Larry W4CSC wrote:

Steve wrote in :


Good info.


Measuring open circuit, unloaded voltage of any lead acid battery is no
indication of state-of-charge. If there's any acid left in it, it'll
measure a little over 2V per cell on all cells with some acid left.

Now, put a 20A load on a charged battery and a discharged battery. THEN,
you can see the charged battery hardly varies at all in voltage as the
discharged battery's INTERNAL RESISTANCE causes a substantial voltage drop
due to the lack of electrolyte to react with lead plates. When you remove
the load from the discharged battery, the voltage goes right back up,
unless you've used all the available electrolyte during the discharge.

The ONLY true way to measure the charge on a lead-acid battery is specific
gravity, compensated for temperature properly. Of course, in the sealed-up
sooper-dooper $500 "maintenance-free", this is moot point.

I always though these were called "maintenance - proof" - Matt


For the boater who has to have it all, there's:
http://www.cellcare.com/products_ser...e200/index.asp
even has a data logger to track cell state.
Comes with tracking software for your PC....(c;

A more practical means of measuring "How much is left?" is a Link 10 or 20
from Xantrex, available from Waste Marine or on the net.
http://www.xantrex.com/web/id/97/p/1/pt/5/product.asp
I've just installed another Link 10 in another sailboat after replacing 2-
year-old house batteries he deep cycled, regularly, into oblivion.
Installation is simple. Disconnect the negative terminal from the battery
bank, this means ALL the connections to the negative terminal, not just the
big cable. Put ALL the connections on the load end of the included 500 Amp
shunt. Connect a #2 battery jumper from the battery end of the shunt (not
included) to the negative end of the battery bank. This makes ALL the
current to and from the battery go through this current measurement,
including charging, please! There is a negative power connection to power
the Link 10/20 and two wires, one power and one voltage measurement that go
to the positive terminal (NOT THE BREAKER PANEL OR SOME OTHER PLACE JUST
FOR YOUR CONVENIENCE) of the battery bank. Now that we can measure battery
voltage and all the current in or out, the Link 10's little computer can
keep up with your amp-hour drain and charge, which is a fair way to measure
state-of-charge. But, it is NOT a power company. Battery amp-hour
capacity wanes with time, changes with temperature (which an accessory
temperature probe can make the Link compensate for, but isn't really
necessary.) So, fairly close, you can now discharge it to 50% of capacity,
even set a accessory alarm (not included) to warn you of impending doom to
get the engine charging cranked up. It's close enough for boats and not
thousands of dollars for a commercial gravity meter.

Link or any amp-hour meter runs all the time, so connect its power leads
directly to the battery with a multiconductor cable through two fuses...one
for volts sensing and one for power (2A in the manual). The thing comes
set to turn off the display in factory default to make you feel better, but
all it does is turn off the LED display. It uses no appreciable power and
would take a hundred years to discharge a set of L-16 monsters, probably
using less power than the battery's natural leakage. I always want the
display LEDs on so owners can glance over to see how many AMPS we're
drawing NOW all the time. Battery power lasts longer when they see the
amps screw up from turning all the lights on like at home. It makes them
more self-concious of power usage...There's a 4-bar "meter" the computer
provides that goes from green to yellow to red as the bars drop to make him
feel even more guilty...(c;

Matt Colie wrote in
:

I always though these were called "maintenance - proof" - Matt


Excellent! I totally agree....AGM, GelCell, car batteries....