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Old 08-12-2018, 03:27 PM   #1
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12V in parallel, will they charge?

New WD 1790 owner here with like next to zip RV experience, so bear with me. I had wanted to install 2 6 Volt batteries wired in series to give me a bit more electricity duration, but find the front end / tongue being so small, in combination with the contour of the actual trailer wall, leaves me a bit short - the 6 volt batteries are just a bit taller tun the 12 V, so they will not fit, fully boxed up and safe.

Had my handy neighbor craft a bracket (pictured below) so I can mount a 2nd 12V battery on the tongue assembly with the idea of wiring these two batteries in Parallel. As long as the trailer power wires are long enough their shouldn't be an issue, I hope.

Question is, will the batteries grouped together like this continue to allow trickle charge while on hookup power, and as I tow down the road with my fully equipped Ford F-150?
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Old 08-12-2018, 04:12 PM   #2
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Assuming your power consumption stays the same, you shouldn't notice a difference.

You're doubling your amp-hours of battery capacity so, all things being equal, two will take twice as long to charge from the same % discharge as one. On the other hand, for a given level of usage, they will discharge only half as much which will compensate in the other direction.

For example, two 80 amp hour batteries discharged to 80% capacity will take twice as long to charge to 100% as a single, 80 amp hour battery discharged to 80% capacity.

On the other hand, the amount of use necessary to discharge the single 80 amp hour battery to 80% will only discharge two 80 amp hour batteries to 90% of capacity.

Therefore, in a perfect world, two 80 amp hour batteries will recharge from 90% to 100% in the same amount of time as a single 80 amp hour battery will recharge from 80% to 100%.
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Old 08-12-2018, 05:44 PM   #3
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Suggest you jumper the first battery to second one with 0 gauge cables instead of 2 gauge.

Your OEM +12 and ground is terminated on the first battery and there will be tiny bit more resistance between the battery charger / loads to the 2nd battery than to the OEM first one. So the first will take lightly more charge and deliver slightly more current than the 2nd one. Not much difference, but you want to minimize that difference by using large gauge cables.

Best practice for parallel 12 volt batteries is to have the +12 cable on battery #1 and the -12 (Ground) on battery #2 and then jumpers + to + and - to - that way the same amount of wire is being used for each battery and they are "balanced".

That said I don't think you need to go to the trouble to do this unless you make more changes and put in a big 1000 watt or more inverter. Then I would do it.
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Old 08-12-2018, 06:55 PM   #4
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Quote:
Originally Posted by powercat_ras View Post
That said I don't think you need to go to the trouble to do this unless you make more changes and put in a big 1000 watt or more inverter. Then I would do it.
Randy, I was with you all the way until your inverter comment. If you spend any time camping away from shore power, additional battery capacity is a good thing with or without an inverter. This is why I've always used golf cart batteries in my travel trailers and now my motorhome. What am I missing?
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Old 08-12-2018, 08:08 PM   #5
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Voltage drop thru 12 volt cables increases with current. The most current flow to/from the batteries in a WD 1790 exclusive of a large inverter will be in the 45 amp range. So the voltage drop across the jumpers from battery 1 to battery 2 are tolerable with 0 gauge wire in that situation.

Now add a large inverter, and say it's tied across battery 1. For example I run a 1,500 Watt Microwave off my large inverter and the 12 volt draw when it's running is 140 amps, over 3X more. Then the in-balance becomes a problem if the battery cables are not balanced.

Yes 2 six volt golf cart batteries in series is the best affordable solution for a given amount of space IMHO. But the original poster stated they would not fit. So my advice is trying to make the best of the situation.
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Old 08-13-2018, 07:40 AM   #6
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Randy,

Thanks for the clarification. I mis-read your first post. I read your comment "That said I don't think you need to go to the trouble to do this" as referring to adding an additional battery, not beefing up the cables, therefore concluding that you recommended he didn't need the additional battery unless he added the larger inverter.

Mea culpa.
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Old 08-13-2018, 11:36 AM   #7
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Is it a big deal if the batteries are different sizes - long and short? All wired up and I read 12.8 volts
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Old 08-13-2018, 01:41 PM   #8
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12.7v is fully charged, so 12.8v is about right:

https://www.solar-electric.com/learn...ery%20Voltages

Here's why it's not a good idea to combine different battery sizes:

https://www.homepower.com/articles/s...eries-parallel
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Old 08-13-2018, 01:53 PM   #9
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They are wires in parallel, so looks likes its not totally unacceptable. From your quoted article :
"Connecting two banks with different capacities in parallel is technically fine since the batteries will be operating at the same voltage. Charge and discharge current will be shared, based on capacity. It is best if the batteries are of the same type and age. For example, avoid combining a sealed (gel or absorbed glass mat) battery with a flooded (conventional) battery because they have different charging setpoints. Broadly speaking, you can parallel batteries without problems, and the charge controller will look after them. Just make sure you give them plenty of charge. If the system tends to operate at less than a full state of charge, adding new batteries to old will probably just result in the old ones pulling the new ones down and everything getting sulphated."

Both of my batteries are new. Thanks for helping out.
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