Here is some info from a forum that may be of interest.

"ALTERNATORS _David Harwood_ One horsepower equals 746 watts. A
perfectly efficient alternator would require one horsepower of
mechanical input to produce 746 watts, but since there are friction
and eddy current losses it takes more than one hp to produce 746
watts. Dividing 746 by the 13.8 volts that most regulators are set up
for, one perfectly converted hp would equal about 54 amps. So a 55 amp
alternator that is actually putting out 55 amps will reduce the
engine's output by something more than one hp. But as the charge is
restored to the battery, the current goes down until it the battery is
fully charged and the alternator supplies only the minimal current
needed to run the instrumentation and spark a gas engine. At that
point the engine is able to put out its rated power. If you now turn
on the stereo/GPS/depth sounder/cabin fan etc, the alternator must
supply the current for those consumers, so the hp at the shaft will go
down. The advantage of a larger alternator is that it recharges the
battery faster (or a bigger one in the same time). One horsepower
equals 746 watts. A perfectly efficient alternator would require one
horsepower of mechanical input to produce 746 watts, but since there
are friction and eddy current losses it takes more than one hp to
produce 746 watts. Dividing 746 by the 13.8 volts that most regulators
are set up for, one perfectly converted hp would equal about 54 amps.
So a 55 amp alternator that is actually putting out 55 amps will
reduce the engine's output by something more than one hp. But as the
charge is restored to the battery, the current goes down until it the
battery is fully charged and the alternator supplies only the minimal
current needed to run the instrumentation and spark a gas engine. At
that point the engine is able to put out its rated power. If you now
turn on the stereo/GPS/depth sounder/cabin fan etc, the alternator
must supply the current for those consumers, so the hp at the shaft
will go down. The advantage of a larger alternator is that it
recharges the battery faster (or a bigger one in the same time). _Ted
Andresen, /Gypsy/, #75_ Just wanted to insert a change I made to my
alternator circuit that may be of value to others. When I installed
the alternator, I put a 50 amp meter in the output leg to measure the
charging current. I was surprised to find that my 50 amp alternator
rarely charged the batteries above 10 amps for more than a few
minutes. Then it would decrease to 5 or 8 amps depending on the number
of amp-hours I had drawn out of the battery on the previous day's
sail. I normally use 10 amp-hours at night. At 5 amps it would take 2
hours to put the 10 amp-hours back into the batteries. I did not like
listening to the engine, so I found a way to get more current out of
the alternator. I ran an extra line from the +12V side of the battery
through a 20 ohm 50 watt rheostat directly into the alternator's field
windings. That's where the voltage regulator also connects to the
alternator. Then I could adjust the rheostat and crank up the current
to 10 amps. I don't think it made much difference to the batteries,
since they didn't bubble (hydrogen gas) or get warm. Instead, of
charging them at 60 watts for 2 hours, I was able to charge them at
120 watts in half the time."

(Mic) wrote in :

The advantage of a larger alternator is that it recharges the
battery faster (or a bigger one in the same time).

Simply, unfortunately, NOT true. The chemical reaction of converting lead
sulphate back into lead metal plated on the plates is, unfortunately, a
slow reaction. The slower you charge it, the "better" it does the
conversion. Charging a 330AH golf cart battery at 120A is only surface
charging the plates so the voltage rises. It is also dangerous as 120A
shoved against a 2V cell produces something a little less than 240 watts of
heat and will boil the electrolyte in a few minutes.

Luckily, batteries and alternators are smarter than captains and wannabees.
The initial charge current from low cell voltage drops rapidly off the
alternator's current limit to something more reasonable, then tapers off
safely because the voltage regulator won't allow the voltage across the 6
cells to go over 14.2V, if it's setup properly. The 120A alternator is now
charging the batteries at 20A like it should....plus the load current the
boat is drawing at regulator voltage.

What current should the batteries be charged at, initially?

http://www.batteryuniversity.com/partone-13.htm

"It takes about 5 times as long to recharge a lead-acid battery to the same
level as it does to discharge."

"The charge current for small lead-acid batteries should be set between 10%
and 30% of the rated capacity (30% of a 2Ah battery would be 600mA). Larger
batteries, such as those used in the automotive industry, are generally
charged at lower current ratings."

Good advise.

Why boaters buy huge alternators to charge golf cart batteries remains one
of life's mysteries....

Larry

A charging system needs to be designed around the battery bank capacity and
type. On a sailboat you want to minimize the charging time and diesel fuel
so a big alternator is often the most efficient way to do it.

A large deep discharge wet cell bank should be bulk charged at about 25% of
capacity then taper off during the absorption stage and top off at a very
low rate. AGMs can take a slightly higher rate. I will be running four
L16-HC 6V batteries in a series/parallel configuration for a total of 850
amp hours at a nominal 12V. It will be operated at between 85% and 50% of
charge so the majority of the charging will be done in the bulk stage at
about 210 amps. The alternator is a 250 amp hot rated Niehoff so I can
charge at the maximum safe rate for about 80 minutes and still have power
left over to run the boat's systems.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Larry" wrote in message
...
(Mic) wrote in :

The advantage of a larger alternator is that it recharges the
battery faster (or a bigger one in the same time).

Simply, unfortunately, NOT true. The chemical reaction of converting lead
sulphate back into lead metal plated on the plates is, unfortunately, a
slow reaction. The slower you charge it, the "better" it does the
conversion. Charging a 330AH golf cart battery at 120A is only surface
charging the plates so the voltage rises. It is also dangerous as 120A
shoved against a 2V cell produces something a little less than 240 watts
of
heat and will boil the electrolyte in a few minutes.

Luckily, batteries and alternators are smarter than captains and
wannabees.
The initial charge current from low cell voltage drops rapidly off the
alternator's current limit to something more reasonable, then tapers off
safely because the voltage regulator won't allow the voltage across the 6
cells to go over 14.2V, if it's setup properly. The 120A alternator is
now
charging the batteries at 20A like it should....plus the load current the
boat is drawing at regulator voltage.

What current should the batteries be charged at, initially?

http://www.batteryuniversity.com/partone-13.htm

"It takes about 5 times as long to recharge a lead-acid battery to the
same
level as it does to discharge."

"The charge current for small lead-acid batteries should be set between
10%
and 30% of the rated capacity (30% of a 2Ah battery would be 600mA).
Larger
batteries, such as those used in the automotive industry, are generally
charged at lower current ratings."

Good advise.

Why boaters buy huge alternators to charge golf cart batteries remains one
of life's mysteries....

Larry

BTW, my charging system was figured out personally by Ol' man Surrette
himself and there are few people in the world who know more about wet cells
than him.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

Subject

The following was given to me by a Trojan application engineer who at
the time was older than dirt and had been at his trade for a long time.

1) Every 100 AH consumed from a wet cell battery requires 125 AH to
replace it.

2) From a design stand point, a battery bank will absorb on average,
about 15% of the total AH capacity, from the alternator over an extended
period.

Want to supply 150A from the alternator, you need a 1,000 AH bank to
accept it.
If you consume 500 AH and then want to replace them, you must supply
500*125%=625 AH which will require 625AH/150A=4.18 hours.

No matter how you cut it, wet cell batteries are not efficient, but they
are convenient.

Having myself been involved in the low voltage DC business for a while,
thought he gave me pretty good advice.

Lew

Lew Hodgett wrote in
nk.net:

Having myself been involved in the low voltage DC business for a while,
thought he gave me pretty good advice.

Lew


What always amazes me is how a sailboater thinks he can recharge his house
batteries in 20 minutes with a 500A alternator.....It just isn't gonna
happen!

--
Larry

"AGMs can take a slightly higher rate"

AGMs can be charged at a rate quite a bit higher than wet batteries. Gels
are 50% of AH rating, AGMs are at 100% or more.

Doug

"Glenn Ashmore" wrote in message
news:SpqRe.7787$dm.1553@lakeread03...
A charging system needs to be designed around the battery bank capacity and
type. On a sailboat you want to minimize the charging time and diesel fuel
so a big alternator is often the most efficient way to do it.

A large deep discharge wet cell bank should be bulk charged at about 25%
of capacity then taper off during the absorption stage and top off at a
very low rate. AGMs can take a slightly higher rate. I will be running
four L16-HC 6V batteries in a series/parallel configuration for a total of
850 amp hours at a nominal 12V. It will be operated at between 85% and
50% of charge so the majority of the charging will be done in the bulk
stage at about 210 amps. The alternator is a 250 amp hot rated Niehoff so
I can charge at the maximum safe rate for about 80 minutes and still have
power left over to run the boat's systems.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Larry" wrote in message
...
(Mic) wrote in
:

The advantage of a larger alternator is that it recharges the
battery faster (or a bigger one in the same time).

Simply, unfortunately, NOT true. The chemical reaction of converting
lead
sulphate back into lead metal plated on the plates is, unfortunately, a
slow reaction. The slower you charge it, the "better" it does the
conversion. Charging a 330AH golf cart battery at 120A is only surface
charging the plates so the voltage rises. It is also dangerous as 120A
shoved against a 2V cell produces something a little less than 240 watts
of
heat and will boil the electrolyte in a few minutes.

Luckily, batteries and alternators are smarter than captains and
wannabees.
The initial charge current from low cell voltage drops rapidly off the
alternator's current limit to something more reasonable, then tapers off
safely because the voltage regulator won't allow the voltage across the 6
cells to go over 14.2V, if it's setup properly. The 120A alternator is
now
charging the batteries at 20A like it should....plus the load current the
boat is drawing at regulator voltage.

What current should the batteries be charged at, initially?

http://www.batteryuniversity.com/partone-13.htm

"It takes about 5 times as long to recharge a lead-acid battery to the
same
level as it does to discharge."

"The charge current for small lead-acid batteries should be set between
10%
and 30% of the rated capacity (30% of a 2Ah battery would be 600mA).
Larger
batteries, such as those used in the automotive industry, are generally
charged at lower current ratings."

Good advise.

Why boaters buy huge alternators to charge golf cart batteries remains
one
of life's mysteries....

Larry

"Doug Dotson" dougdotson@NOSPAMcablespeedNOSPAMcom wrote in
:

AGMs can be charged at a rate quite a bit higher than wet batteries. Gels
are 50% of AH rating, AGMs are at 100% or more.

Doug


And will put that damned AGM wonderbattery into thermal runaway, too.

"9.2.7. If a wet battery becomes hot, over 125° F (51.5° C), or if it
violently gasses or spews electrolyte, turn the charger off temporarily or
reduce the charging rate. This will also prevent "thermal runaway" that can
occur with VRLA (AGM or Gel Cell) batteries if the battery temperature is
over 100°F (37.8° C)."

100F.....that's WARM, not hot! AGM batteries have little way to cool
themselves because the electrolyte has no convection flow all squeezed to
death in that coil of plates and fiberglass. Internal heating has to
migrate outward by conduction, alone.

AGM isn't any kind of magic....no matter what the slick salesman told you
to justify the awful price.

Here's some more charging information to stoke this firebox thread:
http://www.uuhome.de/william.darden/carfaq9.htm

--
Larry

Larry wrote:
(Mic) wrote in :


The advantage of a larger alternator is that it recharges the
battery faster (or a bigger one in the same time).


Simply, unfortunately, NOT true. The chemical reaction of converting lead
sulphate back into lead metal plated on the plates is, unfortunately, a
slow reaction. The slower you charge it, the "better" it does the
conversion. Charging a 330AH golf cart battery at 120A is only surface
charging the plates so the voltage rises. It is also dangerous as 120A
shoved against a 2V cell produces something a little less than 240 watts of
heat and will boil the electrolyte in a few minutes.

This may be true, but a slightly different scenario is reality. I use
4 Trojan T105's, for a total of 425 to 450 Amp-hours. My alternator
is a Balmar 110 Amp controlled by a Xantrex 2000R. If the bank is
discharged more than a third, the charge rate is about 100 Amps, and
it will stay over 80 Amps until its at about 85% full. If the bank is
down to 50%, its not uncommon to charge at the 100+ Amps rate for a
considerable time.



Luckily, batteries and alternators are smarter than captains and wannabees.
The initial charge current from low cell voltage drops rapidly off the
alternator's current limit to something more reasonable, then tapers off
safely because the voltage regulator won't allow the voltage across the 6
cells to go over 14.2V, if it's setup properly. The 120A alternator is now
charging the batteries at 20A like it should....plus the load current the
boat is drawing at regulator voltage.

Yes, it would be silly to have a large alternator on a 200 Amp-hour
bank, since it would be unlikely to ever charge at over 60 Amps.
However, it you have a 400+ Amp-hour bank, it works and it makes sense.

BTW, my first set of batteries gave 6 years of hard use - they were
probably done in more by being neglected last winter then by being
overcharged in the summer. But since the replacement cost for the
bank was $280, that's only about $46 a year.



What current should the batteries be charged at, initially?

http://www.batteryuniversity.com/partone-13.htm

"It takes about 5 times as long to recharge a lead-acid battery to the same
level as it does to discharge."

"The charge current for small lead-acid batteries should be set between 10%
and 30% of the rated capacity (30% of a 2Ah battery would be 600mA). Larger
batteries, such as those used in the automotive industry, are generally
charged at lower current ratings."

Good advise.

Why boaters buy huge alternators to charge golf cart batteries remains one
of life's mysteries....

It depends on what you call "huge" and how many golf cart batteries
you have. A 110 Amp alternator is a very good match for 4 T105's.