"Gary Schafer" wrote in message
...
On Mon, 18 Apr 2005 14:13:29 +1200, "Wout B"
wrote:


"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


At least everyone seems to agree that the problem (non existing according
to
some people) is in the re-charging, not in the tapping off of the 12V.
It's interesting why so many people got it wrong. Perhaps it is because
they wrongly use the analogy with water. A water-flow through 2 unequally
filled reservoirs in series will cause the reservoirs to level, but not
an
electric current through batteries. The total current has to flow through
both batteries. The difference with the example of the water reservoirs
is
that the current itself does not charge the batteries, like the water
does
to the reservoirs. It is the chemical EFFECT that the electric current
has
on the batteries that charges them. The two unequally charged batteries
have
different resistances and as they are in series they form a voltage
divider.
During charging, the charger's voltage regulator senses the TOTAL Voltage
of
the two batteries is series and adjusts the current to keep this at say
28.8
Volts during the absorbtion stage. The battery with the lower charge may
be
charged with 14.2 Volts and the untapped one with 14.6 Volts. The latter
will gass and suffer and eventually fail, if this mistreatment takes
place
on a regular basis.
During gassing, the full battery's Voltage will still rise, but slower
than
the battery with the lesser charge.
This means that the lower battery will eventually reach near full charge,
but it will take long and at a great cost to the gassing battery.
To prevent overcharging and gassing of one battery, the charger should
look
at the Voltage x 2 to of the higher Voltage battery. This can be achieved
by
sensing only the untapped battery and using a 2x Voltage multiplier
circuit.
By connecting the sense wire of the voltage regulator to this circuit, a
simple safety mechanism is built. Absorbtion charging will stop when this
battery reaches 14.4 Volts. It will protect the untapped (higher V)
battery,
but it will take a long time for the tapped battery to reach near-full
charge state, as the full battery "puts the brake on". This not fully
charged state is less damaging than the gassing and is acceptable.
One way to compensate for this would be to connect a 12V solar panel to
this
battery to top it up to full charge when the boat is not being used.
Wout



Good explanation. As an added note,
not fully charging a battery is less damaging than overcharging but:

If just sensing the "untapped" battery and the charger shuts down,
goes into the float mode because that battery is fully charged, then
the "tapped" battery will never get a full charge. A battery that
never gets a full charge will sulfate and have a shorter life because
of it.
There is no way to win short of having individual chargers for each
battery. Or don't tap the battery bank.

Regards
Gary

Gary,
Going too early into float-mode is a general problem with chargers. I've
"doctored" my smart 3-stage alternator V-reg to prolongue absorption. If
the batteries are not all that new anymore the V- level jumps up very
quickly. Timed absorption stage with adjustable time is ideal, because the
age and state of the batteries can be compensated for. In my opinion,
provided that the gassing is prevented as described and provided that the
tapped battery is regularly brought up to full charge in some way, the
tapping is O.K., if it is only for electronics. Maybe install a simple
changeover switch to tap the batteries in turn. Very important is of course,
like someone else mentioned before, that the electronics that are run this
way have an isolated -wire, not connected to the 24V system's ground.
Regards,
Wout





Maybe the batteries need replacement a year or so earlier, but I think it
will work out cheaper. Many people leave their boats on the moorings for
weeks with half-charged batteries and they still seem to get years of use
out of them. A bit of "tapping", wisely done, A cheap solution would also
be to install a changeover switch, to tap the batteries in turn, but the
gassing-prevention has to be number 1.

Ok I've risen to the provocation.

You guys have given Neal the validation he seeks by arguing with him and
you've turned a perfectly innocent query into a battleground. As a result he
has managed a troll into uk.rec.sailing which uptill now commonsence has
denied him.

To answer the original query - and all the attempts to explain electrical
theory

Let's make this simple, a battery resists the flow of electricity through
it. Most things resist the flow of electricity, this resistance is
demonstrated by dissipating some of the energy as light (the light bulb), or
heat (the radiant heater) or work (the electric motor). In humans this
resistance is demonstrated by burning (the electric chair). In a battery
this resistance is demonstrated by a chemical reaction that builds up a
charge on one side of the cell, when the capacity of the battery to sustain
that chemical reaction is satiated then the battery dissipates the energy in
heat. In a circuit the battery absorbs energy and shows a high resistance.
What is more a battery isn't really that efficient a lot of the work put in
is wasted as a by-product of the chemical reaction (heat, hydrogen given off
etc)

Resistance is often regarded as as wrong thing, because of the english
connation of the word resistence as somehow negative, it isn't it merely
means the more resistance the more work that has been achieved.

At the start of battery technology you had the basic lead acid cell powering
your radio, and once a week little johnny would go to the chemist who would
top up the acid and recharge the battery cell. At this stage the cell was a
simple device that held a few volts charge, and anyway we could see it
bubbling nicely so it must be magic.

Then along can applications that needed more of the magical oomph (had a
greater resistance) so some clever people put a whole sucession of these
strange devices together. In each one they connected the positive (oomph
end) with the -ve (oomphless end). They put Exide on the outside and retired
rich because they had created the modern battery. One can assume mistresses
and debauchery at this point.
The positive result is that if you put 2 oomphs together (one after the
other) you got 2 oomphs (bit like doubling up the number of horses) (but if
you put the oomphs side by side you only got one oomph for twice as long -
analogy is a bugger).

The battery isn't a magical concept, the word walmart on the outside hasn't
somehow made it sacred, it's a number of little cells connected together,
each one reacting to what's passed through it. Each little cell contributing
ther own bit bof oomph to the party.

Now say you've 24 oomphs harnessed together (a 24 Volt circuit) and you only
need 12 oomphs to move your wagon train. Well you can split the two in half
and take the 12 oomphs away, but they will become tired compared to the 12
oomphs you've left unharnessed.

If the world was fair the 12 unused oomphs would share energy with 12 used
oomphs, but they can't (at least for more than a moment) because to put the
energy back into the 12 used oomphs they'd need both the energy to drive the
chemical reation (+heat, hydtogen etc) plus the energy to provide the
replacement oomphs. They haven't got that extra energy so they see the 12
depleted oomphs as a wall they can't climb.

Then along you come with the charger (and the charger wont take no for an
answer it's got an 'effing great diesel engine diesel engine behind it) ,
because all these little cells are in sucession, you are pushing the same
amount of charge through each. The 12 depleted cells take the charge (+waste
a lot of it on heat and hydrogen), the 12 full cells waste all of the
charge (heat,hydrogen) because they can't store anymore oomph. Because the
world is unfair the charger has put a lot more oomph into the circuit (to
provide the energy for the heat, the hydrogen given off, the basic chemical
reaction, plus the energy for the charge that is stored) than you ever get
out of the battery.

A battery only has so much electrolyte (the thing that is reacted upon to
provide the oomph), if that electrolyte is wasted in generating hydogen and
heat then it ain't being used to provide electricity (oomph). So the battery
effectively dies sooner.

In an ideal world where a battery was 100% efficient then it wouldn't matter
if you were taking off 12 volts from a 24 volt cell. But in the real world
that does matter ('cos in that ideal where there was no resistance a battery
couldn't exist, try imagining a battery at absolete zero - where there is no
resistance to drive the reaction to store energy).

Batteries are a bit like Tax, you put in lots more than you get back.

OK

So to answer the original question
- yes your radio will work fine
- no because the battery is inefficient the charge will not fully equalise
- depending on how you charge your batteries, how often you use your 12 volt
circuit you might find yourself having to replace your batteries sooner
-In other words it ain't a proper job, it'll work, at sometime it'll give
someone a problem.

Lets now go back to ignoring Neal, like Alice he can live in his own fantasy
land where only the simplest rules of physics apply.

Great anology....i think 'oooph' should become a new lab
standard.
I think CN had one good point...engineers regard batteries as
a two-terminal device with a certain transfer function...chemists
working in the battery industry would see them totally differently.
Norm B

On Tue, 19 Apr 2005 03:39:02 +0100, "Nick Temple-Fry" theP wrote:

Ok I've risen to the provocation.

You guys have given Neal the validation he seeks by arguing with him and
you've turned a perfectly innocent query into a battleground. As a result he
has managed a troll into uk.rec.sailing which uptill now commonsence has
denied him.

To answer the original query - and all the attempts to explain electrical
theory

Let's make this simple, a battery resists the flow of electricity through
it. Most things resist the flow of electricity, this resistance is
demonstrated by dissipating some of the energy as light (the light bulb), or
heat (the radiant heater) or work (the electric motor). In humans this
resistance is demonstrated by burning (the electric chair). In a battery
this resistance is demonstrated by a chemical reaction that builds up a
charge on one side of the cell, when the capacity of the battery to sustain
that chemical reaction is satiated then the battery dissipates the energy in
heat. In a circuit the battery absorbs energy and shows a high resistance.
What is more a battery isn't really that efficient a lot of the work put in
is wasted as a by-product of the chemical reaction (heat, hydrogen given off
etc)

Resistance is often regarded as as wrong thing, because of the english
connation of the word resistence as somehow negative, it isn't it merely
means the more resistance the more work that has been achieved.

At the start of battery technology you had the basic lead acid cell powering
your radio, and once a week little johnny would go to the chemist who would
top up the acid and recharge the battery cell. At this stage the cell was a
simple device that held a few volts charge, and anyway we could see it
bubbling nicely so it must be magic.

Then along can applications that needed more of the magical oomph (had a
greater resistance) so some clever people put a whole sucession of these
strange devices together. In each one they connected the positive (oomph
end) with the -ve (oomphless end). They put Exide on the outside and retired
rich because they had created the modern battery. One can assume mistresses
and debauchery at this point.
The positive result is that if you put 2 oomphs together (one after the
other) you got 2 oomphs (bit like doubling up the number of horses) (but if
you put the oomphs side by side you only got one oomph for twice as long -
analogy is a bugger).

The battery isn't a magical concept, the word walmart on the outside hasn't
somehow made it sacred, it's a number of little cells connected together,
each one reacting to what's passed through it. Each little cell contributing
ther own bit bof oomph to the party.

Now say you've 24 oomphs harnessed together (a 24 Volt circuit) and you only
need 12 oomphs to move your wagon train. Well you can split the two in half
and take the 12 oomphs away, but they will become tired compared to the 12
oomphs you've left unharnessed.

If the world was fair the 12 unused oomphs would share energy with 12 used
oomphs, but they can't (at least for more than a moment) because to put the
energy back into the 12 used oomphs they'd need both the energy to drive the
chemical reation (+heat, hydtogen etc) plus the energy to provide the
replacement oomphs. They haven't got that extra energy so they see the 12
depleted oomphs as a wall they can't climb.

Then along you come with the charger (and the charger wont take no for an
answer it's got an 'effing great diesel engine diesel engine behind it) ,
because all these little cells are in sucession, you are pushing the same
amount of charge through each. The 12 depleted cells take the charge (+waste
a lot of it on heat and hydrogen), the 12 full cells waste all of the
charge (heat,hydrogen) because they can't store anymore oomph. Because the
world is unfair the charger has put a lot more oomph into the circuit (to
provide the energy for the heat, the hydrogen given off, the basic chemical
reaction, plus the energy for the charge that is stored) than you ever get
out of the battery.

A battery only has so much electrolyte (the thing that is reacted upon to
provide the oomph), if that electrolyte is wasted in generating hydogen and
heat then it ain't being used to provide electricity (oomph). So the battery
effectively dies sooner.

In an ideal world where a battery was 100% efficient then it wouldn't matter
if you were taking off 12 volts from a 24 volt cell. But in the real world
that does matter ('cos in that ideal where there was no resistance a battery
couldn't exist, try imagining a battery at absolete zero - where there is no
resistance to drive the reaction to store energy).

Batteries are a bit like Tax, you put in lots more than you get back.

OK

So to answer the original question
- yes your radio will work fine
- no because the battery is inefficient the charge will not fully equalise
- depending on how you charge your batteries, how often you use your 12 volt
circuit you might find yourself having to replace your batteries sooner
-In other words it ain't a proper job, it'll work, at sometime it'll give
someone a problem.

Lets now go back to ignoring Neal, like Alice he can live in his own fantasy
land where only the simplest rules of physics apply.

Oomph is as a good as any other term

Practical electricity is about energy, charge and time.

Volts, amps, ohms are just derivations from energy, charge and time.

Until someone actually meets a volt, shakes a hand with an amp or goes out
to dine with the parents of an ohm I'll maintain my heritical views.

Oomph is now the unit of doing something, I patent it and claim copyright.
Volts and Amps are just illegitamate children. Ohms are the result of incest
on a dark night, useful and I'll keep them working out of sight in the
skullery.

Please do not confuse terminolgy and education with reality.

Only 9 angels dance on the top of the pinhead, if I could detirmine the
speed of the angel maybe I could find out what direction the pinhead is
travelling in.


"engsol" wrote in message
...
Great anology....i think 'oooph' should become a new lab
standard.
I think CN had one good point...engineers regard batteries as
a two-terminal device with a certain transfer function...chemists
working in the battery industry would see them totally differently.
Norm B

Nick Temple-Fry wrote:

Resistance is often regarded as as wrong thing, because of the english
connation of the word resistence as somehow negative, it isn't it merely
means the more resistance the more work that has been achieved.

You seem to be confusing more with less. More resistance means less current
and less work. Less resistance means more current and more work.