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 Is my battery fully charged?		  New Topic  Topic Locked
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Bruce Baker
Captain

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USA
402 Posts
Initially Posted - 04/11/2005 :  09:18:09  Show Profile
How can I tell if my battery is fully charged (or nearly drained)? It was fully charged at the end of last season (the charger's green light came on). But how can I tell how fully charged it is now?

Thanks,
Bruce Baker
Falls Church, VA
"Yee Ha" 3573
'83SR/SK
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ClamBeach
Master Marine Consultant

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3072 Posts
Response Posted - 04/11/2005 :  10:03:35  Show Profile
There are two common ways to test battery charge..

1) Measuring the voltage across the battery terminals
2) Use a hygrometer (available at auto supply stores) to measure the specific gravity.

About halfway down the following page you'll find a state-of-charge table.

http://www.e-marine-inc.com/articles/battery_charging.html

If your battery has been sitting for 6 months without any maintenance charge, it's probably pretty well drained (they self-discharge). Letting a battery sit in a discharged state will radically shorten it's service life. "Battery Pal" and similar 'smart-maintenance' chargers are available starting around $20.

If your boat storage area doesn't have any power available, it's best to pull the battery and take it to a location where you can put a maintenance charge on it... or periodically charge it with a conventional charger.
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JimB517
Past Commodore

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USA
3285 Posts
Response Posted - 04/11/2005 :  10:54:13  Show Profile  Visit JimB517's Homepage
voltage across the terminals should be 13.6 fully charged.
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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts
Response Posted - 04/11/2005 :  11:38:34  Show Profile
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by JimB517</i>
<br />voltage across the terminals should be 13.6 fully charged.
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Is it 13.6 or 12.6?

I thought batteries were designed at 2.1v per cell (6 x 2.1v = 12.6v)
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JimB517
Past Commodore

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USA
3285 Posts
Response Posted - 04/11/2005 :  18:06:21  Show Profile  Visit JimB517's Homepage
13.6

Your charger should bring the system up to 14.1

With no charger, turn all loads off and you should see 13.6.

12.5 is 50% discharged approx.

12.25 is 75% discharged.

12.0 is dead.

If your system voltage does not come to full voltage use the battery meter to read the specific gravity of each cell. If one is too low, the battery is no good.
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Sloop Smitten
Master Marine Consultant

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USA
1181 Posts
Response Posted - 04/11/2005 :  18:09:17  Show Profile
Jim,
How do you read specific gravity of each cell with a battery meter?
Joe
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JimB517
Past Commodore

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USA
3285 Posts
Response Posted - 04/12/2005 :  00:01:27  Show Profile  Visit JimB517's Homepage
well, when I said "specific gravity" and "battery meter" I was refering to one of those turkey baster like devices you can get at Pep Boys or the like. Pull off the caps and take a sample of the acid in each cell.
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Ericson33
Admiral

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USA
892 Posts
Response Posted - 04/12/2005 :  02:25:08  Show Profile  Visit Ericson33's Homepage
Cant you just test the battery like a 9 volt :)
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sterngucker
Navigator

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USA
169 Posts
Response Posted - 04/12/2005 :  04:56:15  Show Profile
Yes, but only once.
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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts
Response Posted - 04/12/2005 :  07:30:52  Show Profile  Visit Frank Hopper's Homepage
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Capri25</i>
<br />Cant you just test the battery like a 9 volt :)
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
If my tongue could reach both terminals at the same time I would be sailing a much larger boat!

Chris, did you get my email?
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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts
Response Posted - 04/12/2005 :  10:46:20  Show Profile
From the Exide website...

<b><i>How do I determine the state-of-charge of a deep cycle battery?</i> </b>

A graduated-float hydrometer will display the battery’s specific gravity level. A fully-charged, flooded electrolyte deep-cycle battery will have a specific gravity of 1.265 or higher. Keep in mind that some hydrometers require temperature correction to 80° F. You can check the specific gravity only in batteries that are of the "flooded" acid design with removable vent caps (accessible cells). Using a voltmeter is another option that can be used to check a battery's state of charge voltage. A fully charged deep cycle, lead acid battery will have a voltage of 12.6 volts or higher when measured across the terminals. If the battery is being discharged during the voltage test or has been discharged recently, you will read a lower voltage or state of charge than is actually available. On the other hand, if your battery is being charged when tested or has been charged within a 24 hour period, a "surface charge" effect will play a role indicating a higher voltage or state of charge than is actually available.
Some manufacturers sealed design batteries are "flooded" types and some are valve regulated lead acid (VRLA), AGM or Gel types. If you are checking a sealed or non accessible battery, it is impossible to measure the specific gravity in these battery types. Here you will have to accept the voltage meter reading as your only indicator of state of charge voltage. When using a voltmeter, if a battery has been charged or discharged in the past 24 hours, it helps to let the battery sit disconnected to "stabilize" for 24 hours. This reading will be your most accurate reading with a voltmeter.

NOTE:

Standard "flooded" lead acid batteries state of charge (SOC) is as follows:

12.6=100%, 12.4=75%, 12.2=50%, 12.0=25%, 11.7=0%.

Sealed AGM or Gel VRLA batteries have a state of charge (SOC) as follows:

12.8=100%, 12.6=75%, 12.4=50%, 12.0=25%, 11.8=0%
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