# How to make a car battery charger using a transformer 11.2v

#### Martiio

Joined Apr 2, 2020
43
I have a transformer 11.2v and around 5A. I would like to use it in order to make a car battery charger.How can I increase the voltage to 14v?

#### MrChips

Joined Oct 2, 2009
24,611
A transformer rated at 11.2VAC rms under load will output over 15.5V when rectified. Output voltage with no load will be even higher.

I think 5A output is not enough to charge a car battery. I have to check this.

#### Ramussons

Joined May 3, 2013
1,098
You will need about 14.2 volts to Charge a car battery, 13.8 to keep it in a float condition.
You can use a Boost convertor to raise the voltages, but your current capacity will reduce. 11.2@ 5A = 56 Watts. A convertor with 85% efficiency will bring it down to about 47 watts, and at 14.2 volts the current will be around 3.3 amps.
A dead battery will get some life on a overnight charge at this current, but not likely to be charged to full capacity.

You can make a 13.8 volts float charger to maintain the battery in a charged condition.

#### Martiio

Joined Apr 2, 2020
43
A transformer rated at 11.2VAC rms under load will output over 15.5V when rectified. Output voltage with no load will be even higher.

I think 5A output is not enough to charge a car battery. I have to check this.
The transformer is 11.2v after rectified! Sorry that I did not say that detail!

#### Martiio

Joined Apr 2, 2020
43
You will need about 14.2 volts to Charge a car battery, 13.8 to keep it in a float condition.
You can use a Boost convertor to raise the voltages, but your current capacity will reduce. 11.2@ 5A = 56 Watts. A convertor with 85% efficiency will bring it down to about 47 watts, and at 14.2 volts the current will be around 3.3 amps.
A dead battery will get some life on a overnight charge at this current, but not likely to be charged to full capacity.

You can make a 13.8 volts float charger to maintain the battery in a charged condition.
To be honest I am not sure if the transformer is more than 5A,because i don't have such a load do measure the maximum of the transformer!

#### Ramussons

Joined May 3, 2013
1,098
What is the transformer core area?
How many output voltages does it have?

#### Ramussons

Joined May 3, 2013
1,098
Ok, 4 Amps instead of 3.3 A

#### Martiio

Joined Apr 2, 2020
43
What is the transformer core area?
How many output voltages does it have?
The output voltage is 12v before rectified

#### ElectricSpidey

Joined Dec 2, 2017
1,962
A transformer that size will not continue to output 12 volts @ 5 Amps, if it can even produce 5 Amps in the first place.

The voltage output is good for a battery charger...but not enough current for a car battery.

#### crutschow

Joined Mar 14, 2008
28,170
The voltage output is good for a battery charger...but not enough current for a car battery.
It can charge a car battery.
It will just take longer.

#### ericgibbs

Joined Jan 29, 2010
14,147
The transformer is 11.2v after rectified! Sorry that I did not say that detail!
hi.
TS post #4

E

#### ElectricSpidey

Joined Dec 2, 2017
1,962
It can charge a car battery.
It will just take longer.
This is true, but if you set that supply up as a brute force charger (Transformer, bridge and cap) and connected it to a discharged car battery...I would be worried.

#### Martiio

Joined Apr 2, 2020
43
This is true, but if you set that supply up as a brute force charger (Transformer, bridge and cap) and connected it to a discharged car battery...I would be worried.

#### ElectricSpidey

Joined Dec 2, 2017
1,962
Overheating the transformer.

Of course you could get lucky and the voltage would drop low enough that it wouldn't produce that much current.

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#### MisterBill2

Joined Jan 23, 2018
9,797
To be honest I am not sure if the transformer is more than 5A,because i don't have such a load do measure the maximum of the transformer!
OK, here is the solution which will provide some battery charging current. Use a full wave voltage doubler circuit! That will give you about 22 volts no load but it will drop quite a bit with a load. But it will charge the battery, just not in a hurry.
The voltage doubler looks a whole lot like two half-wave rectifier/capacitor circuits in series, both powered by the same transformer connection. And that is what it is, which is why you get twice the voltage at no load. You will need two capacitors of probably 1000 microfarad capacitance, rated at least 15 volts working, to have a bit of safety margin. The diodes will need to be rated for more current than the transformer will deliver, ten amps should be OK for a 5 amp transformer.And for battery charging some sort of current meter is really a good idea.