Car battery charger - low output?

[Proto]

Checked the output voltage on a battery charger I've had for a while and I'm getting 10.8 volts. I'm assuming I should be getting 14v or so.
I've opened it up to see if I can see anything obviously wrong and there's not a lot in there - a transformer, a bridge rectifier and a 6v/12v, plus a high/low charging switch.

Can't see any discolouring or damage to the transformer so I'm guessing it's working okay, which really only leaves the rectifier. Now, disclaimer, I know nothing about electronics, but is it possible that the rectifier is faulty causing the low output?

New rectifier easy to source, and I'm loath to chuck something away that can be repaired.

Anyone able to advise?

 

[JohnHughes307]

I definitely don't claim to be an expert, but I thought the output voltage responded to the level of charge (IE voltage) of the battery it was connected to. What happens when you connect it a battery?

 

[presta]

You're measuring the mean, the peak will be ~17V. Do you have any reason to think it's not working?

 

[Proto]

I definitely don't claim to be an expert, but I thought the output voltage responded to the level of charge (IE voltage) of the battery it was connected to. What happens when you connect it a battery?

The battery never fully charges, only gets to 10v (ish)

 

[Proto]

You're measuring the mean, the peak will be ~17V. Do you have any reason to think it's not working?

I’m sorry but I don’t understand this. Measuring the dc output at the battery clips.

What AC voltage should I be getting from the secondary coil Of transformer?

 

[CXRAndy]

Checked the output voltage on a battery charger I've had for a while and I'm getting 10.8 volts. I'm assuming I should be getting 14v or so.
I've opened it up to see if I can see anything obviously wrong and there's not a lot in there - a transformer, a bridge rectifier and a 6v/12v, plus a high/low charging switch.

Can't see any discolouring or damage to the transformer so I'm guessing it's working okay, which really only leaves the rectifier. Now, disclaimer, I know nothing about electronics, but is it possible that the rectifier is faulty causing the low output?

New rectifier easy to source, and I'm loath to chuck something away that can be repaired.

Anyone able to advise?

Is the battery you are testing it with fully charged? The reason I ask is a poor battery can pull down the output from a simple charger.

If you're familiar with a digital multimeter you can test the AC voltage entering the bridge rectifier and from there multiply that number by 1.414 to give the DC voltage. An open circuit diode in the bridge would likely reduce the output DC a little due to half wave rectification.

You need to put the charger on a known good battery with a voltage of 12.6V

 

[presta]

I’m sorry but I don’t understand this. Measuring the dc output at the battery clips.

What AC voltage should I be getting from the secondary coil Of transformer?

What you have is a full wave rectified sine wave, which I assume you're measuring with a multimeter on a DC range, in which case it's measuring the DC mean of the waveform. The mean of a rectified sine is 63.7% of the peak, so the peak is 10.8/0.637 = 17V. The RMS voltage on the secondary should be about 12V (70.71%), give or take a bit for the rectifier.

The mean current you get from that will depend on the battery voltage and the circuit impedance, but it shouldn't be dropping to zero unless the battery voltage reaches 17V. Isn't that meter indicating current? What's that switch on the left connected to?

 

[Proto]

Thanks for above replies.

Input to the rectifier is 12.6v AC, and output is 10.8v DC. Does that imply the rectifier is faulty? (I’ve ordered one anyway, only £3 on Amazon)

Switch on left is car/motorcycle, I assume changes charging rate reflecting battery size.

Connecting to a battery makes no discernible difference to meter readings. The charger is unable to charge a battery up to 12v no matter how long it is connected.

 

[CXRAndy]

I would put a 30W 12V bulb on the clips. Then check all the switches for any voltage drop.

You can resistance check or diode test check a bridge rectifier by measuring across each diode in forward bias-DO THIS WITH POWER OFF

 

[Pale Rider]

I'm bound to like the posts from @CXRAndy and @presta even though I don't understand a word of them.

Good news in that it means qualified electricians have a skill from which they can make a living.

 

[presta]

Input to the rectifier is 12.6v AC, and output is 10.8v DC. Does that imply the rectifier is faulty?

No, it's exactly correct, for the reasons I gave above.

12.6V rms is 17.8V peak. If we allow 1.2V for the rectifier, that's about 16.6V peak at the output. As above, you're measuring the mean on the multimeter, so 63.7% (see * below) of 16.6V is 10.59V. You're measuring 10.8V, which is a 2% error: within the likely tolerance on your meter.

Switch on left is car/motorcycle, I assume changes charging rate reflecting battery size.

Yes, but what's it connected to?

What's the meter on the charger reading?