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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems > 13 plus volts on rested battery?
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Thread: 13 plus volts on rested battery? Reply to Thread
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Topic Review (Newest First)
04-02-2013 06:43 PM
jorn85
Re: 13 plus volts on rested battery?

The best way to test the battery is first it needs to be charged up, and then check if it meets the reasonable load. Again recharge the battery and verify if it holds the charge.
12-29-2012 06:50 PM
Maine Sail
Re: 13 plus volts on rested battery?

Quote:
Originally Posted by btrayfors View Post
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
I still find that even with Midtronics equipment you really need to start with a baseline. Once you have a basleine on a "broken in" bank future tests are far more accurate.

Problem with a 20 hour test is that it has to be performed as a 20 hour test and at 77-80F. I built my own tester using a Victron monitor, Omron cube relay and a load. Still not 100% accurate as the current changes slightly when the voltage does but it is close enough for horseshoes.

*Bring battery to 77F
*Charge battery to full
*Program & reset Battery monitor
*Apply 20 hour load for battery (100Ah battery = 5A, 125Ah battery = 6.25A)
*Set Victron internal relay to cut test at 10.5V
*Read Ah's removed from battery

Anything less than a true 20 hour capacity test is only an approximation. I do prefer a Midtronics with "baseline" as it is much easier to track performance. Problem with any analyzer without a baseline is that many batteries produce better CCA or MCA than what they are rated for when new and broken in.

Testing batteries for capacity when cold is going to be inaccurate as Ah capacity goes down when cold. The 20 hour load will actually be higher to the battery capacity than it would be at 77F. In other words that 5A load at 30F is more like a 10A load at 77F.

Always best to mimic the industry test which is Ah capacity divided by 20 = load, until the bank reaches 10.5V. So (100Ah / 20 = 5A load). It should go 20 hours before hitting 10.5V if the battery is hitting its rated capacity. Counting the Ah's consumed before 10.5V is attained will tell you the Ah capacity as the battery sits currently.
12-29-2012 05:10 PM
btrayfors
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
12-29-2012 01:43 PM
Bene505
Re: 13 plus volts on rested battery?

Quote:
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
12-29-2012 10:19 AM
Maine Sail
Re: 13 plus volts on rested battery?

Quote:
Originally Posted by Rockter View Post
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....
12-29-2012 08:39 AM
Rockter
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).
.
12-25-2012 10:33 PM
Barquito
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.
12-20-2012 04:46 PM
Maine Sail
Re: 13 plus volts on rested battery?

Quote:
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"..

12-20-2012 04:24 PM
brokesailor
13 plus volts on rested battery?

Excellent question Wilson! I am wondering the same thing.
12-20-2012 02:04 PM
jameswilson29
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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