I have some 12 volt batteries that were used in alarm systems. They
apparently still have some good life in them. My question is about charging
them. I was told that I can just take a 12 volt DC wall wart and connect it
across the battery= pos to pos or pos to neg??= Is that true? Do I need to
be concerned about current ratings vs. ampere hours of battery? Is there a
way of checking the ampere hours of the battery if not stamped thereon?
Thanks-
email appreciated at

"Mike Muderick" wrote in message
news:VYgQd.21992$ya6.3244@trndny01...
I have some 12 volt batteries that were used in alarm systems. They
apparently still have some good life in them. My question is about
charging
them. I was told that I can just take a 12 volt DC wall wart and connect
it
across the battery= pos to pos or pos to neg??= Is that true?

Maybe... the supply voltage needs to be far enough above battery voltage to
allow it to fully charge.

The supply must be able to provide sufficint current to charge in a
reasonable amount of time (say 10 to 12 houres).

The supply must be current limited so as not to destroy itself or catch on
fire when shorted out (as in charging a depleted battery.

Do I need to
be concerned about current ratings vs. ampere hours of battery?

You need to be concerned about max charging current. too much causes
excessive heating which might lead to more serious issues.


Is there a way of checking the ampere hours of the battery if not stamped
thereon?

part number which leads to MFR spec sheets.

or (less accurately) compare volume and weight of you batteries with ones of
a known value.

For some picture comparisons try http://www.emesystems.com/batcharg.htm

Thanks-
email appreciated at

you can get cheap chargers designed for Lead Acid's at your local hobby
store as must modellers use one
"TimPerry" wrote in message
news

"Mike Muderick" wrote in message
news:VYgQd.21992$ya6.3244@trndny01...
I have some 12 volt batteries that were used in alarm systems. They
apparently still have some good life in them. My question is about
charging
them. I was told that I can just take a 12 volt DC wall wart and connect
it
across the battery= pos to pos or pos to neg??= Is that true?

Maybe... the supply voltage needs to be far enough above battery voltage
to
allow it to fully charge.

The supply must be able to provide sufficint current to charge in a
reasonable amount of time (say 10 to 12 houres).

The supply must be current limited so as not to destroy itself or catch on
fire when shorted out (as in charging a depleted battery.

Do I need to
be concerned about current ratings vs. ampere hours of battery?

You need to be concerned about max charging current. too much causes
excessive heating which might lead to more serious issues.


Is there a way of checking the ampere hours of the battery if not stamped
thereon?

part number which leads to MFR spec sheets.

or (less accurately) compare volume and weight of you batteries with ones
of
a known value.

For some picture comparisons try http://www.emesystems.com/batcharg.htm

Thanks-
email appreciated at

"TimPerry" bravely wrote to "All" (15 Feb 05 02:11:34)
--- on the heady topic of " battery charging"

Ti From: "TimPerry"
Ti Xref: aeinews sci.electronics.repair:10570

Ti Maybe... the supply voltage needs to be far enough above battery
Ti voltage to allow it to fully charge.

Ti The supply must be able to provide sufficint current to charge in a
Ti reasonable amount of time (say 10 to 12 houres).

Ti The supply must be current limited so as not to destroy itself or
Ti catch on fire when shorted out (as in charging a depleted battery.


Do I need to
be concerned about current ratings vs. ampere hours of battery?

Ti You need to be concerned about max charging current. too much causes
Ti excessive heating which might lead to more serious issues.

Lately stores have been selling portable car battery backup systems in
various sizes. One such model claims 500ccA from a 12V 18aH sealed
battery. I suppose it might work however the charging time is spec'd
at between 36 to 72hrs. Reading the manual a little more closely I
finally found that the charging current from the AC line is but a mere
0.5A. No wonder it takes so long to charge up! 18aH/0.5A=36hrs but
IIRC a lead-acid battery requires 10 to 20% more time to achieve full
capacity.

The manual also hints that the 12V power socket can be used for
charging directly from the car's lighter socket however it warns not
to exceed 4hrs. The current is thermally limited to 10A at the 12V
socket but the connecting cable seems a little flimsy.

Anyways I'm wondering if increasing the AC charging current a little
say to 1 or 2 amperes would be safe for the sealed battery?

BTW I've never seen a car with a dead battery actually start with one
of these gadgets. So far it stands at 3 for 3 no go...

Comments?

A*s*i*m*o*v

.... Electrical engineers deal with current events.

Hi!

BTW I've never seen a car with a dead battery actually start with one
of these gadgets. So far it stands at 3 for 3 no go...

Well, I have.

This was done with a 1989 Buick Electra with a really dead battery in the
wintertime...the battery was worn out and happened to be so dead that it
would not even illuminate the dome lights. Several smaller examples of these
"jump starters" were tried with no success...so I went and got my "really
big" Vector-branded one. Hooked it up and the interior lights came back on
to normal intensity.

The car turned over *very* sluggishly, but it started. The cables on the
jump starter got rather warm. The jump starter was then removed after
letting the car run for a few minutes. It became obvious at that time how
bad the car's battery really was--the alternator was unable to provide well
regulated power (you could see the lights going up and down in intensity)
and the car stalled a few minutes later.

I don't think this is the appropriate use for one of these though--I think
they are made more for the purpose of starting a car whose battery is
"almost good enough" to do the job itself.

William