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Date:         Mon, 29 Oct 2007 08:29:46 -0700
Reply-To:     Michael Elliott <camping.elliott@GMAIL.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Michael Elliott <camping.elliott@GMAIL.COM>
Subject:      Charging deep cycle batteries with the alternator
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

We have a largish Trojan 130 amp-hour auxiliary battery under the bench seat in our Westy, Mellow Yellow, and use solar power when camping to keep it charged up (see http://camping.elliott.googlepages.com/aboutoursolarpowerrig for details if curious).

Our solar charge controller is designed to charge deep cycle batteries to full capacity, but the engine alternator is not. Starting batteries have thin lead plates so they can deliver a lot of current quickly, and in routine usage they don't get very deeply discharged. The alternator is designed to provide a constant voltage (usually around 13.8V or so) and a lot of current in order to quickly recharge the starting battery. It's an effective match.

But the alternator isn't very good for recharging deep cycle batteries with their thick plates. They are best charged by multistage charges which take the battery through several steps of voltage and current in order to wring the best performance and operating life, and they like to be taken to more than 14 volts to completely remove sulfite deposits from the plates. The best an alternator can do it take the deep cycle to around 85% to 90% of full charge.

In Mellow Yellow we would routinely arrive in camp with the battery still 16Ah low (as indicated on the Xantrex Battery Monitor -- that's 12% low for a 130Ah battery), whereas the solar charge controller, a three-stage design, takes the battery to 100%, if given enough sunlight and time.

I'd prefer to arrive in camp with a topped-off battery: having an extra 16Ah gives a little extra freedom in not having to deploy the solar panels so soon, it might allow an extra day of camping in overcast conditions, and that missing 12% matters more if we're trying to avoid discharging the battery below, say, 50% to help prolong its life.

So I've been looking for a way to properly charge the deep cycle battery from the alternator while driving and I think I've found a good solution: a four-stage DC input charger which operates off the alternator. It's made -- or distributed -- by Powerstream and the model I'm trying is their PST-BC1212-15 (http://www.powerstream.com/DC-input-chargers.htm).

It operates at input voltages ranging from 10 to 15VDC, and provides up to 15A peak charging current (switchable to 7.5A) and trimpot adjustable for custom values (the online specs don't mention 15A peak current option, but it's there), with trickle, bulk, fill, and float charge functions. It also has low input voltage protection, high input voltage protection, over temperature protection, output short circuit protection, and battery polarity protection. It does everything except get you elected Queen of the May!

In a brief test with an idling engine (13.7 volts from the alternator) the charger took the 10% discharged deep-cycle up to 100% in about an hour. It started by putting in more than 16A @ 14.4 volts, then pulled back the current as the battery reached full charge and went into float mode. Seems to work just peachy.

With just the alternator I wouldn't normally bother to try to charge the battery from the idling engine but with this gadget it's good to know that I can fully recharge our deep cycle when away from 120VAC, parked in camp on a rainy day.

I've mounted the charger in the secret space forward of the potable water tank (the compartment behind the AC outlet and circuit breaker) and will see how it performs over the course of the next few trips.

-- 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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