On Wed, Feb 25, 2004 at 12:46:26PM -0700, Dwayne Reid wrote:
> At 02:59 PM 2/24/2004, Byron A Jeff wrote:
> >On Mon, Feb 23, 2004 at 03:01:52PM -0700, Dwayne Reid wrote:
> >>
> >> The simplest (and one of the most reliable) charger for charging 12V gel
> >> cell battery from a 12V automobile is to use a schottky diode in series
> >> with a current limit resistor.  The 12V supply is already at the correct
> >> voltage.
> >
> >No it isn't. Lead Acid requires a minimum of 2.3V/cell to charge. That
> >means
> >you need 13.8V. Now usually this isn't a problem because the car alternator
> >will produce somewhere in the ballpark of 14V. But if it's a true 12V it
> >won't
> >properly charge.
>
> Hi there, Byron.
>
> I don't mean to start an argument but consider the following: the battery
> in the automobile also uses lead acid chemistry.  The alternator output
> voltage is regulated so as to charge that battery properly.  That's why it
> measures between 14.0 - 14.5V (engine running, head-lights off).

So where's the argument?

>
> I've glossed over a whole bunch of issues (mostly temperature compensation)
> but, in general, a 12V gel cell battery charges very nicely from that
> voltage.  It is important to limit charge current and I'm not real happy
> about the ~0.18V drop of a large sized schottky diode (increases the
> top-off time) but it does work.

At that voltage. I agreed with you there:

"Now usually this isn't a problem because the car alternator will produce
somewhere in the ballpark of 14V."

>
> PS - that's why I worded my message the way I did: "charging a 12V gel cell
> battery from a 12V automobile".

But here's the problem. The OP pointed out that it would be a motorhome
situation where the charging would occur from the motorhome battery only.
So the alternator will not be running.

So the original point is valid: the input voltage will need to be raised from
the nominal 12V of the motorhome battery to charge the gel cell.

Of course all of this could be avoided by buying a solar panel specifically
designed to charge 12V batteries.

BAJ

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