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Old 07-15-2011, 05:19 AM   #1
Rivet Master
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2005 30' Classic
Burlington , Ontario
Join Date: Sep 2008
Posts: 2,265
Battery charging when connected to shore power.

Here's something I've wondered about for a while. Maybe someone who knows more about this subject can set me straight!

In addition to having a meter and low voltage alarm for 110vac in our trailer, I also have a 12v digital meter that I leave plugged into a cigarette lighter plugged in the trailer just so I can now what goes on.

It invariable sits at around 13.7 - 13.8 volts for as long as we are connected to shore power.

What I'm not sure about is whether this same voltage is being seen across my battery terminals or not at all times - I recall putting a meter across the battery on one instance and did measure the same voltage.

I had the feeling that IF this voltage is always present across the batteries it might not be a good thing on a long term basis, so what I have done is to put those quick disconnect knob terminals on my batteries.

If we are on shore power for just a couple of days, I leave the connected, but if I know we will be on shore power for a week or so, then after a day or so, I disconnect the batteries for the balance of our stay.

Anyone know if this is a worthwhile thing to do for longevity of my AGM batteries or am I just wasting my time?

My trailer is a 2005 classic 30, and the charger is the original "Parallax" that came with it. I'm not sure if that qualifies as what folk call a
"smart" charger of if is just of average IQ! I'm sure I have heard that there are better ones, but wasn't really thinking to change t until/unless it becomes defective.


Brian & Connie Mitchell

2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.
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Old 07-15-2011, 07:31 AM   #2
Rivet Master
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2007 30' Classic
Oswego , Illinois
Join Date: Jul 2010
Posts: 8,191
I'd get rid of the Parallax. The TRUE 3 stage chargers will still show 13.XX voltage, but they control AMPERAGE much better. It's not the voltage that kills (within reason), it's uncontrolled amperage.

My Iota will show 13.8 volts (after a charge cycle), but is only trickle maintaining at 100 - 200 mA. (above any parasitic load present)

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Old 07-15-2011, 09:38 AM   #3
Wise Elder
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2010 30' Classic
Vintage Kin Owner
South of the river , Minnesota
Join Date: Dec 2009
Posts: 4,020

In reality much depends upon temperature. The 13.7 output from modern Parallax converters is a compromise voltage that is on the high side for a true "float" state and on the low side for rapid charging. At around 60 degrees and below the battery will not charge at all at 13.7 volts. At higher temperatures overcharging is possible, particularly above 90 degrees or so where battery life is already reduced.

With flooded cells it's enough to just top off the electrolyte from time to time.

With AGMs that isn't possible so maybe in hot weather it is indeed a good idea to disconnect them. I wouldn't worry too much in cold weather.

The best available multistage converters are temperature compensated to deal with this better.
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Old 07-15-2011, 10:12 AM   #4
Rivet Master
1975 29' Ambassador
Reno , Nevada
Join Date: May 2003
Posts: 1,351
One of the best upgrades you can do for your batteries is a converter that is a multiple stage battery charger plus a modern battery maintainer that keeps a full charge and inhibits sulfation.

The Intellipower with Chargewizard, WFCO, and some others seem to fit the bill.

These will use a charging voltage that is safe for attached appliances (usually about 14.4v) and then drop to a float voltage like you've seen. After a while, they'll drop to a maintenance voltage (about 13.2v) to avoid boiling off electrolyte. While in maintenance mode, they will periodically bump the voltage for a few minutes to assure a top charge and stir the electrolyte to inhibit sulfation.

The maintenance mode is critical for those RV's that sit for weeks or months between outings.
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