In article ,
Bruce W...1 wrote:

This is not ham radio related but I know you guys have the answer.

I need to store a car unattended for a winter in the midwest. And I
want to put a tricke charge on the battery.

I'd rather not buy a fancy trickle charger because it would probably get
stolen. On the other hand I have a vast array of wall wart
transformers.

If I connected one of these transformers how many milliamps should it
put out at 12V? Or rather what's the least current that would do the
job?

I would guess that 100 milliamperes would be more than sufficient to
overcome any self-discharge of the battery, as long as the vehicle
doesn't have any electronics onboard which are drawing current.

It seems that lead acid batteries can dissipate too much current as
heat. In other words I could probably feed it one amp continuously.

You really don't want to do that.

The reason is this: in order for a car battery to be drawing 1 amp of
charge current on a continuous basis, you're going to have to boost
the voltage up fairly high. You'd probably exceed the electrolysis
voltage, and much of the current would end up breaking down the water
in the electrolyte into oxygen and hydrogen. If your battery's design
and chemistry don't allow the gasses to be recombined into water fast
enough, you'd be at risk of "boiling" the battery dry.

But would 100 mA at 12V do the job? I'd hate to lose a 1-Amp wall wart.

Lead-acid batteries are happiest if you feed them a well-regulated
charge/float voltage, with suitable current limiting. Using an
unregulated or weakly-regulated 12-volt wall-wart is probably not a
good idea - these wall warts tend to deliver a rather high voltage
(often 16-18 volts) under conditions of little or no load. Depending
on the specific wall-wart you choose, and the condition and type of
your battery, you might end up electrolyzing away the water faster
than the battery can recombine the hydrogen and oxygen. Losing a
wall-wart would be annoying; losing the battery would be worse ;-)

The best thing to do is get (or build) yourself some sort of
well-regulated trickle charger. 100-200 mA is probably more than
plenty for this application, as long as it's provided at a
well-regulated voltage. Most battery companies seem to recommend
between 13.6 and 13.8 volts for a "float charging" application - at
this voltage, the battery will self-regulate the amount of current it
takes and will not overcharge or electrolyze itself. One
knowledgeable amateur I know, recommends sticking to a lower 13.5
volts to ensure safe float-charging under a wide range of charge
conditions and temperatures.

About a year ago I put together a simple float charger to keep the
65-amp-hour glassmat battery in our city's RACES ham-shack properly
charged. It's a simple design, based on the jellybean LM317
three-terminal regulator IC and on the schematics in National
Semiconductor's data sheet for this IC. My version includes reverse
polarity and short-circuit protection, a charge float voltage which is
adjustable over a span of a volt or so, and built-in current limiting
to protect the wall wart (a 200 mA 16-volt unit, if I recall
correctly). It's not a bad one-afternoon project and the components
are the sort of thing most homebrewers are likely to have in their
junk-box.

Schematic is at http://www.radagast.org/~dplatt/hamradio/charger.pdf

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Get the least expensive battery charger you find and put a one of
those cheap little mechanical timers on it. And set the time to
charge the battery for one hour each day. It will be like driving
your car one hour per day.

(Dave Platt) wrote in message ...
In article ,
Bruce W...1 wrote:

This is not ham radio related but I know you guys have the answer.

I need to store a car unattended for a winter in the midwest. And I
want to put a tricke charge on the battery.

I'd rather not buy a fancy trickle charger because it would probably get
stolen. On the other hand I have a vast array of wall wart
transformers.

If I connected one of these transformers how many milliamps should it
put out at 12V? Or rather what's the least current that would do the
job?

I would guess that 100 milliamperes would be more than sufficient to
overcome any self-discharge of the battery, as long as the vehicle
doesn't have any electronics onboard which are drawing current.

It seems that lead acid batteries can dissipate too much current as
heat. In other words I could probably feed it one amp continuously.

You really don't want to do that.

The reason is this: in order for a car battery to be drawing 1 amp of
charge current on a continuous basis, you're going to have to boost
the voltage up fairly high. You'd probably exceed the electrolysis
voltage, and much of the current would end up breaking down the water
in the electrolyte into oxygen and hydrogen. If your battery's design
and chemistry don't allow the gasses to be recombined into water fast
enough, you'd be at risk of "boiling" the battery dry.

But would 100 mA at 12V do the job? I'd hate to lose a 1-Amp wall wart.

Lead-acid batteries are happiest if you feed them a well-regulated
charge/float voltage, with suitable current limiting. Using an
unregulated or weakly-regulated 12-volt wall-wart is probably not a
good idea - these wall warts tend to deliver a rather high voltage
(often 16-18 volts) under conditions of little or no load. Depending
on the specific wall-wart you choose, and the condition and type of
your battery, you might end up electrolyzing away the water faster
than the battery can recombine the hydrogen and oxygen. Losing a
wall-wart would be annoying; losing the battery would be worse ;-)

The best thing to do is get (or build) yourself some sort of
well-regulated trickle charger. 100-200 mA is probably more than
plenty for this application, as long as it's provided at a
well-regulated voltage. Most battery companies seem to recommend
between 13.6 and 13.8 volts for a "float charging" application - at
this voltage, the battery will self-regulate the amount of current it
takes and will not overcharge or electrolyze itself. One
knowledgeable amateur I know, recommends sticking to a lower 13.5
volts to ensure safe float-charging under a wide range of charge
conditions and temperatures.

About a year ago I put together a simple float charger to keep the
65-amp-hour glassmat battery in our city's RACES ham-shack properly
charged. It's a simple design, based on the jellybean LM317
three-terminal regulator IC and on the schematics in National
Semiconductor's data sheet for this IC. My version includes reverse
polarity and short-circuit protection, a charge float voltage which is
adjustable over a span of a volt or so, and built-in current limiting
to protect the wall wart (a 200 mA 16-volt unit, if I recall
correctly). It's not a bad one-afternoon project and the components
are the sort of thing most homebrewers are likely to have in their
junk-box.

Schematic is at http://www.radagast.org/~dplatt/hamradio/charger.pdf

Dave Platt wrote:
It's not a bad one-afternoon project and the components
are the sort of thing most homebrewers are likely to have in their
junk-box.

Schematic is at http://www.radagast.org/~dplatt/hamradio/charger.pdf

Interesting circuit.

I have to beg a fair amount of ignorance about battery behavior. I've
got a couple of the smaller sealed lead-acid batteries - the one on the
desk right now is 4Ah.

Judging from what (little) I know about these batteries, I presume it
would be safe to use your circuit as-is - that the maximum 200mA
charging current, while intended to protect the unregulated source,
would also be low enough to not overheat the battery.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com

In article ,
Doug Smith W9WI wrote:
Dave Platt wrote:
It's not a bad one-afternoon project and the components
are the sort of thing most homebrewers are likely to have in their
junk-box.

Schematic is at http://www.radagast.org/~dplatt/hamradio/charger.pdf

Interesting circuit.

I have to beg a fair amount of ignorance about battery behavior. I've
got a couple of the smaller sealed lead-acid batteries - the one on the
desk right now is 4Ah.

Judging from what (little) I know about these batteries, I presume it
would be safe to use your circuit as-is - that the maximum 200mA
charging current, while intended to protect the unregulated source,
would also be low enough to not overheat the battery.

Should be fine. According to the Power-Sonic Technical Handbook
(http://216.87.171.32/manuals/techman.pdf) charge currents should not
be allowed to exceed 0.20 * C amperes - which would be 800 mA for a
4Ah battery. I prefer to stick to 0.10 * C amperes if I can afford to
wait overnight for the battery to charge.

I mis-stated the actual current limit provided by the schematic I
posted... this schematic is a modified version of what I actually
built, and has a higher current limit which would allow it to
be used with a larger wall-wart. The current limit is set by the
value of R4 - the charger goes into current-limiting when the voltage
drop across this resistor reaches 0.7 volts and turns on Q1. The
1-ohm value in the schematic sets the limiting point at 700 mA (not a
bad value to use when charging a standard 7Ah gel cell). 3 ohms would
give you a bit more than 200 mA.

In actual use, when charging gel cells, you're likely to find that the
charging circuit only goes into current-limit if the battery is quite
deeply discharged. Once the battery charges up to a significant
fraction of its total capacity, its terminal voltage will rise up to
13.5 (or whatever float voltage you've tweaked the charger for) and
the current flow will decrease, bringing the charger out of limiting.
It will then charge at a slower (and steadily decreasing) rate until
fully replenished, and will then draw perhaps 0.01 * C amperes of
float/trickle charge current indefinitely.

In order to charge the battery more rapidly, you'd need to set the
charge voltage up to 14.4 volts or so (the "fast charge" voltage
region) and then set it back down again once the current flow had
dropped below 0.01 * C. Commercial battery chargers often use this
sort of dual-voltage charging scheme to allow for faster recharge.

Note that you should _not_ use this sort of "higher voltage for fast
charging" technique when charging starved-electrolyte (glass mat)
lead-acid batteries - they are rather intolerant of higher charge
voltages and should not be charged at above 13.8 volts. However,
using a higher charge voltage really isn't necessary with these
batteries... they'll soak up charge amazingly fast at 13.6 - 13.8
volts.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

In article ,
Doug Smith W9WI wrote:
Dave Platt wrote:
It's not a bad one-afternoon project and the components
are the sort of thing most homebrewers are likely to have in their
junk-box.

Schematic is at http://www.radagast.org/~dplatt/hamradio/charger.pdf

Interesting circuit.

I have to beg a fair amount of ignorance about battery behavior. I've
got a couple of the smaller sealed lead-acid batteries - the one on the
desk right now is 4Ah.

Judging from what (little) I know about these batteries, I presume it
would be safe to use your circuit as-is - that the maximum 200mA
charging current, while intended to protect the unregulated source,
would also be low enough to not overheat the battery.

Should be fine. According to the Power-Sonic Technical Handbook
(http://216.87.171.32/manuals/techman.pdf) charge currents should not
be allowed to exceed 0.20 * C amperes - which would be 800 mA for a
4Ah battery. I prefer to stick to 0.10 * C amperes if I can afford to
wait overnight for the battery to charge.

I mis-stated the actual current limit provided by the schematic I
posted... this schematic is a modified version of what I actually
built, and has a higher current limit which would allow it to
be used with a larger wall-wart. The current limit is set by the
value of R4 - the charger goes into current-limiting when the voltage
drop across this resistor reaches 0.7 volts and turns on Q1. The
1-ohm value in the schematic sets the limiting point at 700 mA (not a
bad value to use when charging a standard 7Ah gel cell). 3 ohms would
give you a bit more than 200 mA.

In actual use, when charging gel cells, you're likely to find that the
charging circuit only goes into current-limit if the battery is quite
deeply discharged. Once the battery charges up to a significant
fraction of its total capacity, its terminal voltage will rise up to
13.5 (or whatever float voltage you've tweaked the charger for) and
the current flow will decrease, bringing the charger out of limiting.
It will then charge at a slower (and steadily decreasing) rate until
fully replenished, and will then draw perhaps 0.01 * C amperes of
float/trickle charge current indefinitely.

In order to charge the battery more rapidly, you'd need to set the
charge voltage up to 14.4 volts or so (the "fast charge" voltage
region) and then set it back down again once the current flow had
dropped below 0.01 * C. Commercial battery chargers often use this
sort of dual-voltage charging scheme to allow for faster recharge.

Note that you should _not_ use this sort of "higher voltage for fast
charging" technique when charging starved-electrolyte (glass mat)
lead-acid batteries - they are rather intolerant of higher charge
voltages and should not be charged at above 13.8 volts. However,
using a higher charge voltage really isn't necessary with these
batteries... they'll soak up charge amazingly fast at 13.6 - 13.8
volts.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Get the least expensive battery charger you find and put a one of
those cheap little mechanical timers on it. And set the time to
charge the battery for one hour each day. It will be like driving
your car one hour per day.

(Dave Platt) wrote in message ...
In article ,
Bruce W...1 wrote:

This is not ham radio related but I know you guys have the answer.

I need to store a car unattended for a winter in the midwest. And I
want to put a tricke charge on the battery.

I'd rather not buy a fancy trickle charger because it would probably get
stolen. On the other hand I have a vast array of wall wart
transformers.

If I connected one of these transformers how many milliamps should it
put out at 12V? Or rather what's the least current that would do the
job?

I would guess that 100 milliamperes would be more than sufficient to
overcome any self-discharge of the battery, as long as the vehicle
doesn't have any electronics onboard which are drawing current.

It seems that lead acid batteries can dissipate too much current as
heat. In other words I could probably feed it one amp continuously.

You really don't want to do that.

The reason is this: in order for a car battery to be drawing 1 amp of
charge current on a continuous basis, you're going to have to boost
the voltage up fairly high. You'd probably exceed the electrolysis
voltage, and much of the current would end up breaking down the water
in the electrolyte into oxygen and hydrogen. If your battery's design
and chemistry don't allow the gasses to be recombined into water fast
enough, you'd be at risk of "boiling" the battery dry.

But would 100 mA at 12V do the job? I'd hate to lose a 1-Amp wall wart.

Lead-acid batteries are happiest if you feed them a well-regulated
charge/float voltage, with suitable current limiting. Using an
unregulated or weakly-regulated 12-volt wall-wart is probably not a
good idea - these wall warts tend to deliver a rather high voltage
(often 16-18 volts) under conditions of little or no load. Depending
on the specific wall-wart you choose, and the condition and type of
your battery, you might end up electrolyzing away the water faster
than the battery can recombine the hydrogen and oxygen. Losing a
wall-wart would be annoying; losing the battery would be worse ;-)

The best thing to do is get (or build) yourself some sort of
well-regulated trickle charger. 100-200 mA is probably more than
plenty for this application, as long as it's provided at a
well-regulated voltage. Most battery companies seem to recommend
between 13.6 and 13.8 volts for a "float charging" application - at
this voltage, the battery will self-regulate the amount of current it
takes and will not overcharge or electrolyze itself. One
knowledgeable amateur I know, recommends sticking to a lower 13.5
volts to ensure safe float-charging under a wide range of charge
conditions and temperatures.

About a year ago I put together a simple float charger to keep the
65-amp-hour glassmat battery in our city's RACES ham-shack properly
charged. It's a simple design, based on the jellybean LM317
three-terminal regulator IC and on the schematics in National
Semiconductor's data sheet for this IC. My version includes reverse
polarity and short-circuit protection, a charge float voltage which is
adjustable over a span of a volt or so, and built-in current limiting
to protect the wall wart (a 200 mA 16-volt unit, if I recall
correctly). It's not a bad one-afternoon project and the components
are the sort of thing most homebrewers are likely to have in their
junk-box.

Schematic is at http://www.radagast.org/~dplatt/hamradio/charger.pdf

Dave Platt wrote:
It's not a bad one-afternoon project and the components
are the sort of thing most homebrewers are likely to have in their
junk-box.

Schematic is at http://www.radagast.org/~dplatt/hamradio/charger.pdf

Interesting circuit.

I have to beg a fair amount of ignorance about battery behavior. I've
got a couple of the smaller sealed lead-acid batteries - the one on the
desk right now is 4Ah.

Judging from what (little) I know about these batteries, I presume it
would be safe to use your circuit as-is - that the maximum 200mA
charging current, while intended to protect the unregulated source,
would also be low enough to not overheat the battery.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com