Battery charger shorts out?

I guess you old charger contains nothing more than a big transformer and a bridge rectifier. The rectifier contains 4 diodes to convert the AC from the transformer to the DC for the battery. If one of the diodes goes short circuit then you will get the effect you are seeing.

If you really want to resurrect the charger you could probably replace the old bridge rectifier with a new one from Maplins for couple of quid (AR84F looks suitable). But as modern cars and batteries are more susceptible to mistreatment I would be tempted to just get rid of it.

Chris

Older battery chargers are usually very basic, generally consisting of a mains transformer, rectifier, ammeter, and a fuse... I'd guess by the sound of it that the rectifier has gone short circuit.

Thanks for the quick replies. I have now taken the back off and yes it does look very basic inside. I can see the rectifier which has 4 connectors and a hole in the middle screwed into a thin sheet of metal (or is that a heatshield). It has writing on the rectifier which says;

S4VB

10 (Hole) 84

I'm guessing that the 10 refers to 10amp (84 could be the year?). I've looked at the Maplin code for AR84F and that is 25A. Would it cause any problems using a higher amp or is it best to keep with a 10A? Cheers.

Nope, that'll be fine.

That wil be a heat*sink* - deisnged to help cool the rectifier. You will need to fit something similar to the replacement part.

Bigger (in this application) will be fine.

S4VB is a 4A bridge rectifier, and the 10 means 100V working.

A 10A or higher (50V or higher) bridge rectifier will be fine. (A 4A bridge rectifier was obviously inadiquate.) I would also get some heat sink compound to smear between the new rectifier and the heatsink.

Got the rectifier from my local Maplins and is now fitted. It's twice the size of the original with bigger spade connectors so had to replace the ends of the 4 wires. It was a bit confusing knowing which way round the - wires go as the only markings on the new rectifier only displayed on one side showing the AC & +. Going round in a clockwise direction, if the 1st terminal is AC+, the 2nd terminal is DC+, I connected the 3rd terminal to AC-, and the 4th terminal to DC-

I've not yet tried it connected to a battery but it now powers on without clicking and connecting a DVM to the croc clips shows around

12.06v or 6.02 on the 6 volt setting. I shall give it a test on a battery tomorrow.

That should be right.

Is your meter reasonably accurate? These voltage readings seem rather low.

Sylvain.

I think the battery charger voltage increases when connected to a battery, I will check it out tomorrow. cheers.

Most bridge rectifier are usually marked: ~ ~ (ac in) + - (dc out).

Looked all round using a magnifying glass but the only marks I could find were on one edge similar to this pic

IME, only the +ve is normally marked on the square ones. The -ve will be diagonally opposite, and the others are the AC (doesn't matter which way around).

Are the other two terminals on the other side of the rectifier similarly marked AC & - ? ....if so then it's simply a matter of the mains transformer connecting to the two terminals marked AC and the ones marked + & - eventually finding their way to the respective battery terminals.

The terminal marked AC is one AC connection, the other AC connection goes to the corner diagonally opposite. The terminal marked "+" is the DC+, the terminal opposite that is DC-.

If there's no battery connected it'll be trying to measure a non-dc waveform, how it does this will give a somewhat arbitrary result.

I'm afraid not. With an unregulated supply, the voltage will drop when connected to a load. It would be interesting to know what the transformer voltage (AC) is with the rectifier disconnected. Those people more knowledgeable than myself should be able to work out the expected DC voltages (2 diodes in series dropping 1.5 VDC each ?). This would also check whether the rectifier is correctly connected.

Sylvain.

>

That would be a clever trick. ;-) Unless there is some form of electronics regulating the output the open circuit voltage should be very much higher than that. Have you measured the AC voltage direct across the transformer

- ie the input to the rectifier?

I seem to recall my cheap n nasty charger puts out around 12v when not connected to a battery, but I've seen it reach 17v when charging a failing battery after 12 hours or so. Which I don't think was good for the battery but at least it was an old and tired one.

I'm thinking about Heath Robinsoning something using an old 240v AC motor and spare alternator now...