"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...
snip


The other possibility is that due to a deeper discharge of one battery,
it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.
Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The amount
of charge required controls the charge time. Whether OP is running his
radio off one battery or both makes no difference to the ah used, and
therfore no difference to the charge time (except, of course, that if he
uses a converter he will be consuming more ah for a given usage than if he
uses just one battery).

Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the partially
charged battery can get more charge is by overcharging the other. That
overcharge is "energy" which cannot be stored in the battery, but has to go
somewhere, and that somewhere is either/or elctrolysing the battery water to
oxygen and hydrogen, and heat. If the discrepancy is small, it probably
doesn't matter, but if not, then it certainly does.

Forget for a moment the issue of connecting a 12 volt load
to a 24 volt system.

Consider only the problem of properly charging two 12 volt
batteries in series with a single 24 volt charger. My
understanding is that unless the two batteries are identical
(goes way beyond same make or capacity) they will not be
charged to the same voltage. Two electrically non-identical
batteries will have different internal resistances (by
assumption)and thus charge to different voltages. As I
mentioned in an earlier post, Ample Power shares this
understanding and offers a 24 volt charger that is in
reality two 12 volt chargers, one connected to each of the
two 12 volt batteries (which are in series, of course).

If my understanding is incorrect, that would mean that a 12
volt marine battery and 8 AAA's in series would all be
properly charged by a 24 volt charger (with current limited
so as not to destroy the AAA's). An attempted reductio ad
absurdum.

My point is simply that with a real-world installation of
two 12 volt batteries in series, the two batteries will
probably measure different voltages. The greater the
differences between the batteries, the greater the potential
problem (no pun).

Without knowing the internal resistances, etc. of the two
batteries, it is not possible to know the precise effect of
placing a different load on one of the batteries. Doing so
would effectively place the added load resistance in
parallel with the internal resistance of the affected
battery. That could make the two batteries either more or
less alike, depending on the values involved.

All of this is hand-waving, of course, because a battery's
internal resistance is really not a constant, but a function
of the battery's chemistry, state-of-charge, etc. It is a
non-linear resistance, in that it varies with current. I
don't believe we miss the mark here by ignoring these
factors, however.

Regards,

Chuck









Pete Styles wrote:
"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...

snip

The other possibility is that due to a deeper discharge of one battery,
it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.

Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The amount
of charge required controls the charge time. Whether OP is running his
radio off one battery or both makes no difference to the ah used, and
therfore no difference to the charge time (except, of course, that if he
uses a converter he will be consuming more ah for a given usage than if he
uses just one battery).


Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the partially
charged battery can get more charge is by overcharging the other. That
overcharge is "energy" which cannot be stored in the battery, but has to go
somewhere, and that somewhere is either/or elctrolysing the battery water to
oxygen and hydrogen, and heat. If the discrepancy is small, it probably
doesn't matter, but if not, then it certainly does.

On Sat, 16 Apr 2005 10:12:46 GMT, "Pete Styles"
wrote:


"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...
snip


The other possibility is that due to a deeper discharge of one battery,
it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.
Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The amount
of charge required controls the charge time. Whether OP is running his
radio off one battery or both makes no difference to the ah used, and
therfore no difference to the charge time (except, of course, that if he
uses a converter he will be consuming more ah for a given usage than if he
uses just one battery).

Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the partially
charged battery can get more charge is by overcharging the other. That
overcharge is "energy" which cannot be stored in the battery, but has to go
somewhere, and that somewhere is either/or elctrolysing the battery water to
oxygen and hydrogen, and heat. If the discrepancy is small, it probably
doesn't matter, but if not, then it certainly does.


Pete is right on the mark. Reread his prior post. Also go back and
read Bruce's post on the subject. It is bad news to tap a battery
bank.

You can get away with it for a time but it will shorten battery life
because the two batteries do not get charged equally.
As Pete says, "in order to get the battery that is used the most
charged fully you must overcharge the other". There is no way around
that.

Overcharging a battery not only cause water loss but it can warp the
plates in the battery among other things.

Overcharging is the biggest killer of batteries.
Just ask anyone that still uses a feroresonant charger. They will tell
you batteries last only a couple of years.

Do it properly and use the inverter.

regards
Gary

"Gary Schafer" wrote in message ...
On Sat, 16 Apr 2005 10:12:46 GMT, "Pete Styles"
wrote:


"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...
snip


The other possibility is that due to a deeper discharge of one battery,
it
takes longer for the total voltage to reach the point where the charger
goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.
Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The amount
of charge required controls the charge time. Whether OP is running his
radio off one battery or both makes no difference to the ah used, and
therfore no difference to the charge time (except, of course, that if he
uses a converter he will be consuming more ah for a given usage than if he
uses just one battery).

Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the partially
charged battery can get more charge is by overcharging the other. That
overcharge is "energy" which cannot be stored in the battery, but has to go
somewhere, and that somewhere is either/or elctrolysing the battery water to
oxygen and hydrogen, and heat. If the discrepancy is small, it probably
doesn't matter, but if not, then it certainly does.


Pete is right on the mark. Reread his prior post. Also go back and
read Bruce's post on the subject. It is bad news to tap a battery
bank.

You can get away with it for a time but it will shorten battery life
because the two batteries do not get charged equally.
As Pete says, "in order to get the battery that is used the most
charged fully you must overcharge the other". There is no way around
that.

Overcharging a battery not only cause water loss but it can warp the
plates in the battery among other things.

Overcharging is the biggest killer of batteries.
Just ask anyone that still uses a feroresonant charger. They will tell
you batteries last only a couple of years.

Do it properly and use the inverter.

regards
Gary

I disagree. Two 12v batteries in series to make a 24v battery will self-level
with respect to charge.

They will self level even if one is tapped to run a VHF. No harm will come
to either battery. Voltage can be likened to pressure.

I hope this helps.

CN

"Capt. Neal®" wrote in message
...

"Gary Schafer" wrote in message
...
On Sat, 16 Apr 2005 10:12:46 GMT, "Pete Styles"
wrote:


"James Hahn" wrote in message
news "Meindert Sprang" wrote in message
...
snip


The other possibility is that due to a deeper discharge of one
battery, it
takes longer for the total voltage to reach the point where the
charger goes
from bulk to float. So while for the more empty battery the charge is
still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.
Deeper discharge of some cells in the string will not, of itself, mean
that the charge takes longer. The _difference_ doesn't matter. The
amount of charge required controls the charge time. Whether OP is
running his radio off one battery or both makes no difference to the ah
used, and therfore no difference to the charge time (except, of course,
that if he uses a converter he will be consuming more ah for a given
usage than if he uses just one battery).

Go and read my last post again. The charge doesn't magically share itself
according to need between the two batteries connected in series. Instead,
the charging current supplies equal charge to both batteries. If one
becomes fully charged before the other, then the only way that the
partially charged battery can get more charge is by overcharging the
other. That overcharge is "energy" which cannot be stored in the battery,
but has to go somewhere, and that somewhere is either/or elctrolysing the
battery water to oxygen and hydrogen, and heat. If the discrepancy is
small, it probably doesn't matter, but if not, then it certainly does.

Pete is right on the mark. Reread his prior post. Also go back and
read Bruce's post on the subject. It is bad news to tap a battery
bank.

You can get away with it for a time but it will shorten battery life
because the two batteries do not get charged equally.
As Pete says, "in order to get the battery that is used the most
charged fully you must overcharge the other". There is no way around
that.

Overcharging a battery not only cause water loss but it can warp the
plates in the battery among other things.

Overcharging is the biggest killer of batteries.
Just ask anyone that still uses a feroresonant charger. They will tell
you batteries last only a couple of years.

Do it properly and use the inverter.

regards
Gary

I disagree. Two 12v batteries in series to make a 24v battery will
self-level
with respect to charge.

They will self level even if one is tapped to run a VHF. No harm will come
to either battery. Voltage can be likened to pressure.
I hope this helps.

CN


Capt. Neal thinks I'm wrong.
I rest my case!
:-)
P.

"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...
snip

There are two possible scenarios: one battery wears more quickly than
the
other, eventually reaching a state where the cell voltages reach higher
values due to increasing internal resistance. Thus the charges shuts off
too
early, leaving the better battery not topped up.
The charger does not monitor cell voltages. It monitors the voltage
across
all the cells in series.

That is exactly what I meant and exactly the reason why it can go wrong. So
I would like to invite you to shoot holes in my following argument:

The charge of a battery is the product of current x time.

Both batteries are in series and one load is connected to the set, operating
at 24V. Another load is connected across only one battery, operating at 12V.
So it is evident that one battery is discharged more than the other.

I start to charge the set in series, so the current through both batteries
is exactly the same. Since one battery is discharged more than the other and
the current throug both is the same, one battery must be charged longer that
the other. Exactly how am I going to achive that with the same current
through both batteries?
One battery will reach the full state before the other but is still being
charged with full current because the other battery hasn't reached the
voltage that corresponds with full charge.

Now, if you can come up with an valid argument why this shouldn't damage one
battery, I bow and take my hat off.

Meindert

Hello Meindert,

I agree with your analysis and go a step beyond: even if
there is no differential load across one of the batteries,
uneven charging is possible (or likely) because of
differences in the batteries themselves.

Regards,

Chuck

Meindert Sprang wrote:
"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...

snip

There are two possible scenarios: one battery wears more quickly than

the

other, eventually reaching a state where the cell voltages reach higher
values due to increasing internal resistance. Thus the charges shuts off
too
early, leaving the better battery not topped up.

The charger does not monitor cell voltages. It monitors the voltage

across

all the cells in series.


That is exactly what I meant and exactly the reason why it can go wrong. So
I would like to invite you to shoot holes in my following argument:

The charge of a battery is the product of current x time.

Both batteries are in series and one load is connected to the set, operating
at 24V. Another load is connected across only one battery, operating at 12V.
So it is evident that one battery is discharged more than the other.

I start to charge the set in series, so the current through both batteries
is exactly the same. Since one battery is discharged more than the other and
the current throug both is the same, one battery must be charged longer that
the other. Exactly how am I going to achive that with the same current
through both batteries?
One battery will reach the full state before the other but is still being
charged with full current because the other battery hasn't reached the
voltage that corresponds with full charge.

Now, if you can come up with an valid argument why this shouldn't damage one
battery, I bow and take my hat off.

Meindert

"Meindert Sprang" wrote in message ...
"James Hahn" wrote in message
news
"Meindert Sprang" wrote in message
...
snip

There are two possible scenarios: one battery wears more quickly than
the
other, eventually reaching a state where the cell voltages reach higher
values due to increasing internal resistance. Thus the charges shuts off
too
early, leaving the better battery not topped up.
The charger does not monitor cell voltages. It monitors the voltage
across
all the cells in series.

That is exactly what I meant and exactly the reason why it can go wrong. So
I would like to invite you to shoot holes in my following argument:

The charge of a battery is the product of current x time.

Both batteries are in series and one load is connected to the set, operating
at 24V. Another load is connected across only one battery, operating at 12V.
So it is evident that one battery is discharged more than the other.

I start to charge the set in series, so the current through both batteries
is exactly the same. Since one battery is discharged more than the other and
the current throug both is the same, one battery must be charged longer that
the other. Exactly how am I going to achive that with the same current
through both batteries?
One battery will reach the full state before the other but is still being
charged with full current because the other battery hasn't reached the
voltage that corresponds with full charge.

Now, if you can come up with an valid argument why this shouldn't damage one
battery, I bow and take my hat off.

Meindert

Duh! the two batteries are in series. The one with the 12v tap cannot become
more discharged than the one without because the battery without the tap
will charge the one with. Or the one with will discharge the one without
until they are both the same. This is the same thing that happens with
individual cells in a 12v battery. With two 12v batteries in a series one
simply has more cells in a series.

CN

Why all this ranting over a proven fact that drawing 12V off of one battery
of a 24V bank is not a good idea? A 40 amp 24/12 DC/DC converter capable
of running all the electronics on the boat can be bought for the price of a
couple of golf cart batteries.

This is nonsense!

--
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

"Nigel" wrote in message
...
I have a 24v electrical set up on my boat, but need a 12v supply for my VHF
(amongst other things). Is there any reason why I can't just run a 12v
supply from just one of a pair of batteries, or do I need to use a 24v/12v
converter. I release this will draw charge from just one battery , but
won't the other just top it up and there by drain them both equally
Thanks

In article ,
"Meindert Sprang" wrote:

That is exactly what I meant and exactly the reason why it can go wrong. So
I would like to invite you to shoot holes in my following argument:

The charge of a battery is the product of current x time.

Both batteries are in series and one load is connected to the set, operating
at 24V. Another load is connected across only one battery, operating at 12V.
So it is evident that one battery is discharged more than the other.

I start to charge the set in series, so the current through both batteries
is exactly the same. Since one battery is discharged more than the other and
the current throug both is the same, one battery must be charged longer that
the other. Exactly how am I going to achive that with the same current
through both batteries?
One battery will reach the full state before the other but is still being
charged with full current because the other battery hasn't reached the
voltage that corresponds with full charge.

Now, if you can come up with an valid argument why this shouldn't damage one
battery, I bow and take my hat off.

Meindert

One thing you left out, Meindert, is the fact that the 12Vdc load is
still there, and consuming part of the charging current across that
battery, which only makes the problem worse.....

bruce in alaska
--
add a 2 before @