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Old 01-18-2016, 10:05 AM   #1
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Battery voltage drops to 8 volts then returns to normal

We have a 2002 HR Vacationer, with 2 each 12 volt house batteries wired in parallel, with no inverter and a 30 amp power converter. Connected to good shore power, we have recently had some instances where the house voltage will instantly drop to 8 or so volts, with very little load on the system, only a few lights. The lights will go dim, I'll look at the voltage on the control panel and see the voltage is well below normal and then hop outside to the batteries to start the troubleshooting. I put my voltmeter on the batteries and sure enough they read 8 volts, I then read the output of the power converter and it reads 12-13 volts, then I go back to read the batteries and they are back to 12-13 volts, and now the lights are back to normal brightness. This all happens in about 1-2 minutes. The batteries are clean and connections are tight. This has now happened at least 5 times but the condition doesn't exist long enough to figure out the cause. Any suggestions or ideas?
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Old 01-18-2016, 10:19 AM   #2
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One or both batteries are probably bad. There may be a circuit breaker between the charger and batteries. The battery acts up and the circuit breaker does its job and opens to relieve the load. The voltage is normal on the converter because of the open circuit to the batteries. The breaker cools off and hits the battery with a sudden inrush of current and wakes up the bad battery.

Any battery going from 13.5 to 8 volts, assuming you are measuring on the center of the posts versus the connections, is bad.

I had a car battery do that and you could set your watch by the time interval. In twenty minutes the voltage would drop like a rock. Jump start the vehicle and it acted fine until turned off for 20 minutes.

Batteries can do some very strange things. I have diagnosed thousands of battery related issues in 40 years and I learned to never say never.

Even if the batteries are new they can fail. They have many mechanical connections inside and can be bad from the factory.
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Old 01-18-2016, 11:30 AM   #3
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Pull the shore cord and check the volts.

If it is 8 volts YC1 is correct.

If it's 12.6 or more, you may have a big short.

Can't imagine what would draw down that much, though. Battery cable to generator, maybe.
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Old 01-18-2016, 07:07 PM   #4
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Thanks for the ideas. I plan to pull the batteries out and have them load tested. Question, as long as I'm on shore power can I operate on the power converter only for a day or two to see if I get a voltage drop without the batteries? I was thinking like you twin boat, the I have a big short somewhere or I have something coming on for a minute then kicking off due to the low voltage, like the bay heaters which by the way are off.
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Old 01-18-2016, 07:12 PM   #5
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Quote:
Originally Posted by skbrla View Post
Thanks for the ideas. I plan to pull the batteries out and have them load tested. Question, as long as I'm on shore power can I operate on the power converter only for a day or two to see if I get a voltage drop without the batteries? I was thinking like you twin boat, the I have a big short somewhere or I have something coming on for a minute then kicking off due to the low voltage, like the bay heaters which by the way are off.
Yes, power converters are designed to supply 12 volts, up to their rated amps.

Did you check the battery volts with the shore cord plug out ?
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Old 01-19-2016, 07:58 AM   #6
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Could also be a bad connection.

Check voltage at converter and battery as well as across connections to confirm all.

Do not remove wires from battery until you confirm connection good by placing one meter lead on post and other on wire then turn lights on or have helper try to start it.
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Old 01-19-2016, 10:14 AM   #7
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A short severe enough to drop the batteries to 8 volts would be very easy to find. Just find where the smoke is coming from.

Since you are measuring on the batteries this leaves out a poor connection down the line which would not be a short but a resistive connection or an (open).
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Old 01-19-2016, 02:03 PM   #8
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Okay reread op and it sounds like 2 problems.

I understand you are on shore power and lights go dim and while dim you check batteries and measure 8 volts then everything comes back to normal 13 volts.

Is this correct?

If you had an intermittent converter AND bad batteries you could have this.

First let's review basic battery.

Imagine a big gulp full of water no ice and a straw.

Takes a bit of time to drink all of the water.

Now ask it with ice and you can drink it all fairly fast.

As a battery ages it can sulfate or degrade where the capacity is diminished.

It will take and hold a charge just fine and hydrometer indicates all is well but due to smaller effective plates the capacity is just a fraction of normal.

So in your case...bad battery connected to shore power and battery floating.

Converter steps working and batteries supply load but not for long.

Converter comes back on and due to batteries being bad voltage comes right up.

A good battery discharged to 8 volts will take all the current your converter can supply and require quite some time to rise back up.

Tasks to do.

Check batteries for water and have them tested.

Look for loose connections between converter and 12 volt power center.

Check your disconnect system for loose connections.
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Old 01-21-2016, 11:30 AM   #9
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I have the exact system that you have. I would maintain a reading of 12.6 and then it would drop to about 8.5. then back up to 12.6. I first started looking at batteries and the were fine. Cleaned all terminals and found the negative cable loose. Tighten it and problem solved. Hope this helps.
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