How do I charge a 12V battery from a 12V 30A power supply?

This is my first post ever and I am completely new to this but would like to be able to charge a sealed lead acid battery. The battery is 12V12Ah / 20HR.  Is it possible to charge directly from power supply which is 12V30A or do I need a resistor to limit current? Besides a power supply is it also possible to recharge the battery a different way? I tried online and made a circuit that didn't really output correct voltage. Can anybody please help?

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steveastrouk5 months ago

That little yellow thing with a Phillips screw in it, on the left is a trim pot. If you can adjust it to 13.2 Volts you can charge the battery directly.....

Etronix (author)  steveastrouk5 months ago

Thanks. I was thinking about trying that. But there's still the issue of applying too much current into the battery? If I apply 13.3V will the current output 30A or will it just supply the necessary current?

The output current is the MAX the supply can give. Electrical things draw as much as they need. You should be OK.

You will need to monitor the battery when charging to ensure it doesn't over charge though.


A whole load of battery charger ccts here.

Rajkumar25065 months ago

You can also use this circuit.Click on the below image to enlarge it.

Etronix (author)  Rajkumar25065 months ago

At that point, couldn't I just apply 15V directly to the battery? Do I need a transformer? Is this to limit the current? Sorry for the many questions.

Yes definitely you need a 12V transformer.The capacitor 2200uF will amplify the 12V to 16.4V. After that you can have 15V by adding 7815

Amplify is the wrong word for what's happening.

Your maths is wrong too, and your method is ALSO wrong, because the 12V is the onload rating of the transformer, in AC volts. Its AC output at unloaded condition depends on the regulation of the transformer, but could be easily 20% more.

And a 7815 needs more than 1.4 V to regulate the load, at least 2V, probably 2.5.

2200uf is a filter capacitor.It rejects ripples.When the ripple is rejected as a result voltage will be raised .If you add capacitor after 7815 there will be no raise in voltage because there is no ripple exists.After 7815 the voltage will be a pure straight line DC

Better than "amplifies".

Downunder35m5 months ago

Batteries need to be charged to their specifications.
As said by other already a dedicated charger is what you need.
And you can't really charge a 12V battery with a 12V supply as you need at least 13.2V to reach a full charge, 13.8V is a common standard for the charging voltage.

Etronix (author)  Downunder35m5 months ago

My other power supply actually outputs a little over 15V. Could I just lower the voltage to acheive the 13.8V to charge the battery? Does current matter or is it just the voltage being applied to the battery?

Yonatan24 Etronix5 months ago

I've heard that they charge at 14.4V. The voltage probably drops from 15+ to around 14 once you connect it.

Etronix (author)  Yonatan245 months ago

So do you think I can apply 15V 1A charger to the battery directly?

Yonatan24 Etronix5 months ago

I don't know, though I'd say yes only in an emergency. The power supply will probably heat up a lot...

Etronix (author)  Downunder35m5 months ago

My power supply has a variable voltage that will output a little over 15V. My LM317 will regulate to the charging voltage. Hopefully I can just apply 13.8V and see how that goes.

Rajkumar25065 months ago

The 12V battery looks like it cannot withstand 30A current.If you connect 30A your battery will either leak or get wasted.It can only withstand 2A.So you need to add resistor to limit the current.Reducing 30A current to 2A current cannot be done by a normal resistor.You need to increase the resistor watts(W).Other wise your resistor will be heated and damaged.A 12V battery cannot be charged by 12V.In order to charge you need to have atleast 15V, 2A.To make 15 V,2 A.Use 12V,2A transformer and connect the transformer to bridge rectifier,at the output of bridge rectifier add 1000uf capacitor in parallel.If you do this your 12V,2A will be ampliied to 16.4V, 2A.Then add 15 Volt regulator 7815 at the output of capacitor.Now you have got 15V, 2A.Add heat sink to 7815 because the regulator is withstanding 2A,it could be get heated and damaged.

What ? You do NOT want 15V from the charger.

Etronix (author)  Rajkumar25065 months ago

That's what I figured but heard it's extremely inefficient using a lot of heat on the resistors. But why 2A when my battery is 12Ah? I also made a circuit using the LM317 regulator but doesn't seem to work at the moment.

Increasing the current will decrease the charging time of the battery.So,for faster charging 2A current can be used.

rickharris5 months ago

Different way HOW?

Buy a 12 volt batter charger. It will save you a lot of effort and worry.

Etronix (author)  rickharris5 months ago

There's many circuits online. I looked at this part one but not sure if this is working. I did the circuit but it's not outputing the desired voltage.

rickharris5 months ago