Date: Sat, 5 Sep 2009 09:42:14 -0700
Reply-To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Rocket J Squirrel <camping.elliott@GMAIL.COM>
Subject: Re: Run time to charge Aux battery
In-Reply-To: <976337.19408.qm@web44712.mail.sp1.yahoo.com>
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Hi Malcolm.
A 24-hour run of your refrigerator, if it draws 7A, and has a duty cycle
of 2/10 (two minutes out of ten) will consume 4.8 amp-hours (Ah) per day.
So if your numbers are correct, and if you wanted to never discharge your
deep-cycle batteries too deeply (say never less than 80% discharged so
they last longer), then a five-day run of (call it) 5Ah per day would
require 25Ah, and a battery with a 30Ah rating should do the job.
But the "low battery" indicator on your refrigerator came on after five
days, and you appear to have heftier batteries than that. This doesn't
match up, does it? There may be several reasons for this.
1. If the wiring to your reefer is fairly skinny, then a 7A draw would
result in some voltage being lost in the wire, so the reefer's voltage
detector would see a lower voltage than is actually present at the
battery's terminal. This could cause it to trigger prematurely even though
you have a lot of amp-hours still in the battery.
2. You may be consuming more power than you think. Consider what other
things are connected to your aux battery and see what their power
requirements are. A radio that wants 2A and is run 12 hours a day will
consume 24 Ah.
3. Your deep cycle batteries are probably not fully-charged in the first
place. Your engine's alternator isn't set up to fully charge your
deep-cycle batteries. It does a bang-up job of topping off your starter
battery, but that thing is built differently than the deep-cycle guys on
account of it has a different job to do (provide a whacking great amount
of current for a very brief period vs a deep-cycle's job of providing lots
of hours at relatively low current). The best you might achieve even after
driving all day will be an 80% charge in the deep cycle batteries. And it
ain't a matter of amps, so much, as it is of volts: the regulator in the
alternator sets the charge voltage too low to ever top off a deep-cycle
battery.
As to how much refreshing of the aux battery you achieve by running the
engine 30 minutes a day, well, here's what we know about that: a. without
an ammeter in line with the aux battery to measure the charge current
going into it, we can't determine how many amps/hour it's getting. And b.
the amperage going into the battery will naturally taper off well before
the aux battery is fully-charged because, as mentioned, the alternator
isn't set up to give a deep-cycle battery a full charge, its output
voltage is too low and it doesn't have the smarts to take the battery
through the needed three charging stages needed to top that thing off and
keep it there (good deep-cycle chargers add even a fourth stage to keep
the battery's guts fresh). If you were to change the regulator in the
alternator to one tailored for deep-cycle batteries you would probably
bake the starter battery. Different designs for different applications.
Reworded: Amps isn't the problem -- the Bosch 90A alternator has plenty
enough current to put 80 or so Ah into a battery with 1-hour run time ...
if you could stuff that much power into a battery in one hour without
baking it, and you can't -- 80 amps at 14 volts is 1,120 Watts . . . do I
hear a battery boiling?.
More battery capacity could be obtained with proper charging of the
deep-cycle batteries. Charging a deep-cycle battery from the alternator is
a while 'nuther subject, and the tech literature for sailors who depend on
their aux batteries for days on end and charge them from their engine
alternators is worth a read.
So the best the alternator can do is charge the aux battery up to about
80% of full. So how much capacity does that provide? If you elect to
discharge the aux battery to no deeper than 80% discharged to prolong its
live, you'll get about 60% of the full-rated battery capacity to work
with; If you choose to allow the battery to discharge to 0% you'll have
80% of your battery capacity to work with. But on a more practical level
since you probably don't have an easy way to measure the batteries' state
of charge, you'll be discharging it until the low battery indicator on
your reefer triggers (do we know what voltage its trigger point is? should
be in the manual or available from the manufacturer) and understand that
the voltage that the reefer is seeing is doubtless lower than what's at
the battery terminal because of wire losses.
So your working range is somewhere between an 80% full battery at the top
end, and the low battery trigger point of the reefer. Whatever that is.
Fatter wire to the reefer might let it better see that actual battery
terminal voltage, can't tell without knowing what voltage is at the reefer
with it is running, and what voltage is at the battery terminal when the
reefer is running. The difference is the wire loss. Even a couple tenths
of a volt could cause the indicator to trigger prematurely. So this is a
worthwhile pair of measurements to take.
Enough background. On the more practical level, you just have to get a
feel for how long you need to run your engine to put enough juice back
into the battery to get you through another 24-hour period. Your numbers
say that all you need is 5Ah per day to top off the battery, and that is
easy-peasy for the system you have. It only needs a little more than 10A
continuous to stuff 5Ah into the battery in a 30-minute run time. Should
work.
Consider using a hydrometer to measure the batteries' state of charge
every couple days before you run the engine to see if you're keeping up
with your consumption or not.
http://cruising.stuffiminto.com/electrical-system/136-testing-deep-cycle-batteries-specific-gravity-hydrometer.html
--
Mike "Rocket J Squirrel" Elliott
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
Bend, OR
KG6RCR
On 9/5/2009 2:03 AM Malcolm Stebbins wrote:
> I know very little about electricity! I checked the archives and I
> found this article, but it did not exactly answer my question:
> http://gerry.vanagon.com/files/motorhome/Motorhome-Alternator-Charging-Systems.txt
>
>
> I have 2, 6 volt deep cycle batteries hooked up and I have about
> 200-ish amps (I think) in the aux batteries. This is used to run my
> Waeco fridge, which takes about 7 amps when it runs, but only runs
> about every 10 minutes for 2 minutes-ish.
>
> 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
>
