On Wed, 26 Nov 2003 01:03:11 -0500 "Bruce W...1"
wrote:

Now two weeks later I check the battery. Its voltage is 12.7V. The
charger circuit measures 13.7V. And I measured the drain, from the
alarm and radio, it is 10mA.

That means that there is 100 Ohms between the PS and the battery. It's
likely that this is the resistance of the meter that you used to
measure the 10mA, and that the actual current without the current
meter was more.

But I still don't understand how you could read 13.7V at the PS and
12.7V at the battery unless there is a significant resistance between
the two. Note that this resistance could be in the ground leg, too.

OTOH, holding the battery voltage at 12.7 will be just fine for long
term storage. Higher voltages will keep it topped up at full charge,
but they also do some long term damage and convert water to hydrogen
and oxygen via hydrolysis.

You're really better off at the lower voltage, and 12.7V is just fine.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

"Bruce W...1" wrote:

Not long ago and in another thread many of you gave me great advice on
how to make a car battery float charger. I wanted to just connect a
properly sized wall wart, but everyone recommended voltage regulation.
So I connected a voltage regulator (13.6V) to a 500mA wall wart. The
wall wart has an open-circuit voltage of 18V and is rated 500mA at 12 V.

Further background, I built this charger to prevent my having to start a
friends car once a week while they're on extended vacation.

Now two weeks later I check the battery. Its voltage is 12.7V. The
charger circuit measures 13.7V. And I measured the drain, from the
alarm and radio, it is 10mA.

The ambient temperature on average is about 40F.

What went wrong? Why is the battery only 12.7V instead of 13.7?

Lacking a better solution from you guys it seems we need more power,
ugh, ugh. 2A ought to do it.

Spec's say that car batteries (at room temperature) are best regulated
at 13.3V. For 32 degrees F 14.2V is better.

Yet the failure analysis remains incomplete. Where did we go wrong?

Thanks for your help.

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.

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.

"Bruce W...1" wrote:

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.

"Bruce W...1" wrote:

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.

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.

Bruce:

Did you measure the voltage with the charger connected, or after you removed
it?
A standard 12v lead-acid automotive battery has a nominal voltage of 12.6
volts. Even after a float charge, once the charging current is removed, the
battery will return to about 12.6 volts fairly rapidly. Thus, the voltage
you measured would be considered normal for a charged battery.

While the trickle charger is connected to the battery, any current in excess
of that needed to fully charge the battery is converted to heat. You will
only read the charging voltage when the charger is actually connected and
operating. about 10ma of the current is going into the loads you've already
measured, with the rest going to keeping the battery at top charge.

Unless you are measuring the voltage with the charger connected, you
probably don't need to have one with more current.

--Rick



"Bruce W...1" wrote:

Not long ago and in another thread many of you gave me great advice on
how to make a car battery float charger. I wanted to just connect a
properly sized wall wart, but everyone recommended voltage regulation.
So I connected a voltage regulator (13.6V) to a 500mA wall wart. The
wall wart has an open-circuit voltage of 18V and is rated 500mA at 12 V.

Further background, I built this charger to prevent my having to start a
friends car once a week while they're on extended vacation.

Now two weeks later I check the battery. Its voltage is 12.7V. The
charger circuit measures 13.7V. And I measured the drain, from the
alarm and radio, it is 10mA.

The ambient temperature on average is about 40F.

What went wrong? Why is the battery only 12.7V instead of 13.7?

Lacking a better solution from you guys it seems we need more power,
ugh, ugh. 2A ought to do it.

Spec's say that car batteries (at room temperature) are best regulated
at 13.3V. For 32 degrees F 14.2V is better.

Yet the failure analysis remains incomplete. Where did we go wrong?

Thanks for your help.

"Bruce W...1" wrote in message ...
Not long ago and in another thread many of you gave me great advice on
how to make a car battery float charger. I wanted to just connect a
properly sized wall wart, but everyone recommended voltage regulation.
So I connected a voltage regulator (13.6V) to a 500mA wall wart. The
wall wart has an open-circuit voltage of 18V and is rated 500mA at 12 V.

Further background, I built this charger to prevent my having to start a
friends car once a week while they're on extended vacation.

Now two weeks later I check the battery. Its voltage is 12.7V. The
charger circuit measures 13.7V. And I measured the drain, from the
alarm and radio, it is 10mA.

The ambient temperature on average is about 40F.

What went wrong? Why is the battery only 12.7V instead of 13.7?

Lacking a better solution from you guys it seems we need more power,
ugh, ugh. 2A ought to do it.

Spec's say that car batteries (at room temperature) are best regulated
at 13.3V. For 32 degrees F 14.2V is better.

Yet the failure analysis remains incomplete. Where did we go wrong?

Thanks for your help.


**** a Duck , Bruce,
this is becoming increasingly metaphysical - the mental effort you
(and everyone else) is putting into debating a car battery is
ludicrous. Let me make a few points.

1. If the battery is more than 4 years old its probably stuffed or
close to it. Sad but true.

2. Go and buy a hygrometer (they are about $3 - people used them
before digital multimeters were invented) - have a look at the SG in
the cells. If its green,
its OK. Check all cells, if 1 or 2 are very different SG then its
stuffed.

3. Do a load test on the thing, turn on all the lights and see how
much the voltage drops. Leave them on for 0.5 hour, if it drops much
below 12v then its stuffed.

How much does a new battery cost anyway?.........

de VK3BFA ANdrew

On Wed, 26 Nov 2003 01:03:11 -0500 "Bruce W...1"
wrote:

Now two weeks later I check the battery. Its voltage is 12.7V. The
charger circuit measures 13.7V. And I measured the drain, from the
alarm and radio, it is 10mA.

That means that there is 100 Ohms between the PS and the battery. It's
likely that this is the resistance of the meter that you used to
measure the 10mA, and that the actual current without the current
meter was more.

But I still don't understand how you could read 13.7V at the PS and
12.7V at the battery unless there is a significant resistance between
the two. Note that this resistance could be in the ground leg, too.

OTOH, holding the battery voltage at 12.7 will be just fine for long
term storage. Higher voltages will keep it topped up at full charge,
but they also do some long term damage and convert water to hydrogen
and oxygen via hydrolysis.

You're really better off at the lower voltage, and 12.7V is just fine.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

"Bruce W...1" wrote:

Not long ago and in another thread many of you gave me great advice on
how to make a car battery float charger. I wanted to just connect a
properly sized wall wart, but everyone recommended voltage regulation.
So I connected a voltage regulator (13.6V) to a 500mA wall wart. The
wall wart has an open-circuit voltage of 18V and is rated 500mA at 12 V.

Further background, I built this charger to prevent my having to start a
friends car once a week while they're on extended vacation.

Now two weeks later I check the battery. Its voltage is 12.7V. The
charger circuit measures 13.7V. And I measured the drain, from the
alarm and radio, it is 10mA.

The ambient temperature on average is about 40F.

What went wrong? Why is the battery only 12.7V instead of 13.7?

Lacking a better solution from you guys it seems we need more power,
ugh, ugh. 2A ought to do it.

Spec's say that car batteries (at room temperature) are best regulated
at 13.3V. For 32 degrees F 14.2V is better.

Yet the failure analysis remains incomplete. Where did we go wrong?

Thanks for your help.

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.