Good Sam Club Open Roads Forum: Checking voltage on connected 6 volt batteries
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 > Checking voltage on connected 6 volt batteries

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LeBout

Ontario, Oregon

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Posted: 03/31/19 04:50am Link  |  Quote  |  Print  |  Notify Moderator

opnspaces wrote:

Yes, depending on what you're testing for. If you want the batteries state of charge you need to disconnect the shore power cord. But then you should turn on a load like the furnace and let it run for about 2 minutes first to remove the surface charge from the shore power.


Should the reading be taken with the furnace still running or after it has run and is now off?


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MFL

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Posted: 03/31/19 05:45am Link  |  Quote  |  Print  |  Notify Moderator

To answer your question...check after it shut off. When a battery has been charging, then just disconnected, it will show a higher state of charge. If you have time to wait, battery at rest, no draw/no charge, for several hours, that is a good time to test. As mentioned, a short load will remove the recent high charge reading, so you don't have to wait to test.

Jerry





Vintage465

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Posted: 03/31/19 07:02am Link  |  Quote  |  Print  |  Notify Moderator

LeBout wrote:

Question: when on shore power, do the batteries need to be disconnected from the trailer before putting a voltage tester on them?


I'm gonna say yes. I can think you'd be getting an accurate reading if you show charge voltage from the converter.


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opnspaces

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Posted: 04/03/19 06:28pm Link  |  Quote  |  Print  |  Notify Moderator

Yes as MFL posted. The batteries will have an abnormally high voltage right after you take them off the charger. But that is what is called a surface charge. It has no real substance and will deplete quickly. So you either have to let them sit with no load or charge for a few hours to dissipate the charge before taking a reading. Or apply a decent load to pull the surface charge off.

With a car the recommendation is to turn on the headlights with the engine off for one to two minutes to dissipate the surface charge. Then turn off the lights and you can take your voltage readings. Since the trailer doesn't have headlights and most interior lights are LED's the furnace is the easiest way to put a decent load on the batteries.

Another way to look at this is picture your battery as a tank of water with the water level indicating the state of charge. Now blow a big soap bubble on top of the water (this is your surface charge). It will suddenly look like there is a lot more charge in the tank. But of course the bubble has no substance so if you touch that bubble it will pop and leave you with the true level in the tank.


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LeBout

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Posted: 04/03/19 09:25pm Link  |  Quote  |  Print  |  Notify Moderator

opnspaces wrote:

Yes as MFL posted. The batteries will have an abnormally high voltage right after you take them off the charger. But that is what is called a surface charge.


Good information. Thank you.

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