Re: [MV] Wanted: Battery Guru's

From: wwd@netheaven.com
Date: Wed Nov 21 2001 - 07:39:17 PST


In <list-1371852@skylee.com>, on 11/21/01
   at 10:39 AM, "Richard Notton" <Richard@fv623.demon.co.uk> said:

>We usually regard the forward diode voltage drop of 0.6V for ordinary
>silicon devices, germanium devices show a lot lower drop but are
>virtually extinct and do not come in high current form. Shottky and
>other modern constructions may show a lot more. It is important to note
>that the forward volt drop of a diode P-N junction is totally independent
>of current flowing through it.

Shottkys only contribute a ~0.3v drop while foreward conducting, which is
a tad less than half the drop of silicon diodes (nominally 0.7v). That is
their 'charm' and is why we choose to use them over the more common
silicon diodes. Thus you can use twice as many Shottkys in a string to
suffer the same drop as silicons. Or you can clamp across relay contacts
with them and cut noise voltage by half.

Of particular interest to MVers might be those simple, lightweight battery
chargers that employ SELENIUM diode rectifiers. Usually they have just an
ammeter and a 6-12volt switch. These are very handy devices and last
forever if kept dry and properly hooked to the batt whilst plugged in.
When first hooked to a 'modestly discharged' car batt the meter will show
approx 4 amps. When that current eventually drops to half (2amps) the batt
is 'fully charged' and can be placed back in service. With a garden
tractor (3amp > 1.5amp) or motorcycle batt (2amp > 1amp) are the
respective ranges. If the batt is severely discharged you have to wait for
it to build slowly from 0.0amp up to full current- then begin the slow
drop to the 1/2 level. The natural characteristics of selenium rectifiers
match the needs of lead-acid batts quite neatly. If you happen to leave
this type of charger hooked up for an extra day or two there is no problem
as it is not much more than a 'trickle-charge'. But try not to make a
practice of this, however.

With a voltmeter you can diagnose batts ok, but putting them thru a full
cycle on a selenium charger, IF they can do it, will give you a much
better picture of a batt's chances of providing future service. The
'modern-type' chargers '2amp-10amp-start' often are unclear about just
what they are doing to your batt and at what point in the charge curve it
is at right now.

Bill

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