Date: Sat, 5 Sep 2009 12:42:21 -0400
Reply-To: Kim Brennan <kimbrennan@MAC.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Kim Brennan <kimbrennan@MAC.COM>
Subject: Re: Run time to charge Aux battery
In-Reply-To: <976337.19408.qm@web44712.mail.sp1.yahoo.com>
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It is probably folly to idle the engine expecting the alternator to
charge the batteries. I don't know what the idle speed is on the
zetec, but most engines are turning slowly enough that the alternator
is putting out minimum amps. In otherwords, just enough to keep the
engine running, but not enough to charge the battery.
Curiously enough, on my Subaru SVX engine, it idles so low (500 rpms),
that the alternator doesn't even put out enough amps for the battery,
and I'm actually running off the battery, instead of charging it when
idling (especially if I have the AC fan running.)
For lead acid batteries a 2 amp charge rate is a nice safe charge
rate. But will take 24 hours to charge the batteries (note that a 70AH
battery, means 70 amps for one hour, or 7 amps over 10 hours.) Since
you shouldn't be draining the batteries past 50 %...
Alternators vary the amount of amperage they put out, depending on the
speed at which they turn (there is a maximum too). You'll be
generating the most amps at your higher rpms. Curiously enough, on
Vanagons, that's around our cruising speeds at 4000 rpms.
The engine doesn't use much current (a few amps at most). Your
headlights, AC fans, radiatior fan, and radio are the major consumers
of power. Your headlights are 55 watts each (watts = voltage x
current). Since we know that we're dealing with a 12 volt system, each
headlight nominally consumes a bit less that 5 amps. Times 4, that's
20 amps. Your AC fans, at max consumption will put about 12 amps
(which curiously is not at the max speed, but that's a whole different
story). Radiator when full on can put 10 amps easily. Most stereos
pull less than 10 amps. Add it up and you aren't even at 50 amps, with
headlights, brights, radio blaring AC blowing, etc.
100 amps should be plenty at cruising speed. And most alternators will
put out most of their amperage, by the time you are up to 2000 rpms.
So back to your question.
if you rev your engine to 2000 rpm, and run it that way for 15
minutes, let us make a few assumptions...
Your alternator puts out 90% of its rated capacity at 2000 rpms.
Your alternator is rated at 100 amps.
Your batteries are 25% discharged. So you need 50 AH of recharging to
top them up.
You keep all the aux stuff off while recharging (no AC, not radio). so
your base power consumption is circa 5 amps, while running the engine
at 2000 rpms.
So you'll be producing 90 amps of power, minus 5 amps of consumption,
for 85 amps. If you did this for an hour you'd produce 85 AH for your
batteries. but since you only need 50 AH...
85/1 x=50/1 (85 Amps per hour time some fraction is equal to 50 amps
per hour) or x = 50/85 = .588 hours. X 60 equals approx 35 minutes.
On Sep 5, 2009, at 5:03 AM, Malcolm Stebbins wrote:
>
> Usually I only park for a day or 2 max, so all is OK with the aux
> battery, but this last trip I stayed parked for 4 or 5 days in a
> row, and the fridge's 'low battery' light came on.
>
> My question is: APPROXIMATELY what is the charge rate IN AMPS for a
> nice good working system. I say good system as I have a new Zetec
> engine with a 100+ amp alternator and the wires are all new and the
> batteries are relatively new.
>
> I use a SUREPOWER 1315 isolator for the 2 battery sets.
>
> When the battery got low, I took to idling the van for about 15
> minutes in the morning, and again 15 minutes in the evening, to help
> the battery a bit and this seemed to help. Is this folly?
>
> Thanks Malcolm
