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Old 01-24-2011, 08:08 AM   #29
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Both voltage and specific gravity measurements need to be properly interpreted for conditions. Measuring specific gravity is unnecessary for most RVers due to the hazmat requirements that should be followed as well as the contamination risks.

The reason most auto shutoff or genset auto starts use 11.5 volts or less is because they assume that the battery is under load when they are called (else why call on the auto or safety function?)

For voltage measures, if you take the measure after the battery has had no significant charging or discharging for at least a half hour, you should be able to use that chart posted above and be 'close enough' for all practical purposes. Note that this is state of charge and not battery condition. You can assess battery condition by watching the voltage as the battery responds to a load. The smartgauge device does this way. Old timers just watch how incadenscent bulbs dim when the furnace or water pump comes on and use their experience.

As for a couple of batteries (i.e. either a 6v or 12v bank of about 150#) handling a furnace - 150# of battery has about 1.5 KwH of usable energy. The furnace runs about 100 watts. That means about 15 hours of runtime. When you add in a few lights and maybe a water pump and such things, it can put a good strain on the battery. Some folks haven't quite learned how to squeeze a battery while off grid camping -- give them time!

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Old 01-24-2011, 09:15 PM   #30
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Your solar system should be bringing your batteries up to 14.8 volts in bulk charge before switching to absorption mode. Your generator should start at 11.9 to 12.0 volts. If you constantly let your batteries go below a 50% discharge they will die prematurely.

Every boondocker should have a Trimetric battery monitor. It will tell you the true state of your battery charge and/or discharge. Without proper instrumentation you are guessing and do not know what is happening in your system.

Ralph & Snickers
2006 3500 Chevy Dually - 8.1 - Allison
2006 30' New Horizon - Solar
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Old 01-24-2011, 10:03 PM   #31
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re: "Every boondocker should have a Trimetric battery monitor." -- I understand the desire but let's not go overboard, what say? Electron counters, like the Trimetric are getting smarter but they are still secondary measures that need proper programming and provide a precision that is way out of line with their accuracy and, thus, can be misleading.

RV solar systems very seldom do adequate bulk charging. By that I mean getting sufficient current through the battery to adequately work on sulfation (not many RV's allow that big a solar system for their battery bank size). That is why many off grid folks use a genset to provide a bulk charge in the morning and then let the solar take care of the absorption stage.

14.8v for a bulk charge target is rather high for an RV. If not carefully controlled, it can cause plate corrosion.

re: "Without proper instrumentation you are guessing and do not know what is happening in your system. " -- when it comes to batteries, even with proper instrumentation you are still guessing. That is because factors such as temperature, cycle to cycle variation, age, and use profile can each influence available battery energy by more than ten percent.

One of the biggest problems I see in these battery discussions is that of metrology and proper understanding and interpretation of measure.
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Old 01-26-2011, 04:55 PM   #32
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We had a 04 Alpine. We would boondock for 2-3 weeks (for about 3 house each time) at a time and only ran the generator every 5-6 days. Bank of 6 deep cell batteries. Had a 125 solar system. Lots of sunshine in summer months, did not need lights in PM for the most part. Only gas frig and water pump used batteris.
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Old 02-01-2011, 08:40 AM   #33
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Folks we are talking deep cell batteries. They are made to hold and give power over an extended period of time. A cranking battery is made for a burst of high amps over a short period of time.

Look at all the charts you want but here is my experience first hand with deep cell batteries.

I have been running bass boats for years with trolling motors that totally drain a deep cell battery in a days use. These batteries if kept properly will last 3 to 4 years before degrading to a point that you need to replace them to get a full day's charge in them. But they still will take a charge. Many boaters when replacing the boat deep cycle take the old boat batteries and put them in their camper.

What kills a battery is heat. Amps and heat have a direct relationship. If you run your trolling motor continuously on high you are pulling high amps and it will overheat a battery or in an extreme case even melt the posts, this will destroy your battery quickly. So in your RV if you need AC, Microwave, block heaters, water heater, space heaters, or anything that generates heat and brings your amp meter over 20 amps for more than a minuite you should have the generator running.
My goal at night when sleeping is under 5 amps. Which is the fan on the heater and the light bulb in the water bay along with a few lights.

The trick to longer lasting deep cycle batteries is to keep them charged and do not subject them to really high sustained discharges.

An AGS is an awsome tool for keeping batteries topped off. Someone told me they had a problem with the cost but if it will extend the life of your batteries, and limit the hours on your generator and the need for service then it pays for itself really quickly. It also keeps you from waking up in the middle of the night and having your OCD force you to check the charge level indicator.

So replace that ceramic heater in the water bay with a 40 watt light bulb, change your interior bulbs to LED's. Leave the factory preset on your AGS alone, 11.2 amps will not kill your deep cell batteries.

Above all check the water level in your batteries often and use only distilled water in them.

Once you have fed your amp OCD you can then work on your Water OCD. And to make it all fun you can even spend a bit of time on your Propane OCD.

An RV'r is someone driving a $300,000.00 Motorhome towing a $40,000.00 car and looking for a free place to park.
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