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Partly charging Car battery via PC PSU

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SimonW

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Dear All,

I was wondering if anyone had any ideas on how to charge a car battery enough to enable the car to start without a charger connected.

I was thinking of using the +12v line of a PC PSU, is it just a matter of connecting the respective pins together and waiting until the voltage reaches around +12v?

Would I need any components in between the PSU and the battery?

To limit the current to 10A, I have calculated a R value of 1.2ohms will the R power rating be 14.4W?

I am unable to push start the car nor do I know anyone to use jump leads with

Your ideas are appreciated.

Thanks
 
The PC power supply will probably just shut down.
The 10 AMP inrush of current would be seen as a short by the power supply, and the starter motor will draw alot more than 10 amps anyway.
Try a couple amps first,( charging ,not starting) but it will never fully charge a 12v battery, a car alternator puts out 13.5-14.5 volts.
 
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I would only use this as a last resort to get the car started. I would not do this regularly. I would not push more than 5 amp into a car battery without using a control circuit as it could potentially shorten the battery life or in extreme cases buckle the plates.

Why did the battey go flat ? Is it a problem with the battery or the cars electrics/charging system? -If it was due to leaving the car lights on then you should be OK.

The voltage of a fully charged car battery will be about 13.6V so a 12V supply will not fully charge the battery. You may be lucky and be able to start the car when the battery is only charged up to 12V, after which you will need to take the car on a run to get the battery fully charged.


A quick easy way to limit the current is to use a 12V car bulbs as resistors:
21W (indicator) will limit to just under 2 amps, 55W (headlight) will limit to about 5 amps, Connecting a headlight dip and main filaments together will give about 10 amps. Nothing more would be needed between the PSU and battery

If you have a regulated voltage supply to the battery the charging current will reduce as the battery voltage rises.

Hope these quick jottings help (I'm on my way out)
 
I strongly advise against using a PC PSU as a charger on the grounds of safety.
There is a possibility of serious damage or even a fire. Lead acid battery chargers are relatively inexpensive anyhow ( under £20) so I really don't see the point given the amount of time a layperson would need to use one.
 
With a bulb as a current limit device what could go wrong Zed? There's no chance of over charging the battery because the voltage is too low and limiting the current with the bulb prevents boiling the battery.
 
You can't over-charge a car battery by applying only 12V to it.

In fact it is difficult to over-charge a car batteryt under any circumstances except serious over-voltage - they are capable of supplying several hundred amps short term and regularly receive up to 10A when charging from the alternator after starting the engine.
 
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