# Help charging lead acid battery

Discussion in 'Power Electronics' started by lucretius, Apr 16, 2014.

1. ### lucretius

3
0
Apr 16, 2014
I need to charge my car battery which is pretty much completely dead. I have a variable voltage power supply thats rated for 5A.

Ive had it hooked up to the battery at about 15V (if measured without anything hooked up to the power supply) And Ive found that around 130mA flows in this config.
if I measure the voltage while its hooked up to the battery I get a reading of around 12 volts.

Ive tried starting the car after around 2 hours of charging and its not really enough.

What i would have thought is that when set up like this my volt meter should give a reading thats the difference between the power supply and the battery(battery gives about 12 volts by itself).

should i turn up the voltage ? how long should this take ? and any other general advice please

Thanks

Also here are some ideas I had about trying to figure out how long itd take:
So whist starting the car draws around 300A aprarently for about 2-3 seconds to start. so at 12V thats (300x12)x3 = 10,800 joules

while charging im providing about 0.13A at 12V (i know the current will vary as it charges but Im assuming its constant) so for that to provide 10,800 J it will take at 1.56W about 115 minutes. So this is assuming that all the energy im putting in is perfectly stored which is isnt. So im going to double it which gives about 3h 51m

Last edited: Apr 16, 2014
2. ### cjdelphi

1,096
104
Oct 26, 2011
You need a higher voltage, 14.2v will be fine, then the current draw will be much higher, supplying the voltage at 12v wont do much at all.

3. ### BobK

7,682
1,688
Jan 5, 2010
Turn the voltage up until you get 1A anyway. At 1A it should charge enough for a start in a couple of hours.

Bob

3
0
Apr 16, 2014
thanks guys

5. ### lucretius

3
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Apr 16, 2014
haha it worked.