Re: (ET) Battery Question

["Christopher Meier" <mr23 mn rr com>]

I was under the impression that PWM charge controllers are essentially
like an EV controller, but with a micro running the throttle to set the
voltage/current out to approximate some charge algorithm (like IUI).
I didn't expect a simple on/off charge controller would work, as you
say it doesn't have any means of current control, and 17v into 13v
would have a significant inrush...  even though it would taper off quick
as the 17 dropped a bit and the 13v came up to 14.x.

-Chris

----- Original Message -----
From: "Travis Creswell" <ozsolar ipa net>
To: <elec-trak cosmos phy tufts edu>
Sent: Wednesday, July 17, 2002 11:42 AM
Subject: Re: (ET) Battery Question


> Hi Christopher,
>
> For what it's worth I don't anything about drag cars.  But I have been in
> the solar business for over 10 years installing and maintaining thousands
of
> batteries and solar panels over that time.
>
> It's not amperage of the loads that would cause the problem that I'm
worried
> about.  Let me take a stab explaining my thought process. The common
charge
> controller has no provision for hooking up loads so the energy for the
load
> would still come directly from the battery (via the cars' on board fuse
> panel).  A charge controller is designed specifically to prevent a solar
> panel from overcharging the battery.  Since solar panels have very 
> limited
> current a charge controller is only designed to handle a limited amount 
> of
> current and essentially has no provisions to deal with high current
sources
> such as batteries, battery chargers, unregulated DC power supplies etc.
Big
> charge controllers are only rated at 40 amps.  The installation manuals
that
> come with charge controllers specifically warn against using them to
control
> anything but solar panels for this reason.
>
> The simpler Charge Controllers close a relay once the voltage of the
battery
> drops to certain level (commonly ~13.3 volts) connecting the battery to
the
> solar panel and charging begins.  After the solar panel brings the 
> voltage
> up to somewhere around 14.5 the relay opens, disconnecting the solar 
> panel
> from the battery to prevent overcharging of the battery.  This process
might
> happen 100's of times day as the battery voltage rises then drops again.
> Charge controllers are specifically sized to the amp rating of the solar
> array.
>
> A battery is capable of delivering 1000's of amps.  Once the charge
> controllers' relay closes to begin the charging process it will see all 
> of
> that amperage as the higher voltage battery attempts to equalize the
> voltage.  If the charge controller survives long enough for the battery
> being charged to reach the "charged" set point it's relay will attempt to
> interrupt a load that could be as large as a thousand amps.  A sudden
> catastrophic failure (an explosion) might happen.  Even if it was under a
> few hundred amps the Charge Controller just isn't designed for large
loads.
>
> Hopefully this explanation clears up my thinking on this.  I might be all
> wet but I would not hook two batteries of different nominal voltages
> together.
>
> Travis
> ----------
> >From: "Christopher Meier" <mr23 mn rr com>
> >To: "Travis Creswell" <ozsolar ipa net>, "Pestka, Dennis J"
> <Dennis Pestka TycoHealthcare com>, <elec-trak cosmos phy tufts edu>
> >Subject: Re: (ET) Battery Question
> >Date: Wed, Jul 17, 2002, 9:14 AM
> >
>
> >I was under the impression that dragster are not cranked via a
> >starter that is run from the 12v battery; they are spun up by a
> >hand held unit that is independently powered.  Please enlighten
> >us as to the starting method used on the dragster in question.