Re: Follow-up: Car battery charger

"Bruce W...1" wrote:
> 
> [EMAIL PROTECTED] wrote:
> >
> > I don't know how you measured things - so I can't say for
> > sure - but you may not have a failure.
> >
> > 1) You need to measure the float charge voltage while the
> > charger is charging the battery. Don't know if you did that,
> > but 13.7 is good if you did.
> >
> > 2) The battery needs to be fully charged before connecting
> > the float charger.  Don't know if it was. If the battery is
> > discharged and you connect your float charger and measure it,
> > you will see a voltage below 13.7  A discharged battery can
> > draw enough current to drop the output voltage of the wall
> > wart down below the 13.7 regulation voltage.
> >
> > 3) A battery removed from the float charge will show a lower
> > voltage than the float voltage.  That is normal.  So it is
> > possible that your charger is working properly and the battery
> > is being held at full charge.
> =============================================================
> 
> The battery was fully charged when the float charging was started.  The
> battery is almost new.  The float voltage measured 12.7V with the
> charger connected.  And the regulator is heat-sinked.
> 
> Someone outside of this thread who is more knowledgeable in this matter
> than I told me the following.
> 
> A float voltage of 13.3V is required to maintain a fully charged state
> (at room temperature).  At lower voltages the battery loses charge,
> regardless of the output of the charger. So if the charger doesn't have
> enough current to keep it at 13.3V, as is the case here, then charge
> will be lost.  If this is true then I should see a lower float voltage
> in the near future.
> 
> It's also become clear that regulating the voltage of an under-sized
> charger is pointless, because the battery never reaches a high voltage
> anyway.
> 
> Bob's point about overloading the charger is certainly valid.  But right
> now it's only pulling a tiny current because the voltage differential is
> so small.
> 
> One conclusion can be drawn from all of this.  The charger I built is
> inadequate for long-term care.  And the wall wart chargers that are sold
> for float charging are not suitable for long-term charging if they can't
> keep the battery at 13.3V.  I'm guessing you need at least 2 Amps to do
> this.  However an under-sized wall wart can certainly reduce the rate of
> discharge by compensating for external loads.
> 
> So what my home-brew charger is doing is just compensating for external
> loads and not adding to the battery charge in any way.
> 
> A lead-acid battery is not damaged until it falls below 12.0V.  How long
> does it take a healthy battery to self-discharge to 12.0V?  This might
> take a year.  I don't have a feel for this at lower temperatures.
> 
> My charger will probably get the battery thru the winter, and certainly
> if I start the car every six weeks or so.  So I think I'll just leave it
> at that.  Thanks all for your help.
> 
> On another battery front, the gel cell in my computer UPS died of old
> age.  Rather than replacing the battery I reconnected the UPS to a 32Ah
> gel cell which I keep around for emergency preparedness.  This kills two
> birds with one stone, it keeps the big battery charged and also gives
> the UPS a whole lot of capacity.  Now that I think about it, an old UPS
> might make a dynamite car battery float charger.


Some points:
1) The input to the regulator must be about 2 volts above the
regulated voltage level. So, if your regulator is set for
13.7, the DC input to the regulator must be about 15.7.
Under no load, what is the DC voltage at the input to the
regulator?  What is it under full load?

2) A wall wart's output voltage will sag under load - the
heavier the load, the greater the sag. The ones that don't
sag have the regulator built in.   How much current is being 
drawn from the regulator when it is connected to the battery?  
What is the wall wart voltage sag, no load to full load?

3) If as you mentioned the voltage differential is so small 
that very little current is being drawn from the charger, then 
there should not be a differential of 13.7 to 12.7, no load to 
full load as measured at the charger terminals that clip
on to the battery.  The problem is that the phrase "very little
current" is undefined. We need the actual numbers.

Bottom line - it sounds like your wall wart may be too wimpy
for this application.  Also, it would be a good idea to
post the details of the circuit.  For example, do you have
a diode in the output between the regulator and the battery?
If not, how do you protect the LM317, and how do you prevent
the battery from discharging through the charger?

In your reply you mentioned:
> It's also become clear that regulating the voltage of an under-sized
> charger is pointless, because the battery never reaches a high voltage
> anyway.

It's not the regulator that's pointless, it's using an 
under-sized charger in the first place, and expecting it
to keep the battery at ~13.7.  When you expect a charger 
to keep a battery at ~13.7, regulation is required.  When
you don't care what the battery voltage is, no regulation
is required. Of course, that's no charger at all - it could 
allow the voltage to go anywhere, and it disagrees with what 
battery manufacturers recommend - voltage regulation for 
trickle charge.