Date: Sun, 25 Jun 2006 17:55:25 -0400
Reply-To: Dennis Haynes <dhaynes@OPTONLINE.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <dhaynes@OPTONLINE.NET>
Subject: Re: Making an alternator a smarter charger? [LVC]
In-Reply-To: <449EBFE6.5060702@gmail.com>
Content-type: text/plain; charset=us-ascii
I have stated many times in the past that the alternator is not the best
method for charging a deep cycle battery. However, the situation is not
that bad. Let me try to explain things a bit further.
Most of the capacity of a battery is returned during the "Bulk" part of
the cycle,(~80%). The current delivered is usually limited by the size
of the charger. The charger should be selected to charge the battery at
an 8-10 hour rate. Now, here is where it gets confusing. An alternator
tries to deliver a fixed voltage, not a specific amount of current. This
voltage is needed to feed all the other loads. If the wiring and
controls are up to snuff, the alternator will probably bulk charge that
group 31 battery at ~ 25-30 amp/Hours, (3-4 hour rate). Guess, what,
that is to fast for long battery life. The cell voltage will rise
quickly and meters will indicate the battery is at a higher state of
charge ten it really is. Now the alternator can't be told to lower the
current because than system voltage can drop and this will cause all
sorts of problems with accessories and lights.
Now using an alternator, we really go from bulk to float. If we float
long enough, it is not an issue to skip absorption as long as we
periodically equalize. 13.8 volts is basically the ultimate float
voltage as it can be withstood almost indefinitely by a healthy battery
as long as the electrolyte is regularly checked. Really smart chargers
will drop the float down to ~13.2
Now for that absorption stage, this is where that alternator controller
shines, however, for automotive use it can have some problems. One
thing, maintenance free starting batteries don't need or like 14.8
charge cycles. Neither do all your lights unless you have voltage drop
issues. Imagine 15 volts on your accessories. The alternator won't like
that load for long also.
For absorption and equalize charging, get a good charger and use it when
shore power is available. The will keep the battery healthy and happy.
If you're driving for extended periods, the float charge will work and
the bulk porting will work the alternator enough. Make sure you have a
good ground directly to the alternator.
My motor home has a 2,500 watt inverter/100 amp charger with true 3
stage operation. On occasion, we will start the generator to operate the
roof AC's while driving. When that charger starts reaching 14.8 it does
a number to all the indicator lights through out the coach. Same problem
if I pull into a campground and plug in while still using everything.
Fix is to change the profile to limit absorption to 14 volts if I am
using stuff. Guess, what, that brings me almost back to basic float
charging.
OK, so what am I saying? That is a great device and it has its
advantages. For Auto/RV use some of those advantages are limited. For
your money, if the battery you have still does not do the job, this
device at best will add ~10-15%, so use your money for more battery
capacity or do with less power.
Dennis
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf
Of Michael Elliott
Sent: Sunday, June 25, 2006 12:55 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Making an alternator a smarter charger? [LVC]
Hi electrical and lead-acid wizards,
I've installed a nice Group 31 flooded cell deep-cycle "house" battery
under the bench seat in Mellow Yellow, along with all the requisite
hefty 6-gauge wiring to reduce voltage loss, and a sturdy relay to
disconnect the house from the engine battery when the engine is not
running. I'd like the house battery to be fully-charged when I pull into
camp, so I've been looking at charging algorithms for lead-acid
batteries and it's clear that an engine alternator is not a very smart
charger.
There are three common stages to charging a lead-acid battery: Bulk,
Absorption, and Float.
http://www.rpc.com.au/products/batteries/car-deepcycle/carfaq9.htm has
some nice charts that show the stages for several types of batteries.
Roughly speaking (I think I have this part correct), after a battery has
been discharged (in the case of the engine battery, this would be due to
starting the engine, in the case of a house battery, this might be after
a day or so of running lights and other loads) the first stage, Bulk,
dumps current into the battery at a fairly high rate until the battery
reaches about 14 to 14.8 volts (depending on battery type). The current
is then tapered as needed to keep the battery from exceeding this
voltage; this is the Absorption stage, and it is here where the battery
is getting "topped off." It is also heating and releasing gas. After a
period of time, an hour or two (according to battery design), the
voltage is dropped to about to a float voltage of 13 to 13.8 volts
(depending on battery type and temperature) to keep from overheating the
battery and damaging it. This Float stage maintains the charge in the
battery that was achieved during the Bulk and Absorption stages. There
is also an optional Equalizing stage which is good for some battery
types -- the battery is occasionally hit with higher voltage to mix
things up and keep the battery happier longer.
That's the (rough) idea. Near as I can tell, alternators are not real
smart battery chargers. They are designed to put out the same voltage,
all the time. They have a high current capacity, when running at higher
rpms, so they do a good job with the Bulk portion of stuffing a charge
into the engine battery when the engine is at high rpms (not at idle),
but they don't have separate Absorption and Float charging stages. If
you want to fully-charge your house battery by setting the alternator to
14 to 14.8 volts (using, maybe, one of those Barton Harness adjustable
regulators that mount on the alternator [thanks Mark Drillock]), the
battery will start to overheat after a few hours (depending on battery
type) because it never drops into the lower voltage Float stage. To
avoid that, the alternator can be set to output 13 to 13.8 volts, but
then the battery never gets that Absorption stage that fully charged it.
So I'm looking Ample Power's Next Step voltage regulator:
http://www.amplepower.com/products/ns2/index.html
It monitors the output voltage of the alternator, and adjusts that
voltage according to how you set its Absorption Voltage, Absorption
Time, and Float voltage trimpots. It also accepts a thermal sensor to
monitor the temperature and modify charging voltages accordingly. It
looks pretty cool to me -- does anyone have any experience with it, or
with a similar device?
--
Mike "Rocket J Squirrel" Elliott
71 Type 2: the Wonderbus
84 Westfalia: Mellow Yellow ("The Electrical Banana")
74 Utility Trailer. Ladybug Trailer, Inc., San Juan Capistrano
KG6RCR
