In message , Andrew Gabriel
writes
In article ,
"Andrew Mawson" writes:

"Tom" wrote in message
...
Hi All,

I've been aiming to buy a new car battery trickle charger for a
while
- I've noticed that there are some very small trickle chargers that
are 500mA output and looks like a plug in power block.

Then I found I had an old adaptor from a defunct jump start unit -
the
jump start was a yellow Rolson model for jump starting 12v car
batteries - but the charger that came with it is, strangely enough,
labelled 15v 500mA.

A subsequent jump starter (same yellow model, different branding)
had
a charger that was labelled 12v. Both these jump starters have gone
west and I no longer have them.

Therefore could I use this little 15v/500mA charger as a trickle
charger for my car battery, like the block trickle charges I've
seen?
If so I think all I would need is a couple of large crocodile clips
to
put on the lead - but for charging purposes which goes on the red
positive battery terminal ?

I have a feeling that the charger's negative goes on the battery's
positive terminal and the charger's positive goes on the battery's
negative terminal. Is this right?

Thanks, Tom

No! negative to negative and positive to positive or your wall wart
will be history pdq

Many of these wall warts do not contain any voltage regulation and
will kill a lead acid battery if connected for significantly longer
than the time to charge it. There may be a clue in the original
instructions for the jump-starter, if it says something like don't
charge for more than 24h (although absence of such a statement is
no guarantee as the voltage regulation may be in the part you're
chucked out).

A friend accidently left a jumpstarter charging for a week on one
of these, and that was the end of the battery. I have the same
model, but I used a proper voltage regulated charger and not the
wall wart which came with it, and some 6 or 7 years later, mine
still works fine.

There's no way that a 500mA wall wart could ever deliver enough current
to kill a car battery.

The important thing would be to check what the charging current actually
was (flat battery and full battery), and see if there was enough / not
too little to be of use / not too much so that it would kill the wall
wart.
--
Ian