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Old 08-25-2009, 03:16 PM   #15
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Be advised if you are looking at 'Gel' cell batteries - they are not tolerant of charging voltages over about 14 volts - you MUST have a charging system that is designed to charge GEL cells, has a 'GEL' cell setting, or you will burn them up!

Ray & Pat; Morada, CA
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Old 08-25-2009, 06:01 PM   #16
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It's over 50 years since I studied DC circuits and ohms law. But I seem to remember that two 12 vot batteries in parallel will present a circuit voltage of twelve volts. The current flow is the same any where in the circuit loop except through the two batteries in parallel. . The current flowing is a function of the load applied and the voltage. With two batteries in parallel the load current drawn from each battery will be one half of the total load drawn. That's the benefit of two batteries, they last lomger between full charge and fully discharged. Maybe I've forgotten something. CharlieE

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Old 08-26-2009, 09:01 AM   #17
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re "I replaced my Univolt with an Intelli-Power 9245" - that seems to be about one of the best things you can do for better battery health over time. I have heard some confusing discussion about AGM's and the sulfation inhibiting 'equalization bump' by the IP - some say it is too much for AGM's, other say modern AGM's have no problem with it, and I have even heard that Progressive Dynamics has a special Charge-Wizard for AGM's. All hearsay so far from my point of view.

Gel Cells seem to have gone out of favor for RV use. Mexray's view is one reason. The Az Wind Sun FAQ and RV pages have some good stuff on this.

re: "DC circuits and ohms law" - the big kicker here when it comes to batteries is named Peukert. Yes, two batteries have twice the energy capacity of one and, in general for nominal RV use, the total energy capacity of the bank is the sum of the capacities of its component batteries. Where it gets interesting is that the 20 Hour energy capacity spec is a 60 watt load for a typical RV battery. That means the ratings for a 2 battery system are centered on a 120 watt load. That is high for 'old style' lights and fans and pumps but rather low for 'new style' entertainment systems and microwaves. That means that 'old style' use will show higher apparent capacity while 'new style' use will benefit from parallel batteries (like diesel engines in modern pickups) due to reduced impedance.
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Old 08-26-2009, 11:43 AM   #18
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I looked at the AZ Wind Sun FAQ. Their objection to GEL cells is primarily that they charge at a slower rate. So what? All wet batteries and every other type I know of with the possible exception of Lithium Ion will benefit from slow charging. The whole concept of a deep cycle battery is that it's designed for a long discharge period and you need to allow a long charging cycle. In a car/Off road vehicle/Class A RV where you're cranking an engine over, running a winch, using an inverter to supply AC voltage to a washing machine then it's a bigger issue.

They also mention that in really hot climates the GEL electrolyte can lose moisture and fail due to voids. Again, heat is the enemy of all batteries. They sell batteries but don't mention in the FAQ the cost differential of GEL and AGM. Even if they last 20% longer when abused (there's no evidence they won't last as long or longer with typical treatment, my experience has been longer) the AGM aren't as cost efficient. AGM does have a big weight advantage. If you're doubling the number of batteries and using the biggest ones you can find then that's an issue.

With two 6V batteries in series if one cell dies (the usual way a battery croaks) you're dead in the water. With two 12V batteries you're back to the capacity of one battery. Charging two batteries in parallel can be a bit tricky. A battery isolator, A/B/A+B is probably a really good idea.
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Old 08-26-2009, 10:34 PM   #19
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re "Charging two batteries in parallel can be a bit tricky." - not in my experience for small banks. The internal impedance of each battery tends to balance things out, even if the batteries are not well matched.

Wiring can be a concern, though, with any battery bank. Smartgauge has an analysis of this.

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