Charging my Car Battery with Lap top AC Adapter.

25 Nov 2005
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United Kingdom
Charging my Car Battery with Lap top AC Adapter.

I have not used my car for the last 3 weeks and the cold weather makes even worse. Hence, the battery is flat and the voltage across terminal reads 11.3V. I butchered my old laptop AC adapter to make my own home made car battery charger. It is supposed to provide (as per label) 15V and 4A output. However, When I connected up to the car battery (the batter still connected in the car) the voltage drops to 14.3V and the current drown is 0.32 A. Can someone tell me why please? Does this mean that battery is partially charged not completely flat and only draws the needed current? If so how long does it need to charge a 70AH battery

I also I connected the above (my home made car battery charger) to an old car battery (60 AH) I have in the garage (3 years old) and the voltage reads the same (14.3V) but the drawn current is 1.7A and I can hear some chemical reaction nose happing in side the battery..
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Basically the battery needs more than your little charger can provide.....voltage and current drop are normal when readings are taken off load.
The chemical reaction you hear from the other battery is normal cell gassing.
I can't see anything particularly wrong with your charger assuming the output is smooth DC (it should be, i would think) and theoretically at a 1 amp charge the battery would take 70 hr to charge up - not including internal losses.
Lets hear what the others say.....where art thou, Peter N?
John :)
Burnerman- thanks for the reply. Yes the output is a smooth DC and the charger is protected with 3 Am, fuse.
This is what I hoped; the adapter would give me between 1 to 4 am, but it is not, it provides 0.32 am which means it will take (281 hours i.e 9 days to recharge). This way the battery is charged slowly and safely if someone can wait.

However, I am experimenting with another laptop charger which is supposed to provide 19V and 4 am current as per label . But when it is connected (onload) the voltage reads 17.1V and the current 3.4am. Is this safe to use and how would I know that I am not over/ripe charging the battery
I think your voltage is on the high side here....although car alternators charge at 14.5v, 17v is overdoing it a bit, although I don't know what chemical effect this would have on the battery. The current rating is ok.
You could experiment with a voltage regulator 'chip' from the likes of Maplins but it would need to be able to handle the high current.
John :)
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Yep, and just because something says 4A on the tin doesn't mean it will put out 4A all the time - it will put out what it needs to depending on the load.
What type of battery is it? Silver calciums need a different type of charger to normal lead acid batteries.
Advice as above - buy the correct charger for the job.
I`ve heard it all now! I know there is a credit crunch on, times are hard & its just after Christmas etc but go get yourself either a new battery or a proper charger before you blow your house up!