"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.
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