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

Rick Frazier wrote:

Bruce:

As others have indicated, you really want to use a regulated charging
voltage to keep the battery up... The voltage is important, however, more
important to getting the design right is knowing how much of a drain the
automobile puts on the battery when "nothing" is going on... Many cars will
drain a good battery in about a month, just from the residual drain that is
the result of such things as clocks and memories in radios and such. Add
anything else and you've got a good chance of needing significantly more
charging current than you might at first think.

You mention that you are worried about using a fancy trickle charger because
it may get stolen, which implies that the car is outside in the elements,
and not in a protected or secured environment. I'd be willing to bet that
if you have to worry about someone stealing a trickle charger, you should
probably be concerned about just about everything else too, once someone
notices the car isn't moving and has a power cord going to it....

In addition, being concerned about a 1-amp wall wart, which are nearly a
dime a dozen, may be false economy, considering the value of the car and
items built or installed in it. Why wouldn't the tires and wheels
disappear, or the battery itself? Same for radio and/or other
accessories... or even perhaps the car itself.

If you do go and build something to keep the battery up, bear in mind that
you are probably going to need something with a voltage around 16 volts or
so, given that most regulators require a couple of volts of headroom over
the regulated voltage. This may take you into a range of supplies or wall
warts that is higher than you may have on hand anyway.

--Rick

================================================== ====

You guys are getting unnecessarily complex me thinks.

Yes a voltage regulator would be nice but not really essential. And
this might get stolen too. Besides, if I was going to go to that effort
I'd just buy a float charger (about $25).

The car is in a garage in a good neighborhood but the owner of the
garage is on and extended vacation. The theives I'm worried about are
neighborhood children. Actually it's the homeowner's car, I'm just
doing them a favor, and saving myself from having to go there to start
the car once a week. So I'd rather not spend money when I have a
plethora of wall warts.

The car does have an alarm but I don't know how much current it pulls.

Case, I've got one wall wart which has a 17V open voltage. When I pull
it down to 12.0 volts the current is 120 mA. Might this work?

I could increase the size of the transformer if needed. It's just that
I haven't yet figured out what's needed.

Lead-acid batteries have a rated current at which they can be float
charged without causing any damage. This is the number I need, but have
not been able to find it.

Here's a related article:
http://www.4unique.com/battery/battery_tutorial.htm

Rick Frazier wrote:

Bruce:

As others have indicated, you really want to use a regulated charging
voltage to keep the battery up... The voltage is important, however, more
important to getting the design right is knowing how much of a drain the
automobile puts on the battery when "nothing" is going on... Many cars will
drain a good battery in about a month, just from the residual drain that is
the result of such things as clocks and memories in radios and such. Add
anything else and you've got a good chance of needing significantly more
charging current than you might at first think.

You mention that you are worried about using a fancy trickle charger because
it may get stolen, which implies that the car is outside in the elements,
and not in a protected or secured environment. I'd be willing to bet that
if you have to worry about someone stealing a trickle charger, you should
probably be concerned about just about everything else too, once someone
notices the car isn't moving and has a power cord going to it....

In addition, being concerned about a 1-amp wall wart, which are nearly a
dime a dozen, may be false economy, considering the value of the car and
items built or installed in it. Why wouldn't the tires and wheels
disappear, or the battery itself? Same for radio and/or other
accessories... or even perhaps the car itself.

If you do go and build something to keep the battery up, bear in mind that
you are probably going to need something with a voltage around 16 volts or
so, given that most regulators require a couple of volts of headroom over
the regulated voltage. This may take you into a range of supplies or wall
warts that is higher than you may have on hand anyway.

--Rick

================================================== ====

You guys are getting unnecessarily complex me thinks.

Yes a voltage regulator would be nice but not really essential. And
this might get stolen too. Besides, if I was going to go to that effort
I'd just buy a float charger (about $25).

The car is in a garage in a good neighborhood but the owner of the
garage is on and extended vacation. The theives I'm worried about are
neighborhood children. Actually it's the homeowner's car, I'm just
doing them a favor, and saving myself from having to go there to start
the car once a week. So I'd rather not spend money when I have a
plethora of wall warts.

The car does have an alarm but I don't know how much current it pulls.

Case, I've got one wall wart which has a 17V open voltage. When I pull
it down to 12.0 volts the current is 120 mA. Might this work?

I could increase the size of the transformer if needed. It's just that
I haven't yet figured out what's needed.

Lead-acid batteries have a rated current at which they can be float
charged without causing any damage. This is the number I need, but have
not been able to find it.

Here's a related article:
http://www.4unique.com/battery/battery_tutorial.htm

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

Although their normal price is about $15, float battery chargers (wall
wart, coiled cord, little black box with LED, two cords with large
alligator clips on the ends) are often advertised in Harbor Freight
catalogs for $7.99. I must have half a dozen of them now, keeping
batteries here and there up to snuff. I've never measured them, but
they seem to do a real good job, and they're hard to beat at that price!

I have no connection to Harbor Freight except as a satisfied customer,

--Myron.
--
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

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

Although their normal price is about $15, float battery chargers (wall
wart, coiled cord, little black box with LED, two cords with large
alligator clips on the ends) are often advertised in Harbor Freight
catalogs for $7.99. I must have half a dozen of them now, keeping
batteries here and there up to snuff. I've never measured them, but
they seem to do a real good job, and they're hard to beat at that price!

I have no connection to Harbor Freight except as a satisfied customer,

--Myron.
--
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

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!

Keeping the battery charged is not the only reason to start the car once
a week.

Wow, that is a revelation. I hope most people reading this group would know
that.
73 Gary N4AST

Keeping the battery charged is not the only reason to start the car once
a week.

Wow, that is a revelation. I hope most people reading this group would know
that.
73 Gary N4AST

On Tue, 04 Nov 2003 15:50:45 -0600, "Clifton T. Sharp Jr."
wrote:

Bruce W...1 wrote:
The car is in a garage in a good neighborhood but the owner of the
garage is on and extended vacation. The theives I'm worried about are
neighborhood children. Actually it's the homeowner's car, I'm just
doing them a favor, and saving myself from having to go there to start
the car once a week.

Keeping the battery charged is not the only reason to start the car once
a week.

Suppose it means that a car battery won't last long if you keep it
steady at top charge level, instead of decharge it down towards 50%,
sense the voltage level and charge it up to 14.4V (for this cold
area), and switch off the charger until it is down to 50% again....

I was told that a car battery wouldn't last long anyway for this
application, so even when my car battery is 6-7 years old, it
wouldnt't last more than a year when trickle charged, while it is far
better economy to buy a leisure type battery

73
Jan-Martin, LA8AK
http://home.online.no/~la8ak/
--
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