[Shop-talk] Trickle Chargers

[Randall]

> The 90 amp alternator 
> that is set at 14.4 volts has always charged it with absolutely no 
> operating problems.

Which of course is exactly as Optima says it should.

>  It isn't hard to build a simple adjustable output voltage 
> limiter/regulator into any simple charger. 

No argument from me, I just wouldn't call the result a "regular" charger.
Perhaps I misunderstood the question, I thought it was "can I use a regular
charger", not "can I modify a charger for this purpose" ?

> I'll bet that there are chips now days that 
> will do the same thing. 

I'm sure it can be done, but there are some caveats.  Most high-current
linear regulators need to have input that stays 2-3 volts above the output
voltage.  An unfiltered supply (like a standard battery charger) isn't going
to do that, so you'll also need to add capacitance to hold the voltage up
... which means you need to derate the transformer and rectifiers because
they now see much higher peak currents.  Don't have my books handy, but as I
recall the derating is substantial.  So starting with a "10 amp" charger may
net you only a 5 amp regulated charger after spending significant money on
the regulator and capacitors and heat sink.  A fun project if you're doing
it for fun; but if you just want to charge a battery, it might not be the
most cost- or time effective way.

> Your meter won't 
> read the 
> 120 Hertz ripple because the battery which acts like a large 
> capacitor 
> when connected to the charger will filter the ripple out.

My meter won't read the peaks whether they are there or not.  Even without
the battery in the circuit, it reads nowhere near the peak voltage (as
confirmed by the o'scope).  And while I don't know what the ESR @ 120Hz is
for an Optima, it's fairly high for an ordinary battery ... it doesn't
filter the ripple as much as you'd think.  I used to even get hum in the AM
radio while charging with a "buzz box".

And while I'm certainly not a battery engineer; as I see it there WILL be
current pulses going into the fully charged battery even if the voltage
doesn't manage to rise much.  I'd guess it's the current after full charge
they are mostly worried about (since that's what causes outgassing, which is
a Bad Thing in a totally sealed cell).

But I haven't tried it ... somehow the logic of spending 300% as much on a
battery that might last 150% longer (with all this special care & feeding)
escapes me.  I get by just fine with the $40 specials at Wally-mart.  And
while 6 years interest on that $80 I left in the savings account won't buy a
new battery, it makes a big dent in it.

YMMV & all that ... if you try it, please let us know what happens.

Randall