On Sat, 20 Dec 2003 19:39:04 +0100, "Ric" wrote:

I have just bought a couple of Delphi Freedom marine deep cycle batteries
for my service bank. Is there a recommended minimum voltage below which one
should not discharge? Also, if the nominal capacity is (say) 100Ah, is that
measured between standard voltages or is it measured from a nominal full
charge (with full charge voltage dependent on the charging system) until
totally discharged - ie 0v? Or is there a standard "never go below" voltage
which manufacturers use?

A dead battery with 6 working cells will show over 12V with no load.
Voltage is not an indication of charge unless you have a standard
load, like the 50A "battery tester" used by mechanics to test
batteries and alternators. Mine cost $19 from Harbor Freight made by
Chinese slaves.

Lead-acid batteries' charge status is shown by a temperature
compensated specific gravity, usually a float inside a glass "baster"
with a thermometer to measure the electrolyte temperature and a
compensating graph to correct the reading on the float. Of course, in
our haste, we sealed up, or at least made it too inconvenient to
measure in most modern batteries. The specific gravity of a fully
charged lead-acid battery cell is 1.270 at 72F. As the battery is
discharged, the heavy acid is converted into lead sulphate as it eats
the plates. The acid is used up, we hope, before it eats
irreplaceable holes in the lead plates we cannot recharge. As the
acid is used up, the resistance of the electrolyte increases, causing
the voltage drop under load you see on your voltmeter. A "discharged"
battery is never discharged fully. If it were, the electrolyte would
be a near insulator preventing us from recharging it! We consider a
specific gravity of 1.150 at 72F to be "discharged" as far as is
prudent. You'll notice the voltmeter drops awful as you load a
battery in this condition, trying to pull electrons through this much
resistance, so you recharge it, IMMEDIATELY if you know what's good
for you and your battery. Lead acid batteries should NEVER be left in
a discharged condition. The lead sulphate in solution in the diluted
acid can be "charged" apart and redeposited on the lead plates
bringing us back to life. IF we let the lead sulphate sit, quietly,
without agitation, it CRYSTALIZES into lead sulphate solids, which
falls out of the solution into the bottom of the battery. This is
BAD. It's very stable this way and won't dissolve so we can charge
it, ever again. Because the acid was used up to create it, when you
charge the battery, there's little acid recovered and little metallic
lead deposited back on the plates. When the acid is mostly used up,
everyone says you have a "dead cell" which won't charge much, and we
trade in the battery for a new one. Recharging immediately reduces
this crystalizing to a minimum and your battery may last for many more
years.

The only real way to measure "charge" is with a hydrometer calibrated
to measure the specific gravity. Even the little float balls
hydrometer is better than a voltmeter as the voltage depends on load
current. By the time the voltmeter drops with no load, it's too late.

The best other way is with an ampere-hour meter you can buy at marine
stores like Waste Marine. It had a counter that multiplies amps times
hours as you charge the battery, then measures amps times hours when
you discharge the battery. It reads out directly in ampere-hours
remaining from the little computer inside or has lights to show E to
F. Of course, it says BOAT on it so the price is tripled, as usual.

I have an intelligent battery charging system on my boat that charges up to
a threshold 14v with an asymptotically decreasing charging current, and a
battery controller with which I can set alarms to warn me of impending doom
on the discharge cycle. At what level should I set the alarm to get best use
out of my batteries?

The less far you discharge lead acid batteries, the longer they will
last but the less power you get to use out of them before recharging.
It's a tradeoff. As the voltage measured is load dependent, it's very
hard to come up with a readable voltage as your load changes. If
there's no load on the discharged battery, it'll read 12.7V, even
though it's discharged as far as it should go. As you add load, this
voltage drops rapidly. So, the best way to judge is to watch the
voltmeter as the load is increased. You'll soon learn to judge when
it's dropping too far for comfort.

The other problem is the way the stupid voltmeters are
connected......at the panel on the other end of the wires carrying all
that current. The voltmeter DOESN'T measure the battery voltage. It
measures the voltage at the breaker panel its mounted in, which is
stupid. As the boat ages, its connections naturally become corroded.
Corroded terminals have increased resistance. When you pull current
through corroded terminals, the connections, ANY connections, cause
the voltage to drop back at the panel where the meter is....even
though the poor battery is fully charged! This worsens as time goes
by, so you think, from the reading, the battery is toast. When you
buy a new battery, the terminals all get cleaned so the voltmeter
problem goes away. You credit the new battery. I, on the other hand,
am standing at the battery boneyard with my 50A test load meter
picking up some real bargains (free!) before the salvors get
there....(c; Thanks!

The voltmeter should be connected by separate, small wires, directly
to the battery under test. Put a 1A fuse in series with the small
wires in case there's a short that would melt them and start a fire.
Without the load current going through the connections the meter is
reading from, you can read the battery voltage, not the panel voltage.
They do it their way because, as usual, it's CHEAP. It's called
"remote sensing" in the power management biz....

I like the "asymptotically decreasing charging current" charger. So
don't boat supply places! Boat is the only place I know where you can
buy a 10A charger for $200....OUCH! Another bad idea is a charger
that's TOO BIG, whether it's "asymptotically decreasing charging
current" or not! You should not charge a lead acid battery over about
25% of its capacity rating....i.e. 25A on a 100AH battery. Even that
is too much as it charges. Regular battery chargers, the cheap ones,
taper off the charge by simply running from a 15V power supply. As
the battery charges, it's voltage rises rapidly at specific gravity
about 1.250, so the battery voltage comes up to the charger's natural
voltage and the current drops a lot. But there's a real problem
charging lead-acid batteries....HEAT.

If I shove 50A through any device with 14V of voltage drop, battery or
load, heat is generated....50 X 14 = 700 watts! When the battery is
charging, from dead, notice how it doesn't get warm until the charge
is nearly full. The energy you're shoving in is being converted to a
chemical change, pulling lead sulphate apart into lead ions and
creating sulfuric acid by a reaction with the hydrogen in water. This
takes up the power. But, as it reaches full charge, we've converted
all the lead sulphates into lead and acid and there's nothing else to
convert, so the current ends up causing HEAT in the cells. If we kept
charging it at 50A, a constant current charger like you use to charge
NiCd or Ni-Metal Hydride batteries, 700 watts would MELT THE PLATES,
which are lead and soft anyways! That's why we want the charge to
taper off, not remain constant. If melting plates EVER touch, the
cell, of course, shorts and you get sprayed with acid in the ensuing
explosion! If you ever see a battery not properly encased explode in
a boat, you'll not soon forget what it does to EVERYTHING in the boat.

The best charger for lead acid batteries is the simply tapering
charger that has automatic shutoff from a voltage measuring circuit.
The charger turns off at 14.2V and back on again at 13.2-13.5V to
replace what you use. Car chargers aren't any good because they are
open to corrosion and flooding. A lot of the mumbo-jumbo in $600 boat
chargers is simply to justify charging $600 for a charger that costs
$60 to make. What I DON'T like about the fancy chargers is all the
NOISE they transmit at the dock to tear up the radios, cellphones,
stereo and TV. Lionheart has a dual 10A charger that just TEARS UP
the HF radio spectrum so bad our HF SSB is totally useless when it's
on. The old saturable reactor charger monsters, like the one my
captain gave me out of Lionheart, a 40A simple 3-step manual French
charger from Amel, makes no noise at all in my shop-stepvan...(c;

Well, hope this did you some good. We have dual 700AH banks of tall
golf cart batteries for house batteries on Lionheart. POWER is our
friend...(c;

Larry
Chief Engineer
S/V Lionheart
Charleston

Sitting under my ham radio & computer desk are 7 special cells in
series, giving me about 16 VDC when the float charger is off. They
are 900 AH "Edison" cells made of Nickel and Iron plates suspended in
Calcium Hydroxide, a base not acid. Ni-Fe cells, properly cared for,
last a lifetime as the reverse chemical process of charging them is
complete. The oldest cell, taken out of a very old Holiday Inn when
their antiquated manual-switchboard telephone system was replaced is
stamped July 8, 1936! The newest cell is 1948. This powers my
station in time of emergency, long enough to get the generators fired
up, even my computer's UPS. I've had them since the mid 1960s when an
old friend, long dead now, gave them to me. He was the Innkeeper, and
a ham.

Too bad we can't have Ni-Fe batteries any more. The nickel is BAD for
the environment as the battery companies that made them (Exide) just
dumped the electrolyte into the ground in many places, like Sumter,
SC, for instance, and the government busted 'em. Going from no load
to 100 amps, drops the stack's voltage about .8V if the terminals are
clean.....amazing power source from the turn-of-the-century.....(c;


Larry W4CSC

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