RadioBanter - View Single Post - Follow-up: Car battery charger

November 28th 03, 04:31 PM

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.