Charging a car battery - can I use any 12V charger?

[johnfm]

I have a number of old 240v in - 12v out chargers in the house.

Is there any reason I cannot cut off the old connector, conenct the +ve and -ve to a couple of alligator clips and use a 12v 1.5A output to charge a flat battery?

Apart from a lack of monitoring, I cannot see the problem.

[andyiley]

For a start a 12v charger will not charge a car battery, it needs to give out more than 12 volt to charge the battery, your alternator will typically give out around 14 volts.

If you are talking about a phone charger or similar, it would also take forever as they give out much less current than a normal car battery charger.

If you have any more information on what you propose to use, post it as it will affect its usefullness.

[johnfm]

Andy

Some trickle charges are rated at 0.5A - which is, I suppose, why they are called 'trickle chargers'.

[andyiley]

Yes you are absolutely correct, they are, alas they give out more than 12 volts.

When you said charge a flat battery I assumed you meant within a reasonable time scale.

If you want it to take all week you MAY get it to 'charge' the battery.

If you want it to be done properly you will need more than 12 volts & more than 1.5amps.

If you actually measure a "flat" battery (ie one that will not start a car) you will probably find it is around 10.5 volts, in other words not flat.

If you measure a fully charged battery you will likely find it measures around 12.5 volts, if you measure a battery on charge you will find it to be around 14 volts.

Therefore your 12v (assumed) phone/laptop/etc charger will not fully charge the battery & will take forever to not fully charge the battery.