# Car battery charger rewiring

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#### saxa

##### New Member
Hi all, I hope I am posting this into the correct forum. I am a newbie in electronics so I want few advises and suggestions on how to do that thing correctly.

I got an old broken car battery charger from my neighbour and I want to make it go again and possibily modifiy ti to be a better one.
Here is a picture of it and of its internals, the circuit is very simple and I think I understand it well, but there are still some things i can not
understand well. So lets go by the design and doubts.

The circuit itself is composed from a transformer, which has different windings yo get out different voltages. These goes connected to a switch for voltage selection, then they go to the rectifier bridges, from there there are 2 electrolitic condensers and from there they flow goes to the battery leads, the negative going directly to the negative of the battery and the positive passes through an AM meter then a fuse and then to the positive battery lead.
Here is a schematic of the whole circuit. Originally probably this went to some more switches, which i dont know why.

Schematic

Here is the photo of the capacitors and their values are 4700uF and 1500uF , if i get it correctly since they are in paralel we can
consider it as one cap of 6200uF , correct ?

One first thing is that I have no idea on why the primary winding of the transformer has another one which divides the main voltage from 128V and give the voltage of 65V , why does it need that ?

The other thing I do not get is why there are 2 rectifier bridges ? BR1 and BR2 are KBPC5010 .

The third thing is that when I rewire it and make the measurement with the capacitors connected the voltage started up to grow and grow.

Lastly I want to know if I choose the correct voltage output from the transformer. The transformer has a lot of output windings rangeing from 0V up to 32V
from what i could measure. I choose the 2 wires which showed on 17,70V. Is that tyoo high for a 12V car or truck battery ?

Will I be able to charge the truck battery with it which has the capacity of 110Ah ?
Here is a picture of how it is right now rewired.

I also plan to insert in a new AM meter as the old one was broken, and possibly also an additional Volt meter. Probably digital ones, opinions ?

Sasa

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Last edited:

#### k7elp60

##### Active Member
Welcome to this forum. If you have a volt meter, I think you can get this charger set up. I would change the connections to the primary of the transformer and connect the ones that has a voltage around 18VDC across the capacitors when the power has been on for about a minute. The maximum voltage you want to charge a normal 12 vehicle battery is close to 14.8 volts. The amount of current drawn by the battery when connected to the charger will cause the voltage across the capacitors to decrease. You can consider the two capacitors in parallel to be one capacitor of 6200 microfarads (uF).
There are digital voltmeters and digital amp meters available in a number of electronic stores.

#### saxa

##### New Member
Hi k7elp60 ! Thaks for your answer. Yeah I have read about the maximum voltage for charging a car battery and I will make sure it doesnt get over. In fact I connected the transformer secondary 2 wires which gave me 17.7V more or less. Quite very stable. It was oscilating a bit between 17,68 and 17,72 I think this is ok.

What kind of connections on the primary you would change ? The primary side of the TR have 2 windings seems, on the central 2 wires I get about 65V. What is
that for ?

I noticed that when measuring the volts after the capacitors I get out a quite high voltage about 25-27V if I recall correctly. This can probably damage the battery.
Now there is some people who is saying that capacitors are not needed, what you think about it ?

#### k7elp60

##### Active Member
Hi k7elp60 ! Thaks for your answer. Yeah I have read about the maximum voltage for charging a car battery and I will make sure it doesnt get over. In fact I connected the transformer secondary 2 wires which gave me 17.7V more or less. Quite very stable. It was oscilating a bit between 17,68 and 17,72 I think this is ok.

What kind of connections on the primary you would change ? The primary side of the TR have 2 windings seems, on the central 2 wires I get about 65V. What is
that for ?

I noticed that when measuring the volts after the capacitors I get out a quite high voltage about 25-27V if I recall correctly. This can probably damage the battery.
Now there is some people who is saying that capacitors are not needed, what you think about it ?
First the voltage you are reading on the capacitor is pretty nornal. With a rectifier and a filter the no load voltage=input x 1.414. In this case 17.7 x 1.414=25v.
I may have missunderstood the transformer connections. The two dials shown on the front panel appear to be some kind of dials. What I would suggest is to get a regular headlight bulb and connect it in series with the + terminal of the battery and the + output of the charger, then measure the voltage on the battery. If the headlight bulb is fully bright then the output from the charger is a large current. If the headlamp is dim then the current is low and the charger my be fine to connect to the battery.

#### saxa

##### New Member
ok, great, thanks for this tip.

The 2 dials on the front, the left and right one have been used for voltage selection one, but the other i have no idea what was used for. They are both kind os switches. The center pin was used as output and on the side of the dial there was connections where
all the wires from the transformer secondary went on, this was permitting to switch the output voltage. I will see if I post the pictures of it.

#### Inquisitive

##### Super Moderator
I highly recommend that you upload all the pictures in this thread to www.electro-tech-online.com. At present you have them linked to a competitors site. This thread will be shutdown if this is not corrected.

#### saxa

##### New Member
ok will be done, sorry, i didnt know this could be an issue.

#### Inquisitive

##### Super Moderator
One issue is only members of the other site can see your photos. People that are members of this site only cannot see your pictures.

#### saxa

##### New Member
Sorry, was pretty busy. Solving it right now.

#### saxa

##### New Member
Ok, I think it is solved now. Sorry but it took me some time to understand that i firstly have to uplod the images and then put them into the text.

#### saxa

##### New Member
One of these days I take some time to map all the wires going out of the transformer. And finish this thingy.

#### shokjok

##### Member
Why does this simple circuit have parallel-connected bridge rectifiers?
If it was two separate transformer windings as you stated, what does the floating voltage feed?

#### saxa

##### New Member
Why does this simple circuit have parallel-connected bridge rectifiers?
If it was two separate transformer windings as you stated, what does the floating voltage feed?

In fact this is what is intriguing me from the beginning. I never understood why those 2 bridge rectifiers are in parallel. After some reading
I could conclude that this would be due to better handle the amperage passing through them if I understood it correctly , but this still implies in a
problem if one of the bridge diodes burn then it overloads the other one. I am not sure if this is correct.

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