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# Lead-Acid battery charger using pc power supply unit

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#### samEEEf

##### Full Member level 2
I've a 12v sealed lead-acid battery. Now I want to make a charger to charge it from our line voltage (230V AC). From my general knowledge, I can use a step down transformer (primary 230v, secondary 12v-0v-12v) as center tapped full wave rectifier to charge. Which is very preliminary circuit.

I've a spare PC power supply unit. Can I use the it to charge the 12v lead acid battery?
12v output of PC power supply cannot be used to charge the 12v battery as a 12v battery needs more than 12v dc to charge. So, can I use PC power supply's 12v in addition with 3.3v(or 5v) to charge it? Can I add the two voltage to get higher voltage to charge it like 12+3.3=15.3v?

#### klystron

Yes, but be carefull with earthing, grounding and the negatives of the two PSUs

samEEEf

### samEEEf

Points: 2

#### samEEEf

##### Full Member level 2
Rating of my PC's PSU is -
12V dc 20A and 3.3V dc 16A
So, if I connect -ve terminal of 3.3v with +ve terminal of 12v I'm supposed to get 15.3v dc from +ve terminal of 3.3v & -ve terminal of 12v with max current of 16A. Am I right?

#### eem2am

##### Banned
Be careful here,

you could blow up battery

you charge battery at C/10 maximum.

ti.com have an excellent switch mode lead acid charger for you.

and excellent app note telling you the information you want.

Lead acid must use "4-stage charger"

1. Light current trckling
2. Heavy current charge
3. Constant voltage.
4. Constant voltage but with current limit to trickle

............you really need to get these conditions right for lead acid

#### SDW

##### Newbie level 1
Hi Guys, Over the weekend I needed to charge my sealed lead acid car battery, so I quickly whipped up the following circuit. the +18v was from a laptop switching power supply. The 10K VR adjusts the pulse width of the signal to the mosfet via NPN transistor which is at about 2khz. I put a DMM in series with the battery and adjusted the VR until it showed 3 amps, and left charging overnight.

After some google'ing, I've realized that I probably needed an inductor and possibly an inline resistor (.5 ohms) in line with the battery. What is the purpose of the inductor? Am I just making hydrolysis in the battery, as even when turned down to around .5 amps, you can still hear the battery bubbling.

Thanks.

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