>
>----- Original Message -----
>From: Keith, Steve <Steve.Keith@compaq.com>
>To: <mil-veh@skylee.com>
>SNIP!
>. The Alternator issue is also
>> another big loss area. You lose up to.6v per diode in the
>alternator and
>> there are two that the voltage must pass thru to get to the
>battery. Up to
>> 1.2v loss on 12v. Like 10%
>
>I guess this is why the current output of alternators on 12v
>systems is usually around 13.5 volts...Am i correct?
>
Antoine
No, not really. The alternator voltage can easily be increased by a few extra
turns on its stator windings and I would expect any "12 volt" vehicle alternator
to actually take the battery to 14V at least. For those of you that may be
familiar with the current and voltage electro-magnet type dynamo (DC generator)
regulators found on older MV's and the like, the alternator has only an
electronic voltage control as the maximum current available is a function of the
design and the magnetic field strength produced by the rotor windings.
Whilst we all call a 6 cell lead-acid battery 12V it actually isn't, this is all
a function of chemistry. At 12V across the terminals its getting towards
discharged, a fully charged battery will have something like 13.2 V at the
terminals at rest, the lab standard used to represent a float charged battery is
13.8V which is the point of gassing too.
It doesn't matter how big the lead-acid cell is made - fork lift trucks to
diesel electric submarines or how small, say 1mm square plates, the terminal
voltage per cell will be a nominal 2.2V.
Automotive applications are considered cyclic and therefore take the battery to
14 - 14.2V under charge to ensure maximum capacity for cranking, batteries used
in float charged stand-by conditions are never charged beyond 13.6V to give
very extended life and to ensure gassing doesn't occur.
You can expect therefore to see 28V - 28.4V on a nominal 24V vehicle with the
engine running and the proposed "36V" future systems will have 43.2V when being
charged.
Richard
(Southampton UK)
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