essentially by
overcharging until the stragglers catch up.  Your charger should support
this function or allow you to adjust it occasionally to do so.


-- 
Christopher Robison
chris at ohmbre.org
http://ohmbre.org          <-- 1999 Isuzu Hombre + Z2K + Warp13!