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post #1 of 12 Old 12-20-2012 Thread Starter
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13 plus volts on rested battery?

I decided to leave my batteries on the boat this winter. Soon after putting the boat up on the hard, the batteries were both at about 12.6 - 12.7 volts. I charged both of them. Now after sitting for a few weeks, I read about 13.5 volts in both. Is this normal? They are clean and dry, with the cables removed. My volt meter was also left on the boat. Would it get wacky at 35 F?

Batteries really make me feel dim.

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post #2 of 12 Old 12-20-2012
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Re: 13 plus volts on rested battery?

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Originally Posted by Barquito View Post
I decided to leave my batteries on the boat this winter. Soon after putting the boat up on the hard, the batteries were both at about 12.6 - 12.7 volts. I charged both of them. Now after sitting for a few weeks, I read about 13.5 volts in both. Is this normal? They are clean and dry, with the cables removed. My volt meter was also left on the boat. Would it get wacky at 35 F?

Batteries really make me feel dim.
While 13.5V is high, and I suspect your volt meter is not quite as accurate as it should be, it is not at all uncommon for batteries in cold weather to take a loooooooong time to self discharge or reach a "resting voltage". The 24 hour "rest" period often talked about applies to 77F..... The colder it gets the longer it takes for the batteries to dissipate the "surface charge".. Batteries can "hang out" in cold temps at over 13V for quite a while and it may take 4-8 weeks+ depending upon temperature for them to even get to a "resting voltage"... When cold the chemical reactions come to a near complete halt thus the battery self discharge in cold temps does too.

I have a battery in my shed right now that I have been monitoring for over three weeks and as of a few minutes ago it was still at 12.96V as measured with a calibrated Fluke 179.. This battery is full at 12.71V to 12.72V.. So even after three weeks, in Maine, in the cold, this battery is still a full 0.25V over "resting full" voltage. Three weeks is a long way from a 24 hour rest to determine SOC..

This is why I have been saying for a very, very long time that leaving batteries on-board, fully charged and 100% disconnected can actually be better for them than storing them in a heated basement.

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post #3 of 12 Old 12-20-2012
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Re: 13 plus volts on rested battery?

I disconnected my solar panel controller when my boat was hauled and reconnected it for my trip down the Bay last week. In the summer, my batteries will charge to above 14 volts with the panel and the alternator. Last week, they were down in the mid 12s voltage. Is there any problem leaving them connected to the solar panel all winter?
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post #4 of 12 Old 12-20-2012
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13 plus volts on rested battery?

Excellent question Wilson! I am wondering the same thing.
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post #5 of 12 Old 12-20-2012
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Re: 13 plus volts on rested battery?

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Originally Posted by jameswilson29 View Post
I disconnected my solar panel controller when my boat was hauled and reconnected it for my trip down the Bay last week. In the summer, my batteries will charge to above 14 volts with the panel and the alternator. Last week, they were down in the mid 12s voltage. Is there any problem leaving them connected to the solar panel all winter?

No problem until it becomes a problem.. By that I mean leaving it connected is fine provided you have a controller that does float but if the panel becomes occulded with snow or ice the controller can actually kill the battery.

These controllers almost always have a small mV draw to them for "self power".. When you disconnect batteries EVERYTHING must be 100% disconnected. Often the easiest method is to break the neg connection to the battery bank. If you disconnected the solar panel but left the controller connected to the battery bank you likely sucked it down..

Here's a prime example of a "disconnected" bank that is not really appropriately "disconnected". The large neg leads have been removed but on the house bank a small back neg wire for the bilge pump is still connected. This boat has an "electronic" bilge switch (self consumption about 0.01A or 1.68Ah's per week) that type of load can drain the batteries over time if that small wire is not also disconnected.... Be sure to really "disconnect"..


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post #6 of 12 Old 12-25-2012 Thread Starter
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Re: 13 plus volts on rested battery?

Thanks Maine Sail. That is good to hear. I'll compare my volt meters to see what is real. Too bad Wisconsin winters won't preserve my body as well as my boat batteries.

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post #7 of 12 Old 12-29-2012
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Re: 13 plus volts on rested battery?

Barquito :

It is notoroiusly difficult to judge a battery condition by measuring the battery voltage, unless the battery is completely dead at (like) 12V or something similar.

Over the years, I have found that the only test is to charge it up, and work it to see if it meets a reasonable load, then re-charge it and see if it holds charge.

When your motor is charging, the battery voltage should be at about 14.1 V (or near to).
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post #8 of 12 Old 12-29-2012
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Re: 13 plus volts on rested battery?

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Barquito :

It is notoroiusly difficult to judge a battery condition by measuring the battery voltage, unless the battery is completely dead at (like) 12V or something similar.

Over the years, I have found that the only test is to charge it up, and work it to see if it meets a reasonable load, then re-charge it and see if it holds charge.

When your motor is charging, the battery voltage should be at about 14.1 V (or near to).
.
This is correct. A "rested" open circuit voltage is very accurate to determining SOC however. What it is NOT capable of is telling you what "capacity" you have left in the battery.

You can have a 100Ah battery reading 12.72V rested that will supply 100Ah. You can also have a 100% full 100Ah battery reading 12.72V that only has 60Ah of capacity left in it. Both batteries are at 100% state of charge but one now has less capacity due to use.

The only true measure of capacity is to do a controlled 20 hour capacity test at 80F... The Midtronics analyzers are also fairly accurate for looking at aging, if a when new baseline was taken, but they cost $600.00 to $2000.00... They still don't do a 20 hour "capacity" test or directly correlate over to capacity....

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Last edited by Maine Sail; 09-14-2014 at 05:36 PM.
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post #9 of 12 Old 12-29-2012
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Re: 13 plus volts on rested battery?

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Originally Posted by Maine Sail View Post
This is correct. A "rested" open circuit voltage is very accurate to determining SOC however. What it is NOT capable of is telling you what "capacity" you have left in the battery.

You can have a 100Ah battery reading 12.72V rested that will supply 100Ah. You can also have a 100% full 100Ah battery reading 12.72V that only has 60Ah of capacity left in it. Both batteries are at 100% state of charge but one now has less capacity due to use.

The only true measure of capacity is to do a controlled 20 hour capacity test at 80F... The Midtronics analyzers are also fairly accurate but cost $600.00 to $2000.00... They still don't do a 20 hour "capacity" test..
Maine,

I'm starting tosuspect that my 6 year old 4D Gell cells have lost some capacity. Is there a simple test I can do to determine the remaining capacity?

I do have the Victron battery monitor that you recommended, so measuring AHs removed is easy. I do leave the batteries on the boat in the winter, on a solar panel. The boat is on the hard, currently at about 30 degrees.

I don't want to do too deep a test and hurt the batteries. Not looking for super high accuracy, within the nearest 5 or 10 AHs would be good enough.

I'm thinking I could let the batteries sit disconnected for a week, noting the curent charge status (AHs already removed) and the voltage. Then I'll reconnect the negative leads and run some lights to get the batteries 30 AH down from where they were. Let them sit disconnected for a week and note the voltage.

The before and after voltage should tell me how much I've gone down in the % charge status. The rest would be simple math.

Would that work?

Regards,
Brad
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Re: 13 plus volts on rested battery?

Brad,

As noted above and elsewhere, the only reliable tests of remaining capacity are either a 20-hour test or the use of a modern (and expensive) tester like the Midtronics series.

Gels are tricky. I've got two golf-cart size gels in my basement shop right now which are pushing 15 years old, the first 10 in use on a sailboat. Been using them for testing and for utility the past five years. One of these still measures more than 90% of original capacity; the other measures a bit lower. I use a Midtronics MDX-650....much easier than the 20-hour tests, but I do those, too.

FWIW,

Bill
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