"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