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Old 07-31-2015, 10:23 AM   #1
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Dual battery set up problem

I have 2 Crown Marine batteries: 20 AH Rate of 75, RC minutes of 105 / 25A

They are both new and hooked up to drain equally with the negative to one and positive to other with them connected together.

Last night started with a full charge. Ran TV off 12v with 300W inverter and watched movie with surround through entertainment unit. Maybe 2 hours. No heater all night, no lights, this morning had heater for 2 hours and TV for hour. Batteries went down to empty.

This seems a bit fast. Esp for not running heater all night like I could have.

Am I wrong?
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Old 07-31-2015, 11:09 AM   #2
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Are these Group 24 batteries or Group 27?

Regardless, if the two batteries were fully charged and then completely flat, I'd say that that is not what I'd expect given the energy your system used.

However. How are you determining the state of charge of the batteries? Are you using the on board led lights on the panel? If so, those led lights are not very informative. More sort of decorative. Sort of like the tank level indicators.

I use an inexpensive digital voltmeter to check my batteries; even that is not especially accurate, but much better than the led lights. The ideal is to measure the batteries when they have been allowed to "rest" for an hour or two, then check the charge with no load on the batteries.

I consider 12.7 volts charged; I discharge my batteries no lower than a reading of 12.1 volts, which is still about half charged; taking them lower can damage the batteries.

Here's a link to lots of battery information: Battery University
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Old 07-31-2015, 11:11 AM   #3
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The total of 3+ hours on the inverter is what ate up the bulk of your charge.


The TV and Stereo are 150+ watts 100% of the time they are running. That's about a 12A draw for 3 hours. Throw in the 7-8A draw of the heater fan....


Remember, stored electricity is not like gasoline. The top 10% of the charge is at at around 13V and then things go downhill. With liquid fuels (gasoline, Diesel, propane, etc.) every drop is the same as every other drop.


If you want to camp off the grid, either:
- Budget your battery power as the precious resource it is
OR
- Buy a generator
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Old 07-31-2015, 11:32 AM   #4
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batteries

Thank you both. I have 2 Honda generators so none of this is really an emergency

I bought the inverter on the advice of someone on the board when I discovered that the TV won't run on the batteries. I can see how it would pull the batteries down.

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Old 07-31-2015, 01:12 PM   #5
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are these 6V or 12V batts? The reason I ask is it sounds like you have the batteries connected in series: batt#1 + to the coach and - to + on batt2. batt2 - to coach ground.
I think that's how you'd connect a pair of 6V batts to get 12V. If you have 2 12V batts and that's how they're connected you're getting 24V.
Series vs Parallel
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Old 07-31-2015, 01:26 PM   #6
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His description does make for wondering if he connect 2 12V batteries in series. It caught my attention too.

1. I don't think they make 6V marine batteries.
2. If he had 24V he would have blown fuses and/or damaged equipment.

Must have been in parallel.

dangerdave, check this out:12 Volt Side of Life
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Old 07-31-2015, 02:06 PM   #7
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The 6 volt batteries are typically called golf cart batteries or something similar, but are deep cycle, typically with more reserve capacity than the same footprint 12 volt batt. Another idea would be to either see if you can hack your current TV's cord to run it on 12v or buy a 12 volt TV. That would eliminate the invertor and its significant power draw. Oh, and if you hook up 24 volts to a 12 volt trailer, it blows a lot of light bulbs (oh, the things you learn)
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Old 07-31-2015, 02:48 PM   #8
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batts

Hey guys,

12V batteries in parallel. with the TT connected to the neg on one battery and the pos on the other so they draw down equally.

I followed this set up on Youtube:


The only unusual thing I do is run that inverter NPower 300W Sine Wave Inverter. I run the TV and entertainment center DVD player. Probably for 3 hours. I know there's a calculation to tell me exactly what to expect but I'm not sure what it is.
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Old 07-31-2015, 02:56 PM   #9
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I think that the standard A = W / V would apply. If true, then 300 watts / 12 volts equals slightly less than 30 Amps. Another site said to divide by ten, but either way by running through the invertor it's a huge power drain. 30 amps power drain for three hours equals low or dead batteries, unfortunately. At least that is my understanding and what I have experienced.
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Old 07-31-2015, 02:56 PM   #10
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The calculations are fairly simple:


Multiply the amps drawn at 120VAC times 10. That'll be about how many amps you are drawing at 12VDC. (the actual DC amps is slightly higher because of losses from conversion). Multiply the amps at 12VDC times 3600 that product is the number of amp-hours you are using.


So 2A draw at 120VAC (24" Flat screen TV) is 20 amps from the battery. In an hour you will have drawn 72,000 amp-hours from your battery pack. Just to watch TV.


Not exactly efficient, is it?
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