From: battan@sequent.com (Jim Battan)
Newsgroups: rec.autos.tech
Subject: Re: battery loading question
Summary: it depends
Date: 5 Oct 92 18:52:19 GMT
Article-I.D.: sequent.1992Oct5.185219.5700
Organization: Sequent Computer Systems Inc.

In article <1992Sep29.055017.6829@sequent.com> I wrote:
>I have a Lexus SC300 that has a 210 watt stereo system.  The manual says
>to not play the stereo "for long periods of time with the ignition off."
>I'm looking for more quantitative data on how long I can keep the stereo
>on (at about half power) without signicantly draining the battery--just
>before the point it wouldn't recharge completely the next morning when
>I drive the car.

Thanks to Andy Huang and John Hassey for their e-mail responses.  Also,
thanks to Mark for his posting.  I knew I wouldn't get much help since
I didn't include the Amp-Hour rating on my battery.  Anyway, I learned
that "it depends", but 1 hour is probably the conservative limit.

Summary follows.

> From andyh@chaos.cs.brandeis.edu  Tue Sep 29 07:54:52 1992

I've been using a 100w/ch ADS and have found that it killed the
battery after >4hrs full volume, playing classical music.

A couple of notes:
    battery capacity varies from model to model, age, state of charge,...
    consumption depends on amp design and signal characteristics
    recharging depends on length and speed of drive; use a low gear
	for more  charging, but you'll kill your mileage.

An analytical approach would be to measure the drain of the amp at the
battery with an ammeter and then calculate how long you could go
without taking more than 10% of the battery rated capacity.  You
shouldn't take more than 10% since automotive batteries are not deep
cycle and can only discharge 50% (?) of rated capacity.  I'm certainly
not an expert, but plan on being conservative.

>From hassey@dg-rtp.dg.com  Tue Sep 29 12:59:41 1992

  Just a guess: 1 or 2 hours.

  I think the only way you'll be able to tell for sure is the measure
  the current your stereo draws.  You probably don't want to draw more
  than 10 to 15 Amp-hours, without rechargind (and that may be pushing it).

  Your battery won't take kindly to being discharged and left sitting.
  Also, figure on a couple of hours driving to completly recharge if you
  do draw it down a fair bit.


> From sequent!uunet!cs.ubc.ca!uw-beaver!fluke!ssc-vax!simnet Fri Oct  2 07:51:33 PDT 1992

What you need to know is the ampere-hour rating of your battery. In the old
days, this was listed on the battery. Now you just get cold cranking amps
and reserve capacity (if you're lucky). Reserve capacity is related to ampere-
hours, but I don't remember the relationship (I think its minutes at a constant
discharge rate of 10 amps).

What you need to know is the ampere-hour rating of your battery. In the old
days, this was listed on the battery. Now you just get cold cranking amps
and reserve capacity (if you're lucky). Reserve capacity is related to ampere-
hours, but I don't remember the relationship (I think its minutes at a constant
discharge rate of 10 amps).

However, in the old days, car batteries were typically 40 to 50 A-H. You really
don't want to use more than 50% of that to prevent damage to the battery plates
and you should be even more convervative if you want enough juice to start your
car (you'll need more when the engine and battery are at or below 40 degrees F).
Heavily discharging a lead-acid battery and letting it sit is the worst thing
you could possibly do to it.

So, assume your battery is 40 A-H and you're willing to use up 15 A-H to run
the stereo. Fifteen amps for one hour will let you supply 15*12 or 180 watts to
your stereo. Your amps will draw more power than they produce, and make sure
to count every amp (left and right, maybe additional pairs for rear and doors,
etc). A total of 50 watts from all the amps should be pretty loud, except for
power required to pound the woofers. Also, are you going to be listening to it
inside the car or out. You'll waste a lot of power trying to hear it out of the
car, so adding a set of external speakers may help out. Otherwise, raise the
trunklid to let more of the sound out.

If you anticipate doing this for an hour or more, I would definitely try
to start the car when you're close to done or every hour. Let the engine
charge the battery for 10 minutes or so, since it can put 20 to 30 amps into
the battery as long as you don't have headlights, window defoggers, 210 WATT
STEREOs, etc stealing all of your alternator's output (which is probably close
to 100A at 2500 RPM, half that at idle). This also helps keep the engine warm
which reduces the amount of electricity needed to crank it over the next hour.

If you don't do this, I'd either buy a decent stereo for the garage, invest
in a battery charger, or hope that it is warm the next morning!

-- 
Jim Battan - Sequent Computer Systems, Beaverton, OR
battan@sequent.com or uunet!sequent!battan
+1 503 578 5129