Date: Sun, 25 Jun 2006 09:55:02 -0700
Reply-To: Michael Elliott <j.michael.elliott@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Michael Elliott <j.michael.elliott@GMAIL.COM>
Subject: Making an alternator a smarter charger? [LVC]
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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
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