Actually a 12 volt battery when fully charged isn't 12 volts. A fully
charged 12 volt battery just off the charger will read above 13 volts and is
considered fully charge down to about 12.8 volts or so (when checked several
hours later). A volt reading of 12.5 to 12.4 is getting around 75% and 12.2
it is 50% or just below. When at 12.0 it is about 25%. Below 11.9, it is
dead and could be history.
To do a quick check of a battery with a volt meter, first give the battery a
good charge. Then unhook the battery from the system so there is no chance
of any draw and after 12 hours check the voltage. It the battery holds the
voltage above 12.8, then it is in excellent shape. If it falls below 12.5 or
12.6 after 12 hours, then it needs further checking and may need
replacement. Of course the only really accurate way to check a battery is to
take it to a shop and have it load tested.
Since I use AGM batteries, I can't check the specific gravity, but a
fully-charged wet-cell battery should have read around 1.265 and a 75%
charged battery will read 1.222. When a battery falls below 1.222 it is
subject to sulfation or hardening of the plates and this eventually destroys
the ability of the battery to generate power.
I usually try to keep my batteries above 12.5 as any battery has a limited
number of charge cycles before it needs to be replaced. Full discharge
cycles will age a battery faster than when they are not allowed to drop much
below 50%. So age and use is a big factor in how a battery will perform.
NOTEI assume your terminals are clean and all your writing and connections
are solid. This could have a significant impact on how well an engine turns
over. Diesel engines, even small ones, take more power to turn over because
of the high compression, so any drop between the battery and the starting
circuit will have a significant effect..
_____
From:
AlbinVega@yahoogroups.com [mailto:
AlbinVega@yahoogroups.com] On Behalf
Of vegatern
Sent: Tuesday, November 07, 2006 7:53 PM
To:
AlbinVega@yahoogroups.com
Subject: [AlbinVega] Battery charge
I'm confused. My batteries show, with the bulb tester, that they have
a 75% charge but with the volt meter, when I make contact with the
terminals, they both show about 12.20 to maybe 12.70 volts. Shouldn't
that be enough to start the engine? The starter and engine turn over
but not enough to get up and go. I sail a couple of times a week but
only run the engine in and out the harbor, so thinking it wasn't long
enough to re-charge the batteries I ran her for a couple of hours, but
it made no difference in either reading. A reading of over 12 volts
on the voltmeter would indicate the batteries are good, right? The
engine is an MD7A Thanks
Craig Tern #1519
