"James Hahn" wrote in message
...
Don't pay any attention to replies that talk about converters.

Obviously you don't know what you are talking about.
With such a setup you achieve two things:
1) one battery will always be drainen slightly more than the other.
2) that same battery will alway be charged a little less than the other
because the charger measures the voltage across both batteries in series.

So eventually that battery will be in a continuous state of being empty.
Imagine what happens when you start the enginge or put another heavy load on
a series of batteries where one of them is empty: the empty one will be
charged by the full one, but with reverse polarity.

Have you ever disassembled a broken battery pack of some device? In 99% of
the cases you will find all cells in perfect condition (charged and all)
except one, which was obviously the weakest in the chain, showing a reversed
voltage.

Meindert

"Meindert Sprang" wrote in message
...
snip

Obviously you don't know what you are talking about.
With such a setup you achieve two things:
1) one battery will always be drainen slightly more than the other.
Correct (although it obviously depends on usage).

2) that same battery will alway be charged a little less than the other
because the charger measures the voltage across both batteries in series.
Incorrect. The charger measures the voltage across both batteries in series.
The voltage it sees is the result of both batteries being there, not the one
with the highest or lowest charge. The charger will charge until the
voltage across the two batteries reaches the correct level. Whether
individual cells (and being in the same case or in different cases makes no
difference) took a longer time to get up to par simply doesn't matter.

So eventually that battery will be in a continuous state of being empty.
Imagine what happens when you start the enginge or put another heavy load
on
a series of batteries where one of them is empty: the empty one will be
charged by the full one, but with reverse polarity.
This is not what happens, and I cannot imagine what scenario you have in
mind that could create this situation. Have you ever replaced one battery
in a two-battery 24v system with a brand new battery? What happened when
you started the engine? I have and I can tell you the answer - the
uncharged battery gradually charged, as you would expect.

Have you ever disassembled a broken battery pack of some device? In 99% of
the cases you will find all cells in perfect condition (charged and all)
except one, which was obviously the weakest in the chain, showing a
reversed
voltage.
You have demolished our own argument. All cells were treated identically,
yet one has collapsed! You can't use that evidence to support a claim that
cells treated differently are more liable to collapse (which I assume is
what you are trying to say).

Collapse of one cell is the most common mode of failure for wet cell
batteries, but it is not associated with differential discharge rates.

For years battery manuafacturers have provided a 12v tap across the first
eight cells of high voltage industrial batteries. There has never been any
evidence that using the tap shortens the battery life, and it has never been
the experience of the users.

"James Hahn" wrote in message
...
2) that same battery will alway be charged a little less than the other
because the charger measures the voltage across both batteries in
series.
Incorrect. The charger measures the voltage across both batteries in
series.
The voltage it sees is the result of both batteries being there, not the
one
with the highest or lowest charge. The charger will charge until the
voltage across the two batteries reaches the correct level.

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 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.

Have you ever disassembled a broken battery pack of some device? In 99%
of
the cases you will find all cells in perfect condition (charged and all)
except one, which was obviously the weakest in the chain, showing a
reversed
voltage.
You have demolished our own argument. All cells were treated identically,
yet one has collapsed! You can't use that evidence to support a claim
that
cells treated differently are more liable to collapse (which I assume is
what you are trying to say).

There is always one cell that collapses first (the weakesrt), due to
tolerances in materials, production, etc. By introducing an extra load
(wear) on part of the cells in a string, they are more likely to collapse
first.

Collapse of one cell is the most common mode of failure for wet cell
batteries, but it is not associated with differential discharge rates.

We all know that a lead-acid battery suffers from deep discharge. And in
this setup, it is a plain fact that the battery "below" the tap is
discharged sooner then the one "above" the tap. And if the charger is on
"the outside" terminals, it can only see the charge state of the entire
string, which is in this case, wrong information.

For years battery manuafacturers have provided a 12v tap across the first
eight cells of high voltage industrial batteries. There has never been
any
evidence that using the tap shortens the battery life, and it has never
been
the experience of the users.

Uhm, 12V across 8 cells? You are talking about non-rechargable batteries
here?

Meindert

"Meindert Sprang" wrote in message ...

No, because the other one is connected in series, not in parallel. So one
battery gets drained faster that the other and that is the success-formula
for wrecking that battery.

No worse, surely, than it would be if there was only that battery and
a 12V system. It has no way of knowing that some of its output is
being boosted by another 12V.

I wouldn't have expected there to be any problem at all with the OP's
idea - but if he was worried, he could always swap the batteries every
season so they get roughly equal use as the 12V supply.

Ian

Ian Johnston wrote:

"Meindert Sprang" wrote in message
...

No, because the other one is connected in series, not in parallel. So one
battery gets drained faster that the other and that is the
success-formula for wrecking that battery.

No worse, surely, than it would be if there was only that battery and
a 12V system. It has no way of knowing that some of its output is
being boosted by another 12V.

I wouldn't have expected there to be any problem at all with the OP's
idea - but if he was worried, he could always swap the batteries every
season so they get roughly equal use as the 12V supply.

mm, the installation and maintenance of most boat batteries is so
sub-optimal that pulling a few watts off one battery probably won't make
any discernible difference, if done within reason.


--
Lithium ion internal and external batteries.
Internal from £30 External from £75 (trade)
All batteries factory new and guaranteed.
http://www.surfbaud.co.uk/
e-mail (www.rot13.com)

In article , chuck
wrote:

Hello Nigel,

Seems you have an interesting situation to begin with. If
you are charging two 12 volt batteries in series, (with a 24
volt alternator or charger) then unless the two batteries
are identical (in age, internal characteristics, etc), they
will be charged to two different voltages. Not really the
best situation.

Now if you connect a load to just one of the batteries, that
will upset the equal distribution of charge voltage across
each battery, as Meindert has suggested. Of course, it could
actually improve the balance, as you can imagine. Just out
of curiosity, have you ever measured the voltage across each
battery while charging? It would be interesting to see what
differences you found, if any.

Ignoring all of that, it is really a matter of degree. If
you run a VHF from just one of the batteries, that would do
a lot less potential harm than if you ran, say, a
watermaker. You might get away with just the VHF. If you had
a lot of time to play with this, you could arrange to put
the same load on each battery. Say a VHF on one and
something that mimicked the VHF's load on the other. That
would require special wiring, or course, for the battery
that didn't have its negative terminal connected to the boat
ground.

Has this been confusing enough?

Good luck.

Chuck

Ok, now listen up folks. There is no difference between, one
24Vdc battery made with 12 cells in series and two 12Vdc batteries in
series, or four 6 Vdc batteries in series, as long as the
interconnection links between cells are of similar low resistance.
Tapping ANY battery system, in the middle IS BAD. The more power
you draw off the tap as a percentage of total power available, the more
damage you will do over time to the battery system.
A single Vhf Radio will draw say 1 amp Standby, 2 amps Receive, and near
7 amps Transmit. Since most useage figures say 75% Standby, 15% Receive,
and 10% Transmit, this would not be a significant load differential
across, a battery system of like 8D's in series. That said, tapping a
battery system, is a BAD IDEA. If you are running a radar, couple vhf's,
GPS or two, Plotter, ECT, then your ASKING FOR TROUBLE, and it WILL find
you. Best Idea is to get a Switching DC to DC Converter, like a Numar
32-12-X where X is the total operational load. One could very easily
put a Group 12 12Vdc onboard, and use the DC to DC converter to keep
the Group 12 battery charged. This is a typical system, that would meet
the USCG Fishing Vessel Safety Act, Radio Power Requirement.

I have no idea where the first replyer got his information, or training
in Marine Electronics, but if he paid any money for it, he should go
back and ask for a refund.


Bruce in alaska
--
add a 2 before @

What was it Tolkien said

"Go not to the Elves for advice, for they will say both No and Yes"

Well the Elves of the newsgroup seem to have spoken.

Actually Nigel I think you've answered your own question - if you are
installing a 12V circuit to supply a VHF radio and "other things" then you
are going to want to install a switch panel, fuses on the circuits to the
radio (+other things etc). In other words you will be crawling over the
boat, cutting holes for the switch panel and the radio (+other things),
poking wires behind the furniture. Generally makeing a mess and getting bad
tempered and irritated - so you may as well do a complete job and put the
converter in.

Chances are once you have a 12V circuit - you are going to want to connect
more bits and bobs to it.

Just thought I'd throw my tuppence hapenth in.


"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

Well ok ... let's apply a testing paradigm or "if you could choose only one
voltage, what would it be" to arrive at an answer ... how about 16 -11 = 5,
divided by 2 gets you 2.5, and adding that to 11 gets you 13.5 ... only 13.8
+/- a tad is more like a real world DC to DC output :-)

"Doug Dotson" dougdotson@NOSPAMcablespeedNOSPAMcom wrote in message
...
A VHF doesn't need regulated DC. They are designed to work off of
any reasonable voltage between 11 and 16.

"bowgus" wrote in message
...
If you use a DC to DC converter, then you know you will be getting a
regulated 12 Volts (or maybe 13.8 for the VHF???) ... just get one with
an
appropriate power output ... i.e., no need to spend big bucks for more
power
than you need. I myself would go with a converter ... that way the VHF
is
guaranteed to work.

"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

I've read this thread carefully, and although I'm no sort of expert here, I
think that only Meindert is getting at the truth. Most of the rest of you
are getting confused between voltage and current. Because both batteries are
connected in series, IRRESPECTIVE of the charging mechanism (eg alternator
or smart charger), then both receive an identical charge. If charging starts
from a state where one battery is more discharged than the other, then the
fuller battery will replenish its charge before the other one, and towards
the end of that charging regime its terminal voltage will rise fairly
suddenly from around 13V to around 14.5. Now, depending on the precise
characteristics of the charging source (different for smart chargers and
alternators), the ONLY way that the undercharged battery can continue to
receive charge is to overcharge the now fully charged one. This implies
applying more than 14.5 volts across the full battery, and this will, of
necessity, cause to battery to gas, perhaps even boil in extreme
circumstances. Enough said?

Guy is probably right in saying that small additional current drain from one
of the two batteries is lost in the noise, as it were. But as a concept,
asymmetric current from series-connected batteries is bad news.

Interestingly, one could get round the problem described above by putting a
14.5V zenner diode around the battery which is not being drained by your 12V
circuit, but this isn't without problems either - the diode would have to
be capable of handling the maximum charge current, lets say 30 amps. 30A x
14.5V = 420watts - thats a pretty big zenner on a pretty big heatsink!!!!
HTH

"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. I don't know what you mean by 'wears', but the
initial charge state of individual cells does not affect the voltage level
at which the charger shuts off.

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).

Have you ever disassembled a broken battery pack of some device? In 99%
of
the cases you will find all cells in perfect condition (charged and
all)
except one, which was obviously the weakest in the chain, showing a
reversed
voltage.
You have demolished our own argument. All cells were treated
identically,
yet one has collapsed! You can't use that evidence to support a claim
that
cells treated differently are more liable to collapse (which I assume is
what you are trying to say).

There is always one cell that collapses first (the weakesrt), due to
tolerances in materials, production, etc. By introducing an extra load
(wear) on part of the cells in a string, they are more likely to collapse
first.
It's not the extra load that causes cell collapse. Mistreatment of the
batteries (either, or both) will shorten their life. If they are operated as
they are designed to be operated then manufacturing differences between
cells are more significant than the fact that some cells have been
discharged to a lower level than others. If you do a proper study of those
dissambled batteries you will find that the collapsed cell is not random -
it has a strong bias towards the poitive terminal.

Collapse of one cell is the most common mode of failure for wet cell
batteries, but it is not associated with differential discharge rates.

We all know that a lead-acid battery suffers from deep discharge.
Who is talking about deep discharge?

And in
this setup, it is a plain fact that the battery "below" the tap is
discharged sooner then the one "above" the tap. And if the charger is on
"the outside" terminals, it can only see the charge state of the entire
string, which is in this case, wrong information.
It is not "discharged sooner". It is discharged to a level slightly below
the other. The charge state of the 'entire string' is not 'wrong'
information. It will indicate the amout of charge required, and any
differential between individual cells, or sets of cells, is not relevant.

For years battery manuafacturers have provided a 12v tap across the first
eight cells of high voltage industrial batteries. There has never been
any
evidence that using the tap shortens the battery life, and it has never
been
the experience of the users.

Uhm, 12V across 8 cells? You are talking about non-rechargable batteries
here?
No. I am talking about standard industrial lead acid wet cells, so I guess
the number is 10. If you need to know it exactly you can take yourself down
to any nearby warehouse and look at the batteries used in a forklift battery
system, and count the number of cells set aside for the 12v tap.