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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]
Comments: To: Michael Elliott <j.michael.elliott@GMAIL.COM>
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


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