should a car battery charger read 12.0v?or more?

does the battery voltage rise with the charger connected? if so then the charger is doing what it should

Testing the output with a meter won't give an accurate reading on a cheaper type battery charger due to the type of regulation and lack of smoothing. There needs to be some form of load, even just a small capacitor but preferably a battery.

Check your battery voltage, then check it again with the charger connected. A rise of some form should be noted, and if the battery is fairly flat obviously a current flow should show as well.

Ideal charging voltage for a lead acid battery is around 13.5-14.5 volts.

Depending on the charger, if the battery is too far flat it may not charge. I once tried to revive a battery which had stood for months and showed less than 2v but my modern charger wouldn't produce any charge. I then connected it to a rusty old 25+ y.o 5 amp charger and the amp meter went off the scale and blew the bridge rectifier, but before going bang it had put just enough juice into the battery to allow the new charger to take over.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Redwood" saying something like:

If you have to do that again, put a resistor in the pos line - just an old tail lamp bulb will do.

A car battery charger puts out unsmoothed DC, so your meter just shows an average voltage of 12v but the voltage is actually varying 100 times a second between about 17v and 0v which good for charging.

You must have learnt your electrical theory in a strange parallel universe...

Chris

Thanks for the tip. I got the old charger working again with a new bridge rectifier from Maplins but I'll remember about the resistor should the situation arise again.

??

That tallies pretty closely with how I understand (cheap) mains car battery chargers work.. what's the issue?

Of course I've only got the HNC in Electronics Engineering and 20 years experience to go on, so I may just be guessing...

I've got a degree in Electronics and 30yrs experience in power electronics so my guess is that your guess is the same as my guess:-)

So a simple battery charger, with an open circuit voltage of 12vdc, will charge a 12v (nominal) lead/acid battery? You learn something everyday.

Chris

Depends how discharged it is...

It won't charge it fully, if that's what you are worried about. Was that the pedantic little nitpick you were concerned about, or was it the general principle of the full wave rectification and RMS output voltages?

(Just to cover myself, I did say 'pretty closely' - I would have expected a somewhat higher RMS (and peak) voltage to charge a '12V' car battery.)

As for 'open circuit voltage' you could get anything from nothing to very high voltages out depending on the exact circuitry involved. It's the voltage while it's actually connected to a battery in good condition that really matters..

It might charge a flat battery, but not fully. It also depends what you used to measure the voltage with. Cheap DVMs may not give a true reading due to the ripple.

I'm not worried in any way... it's not my battery charger!

The OP was specifically asking if the way he/she measured the voltage was an incorrect way to measure the voltage. You attempted to explain the reading he/she was getting, but implied that a reading of 12v was correct. That is wrong, and fundamentally misleading; hardly a pedantic little nitpick IMHO.

In the context of the OP's question, he/she is reading 7.3 volts on the 6v setting, and 12 volts on the 12v setting, tested "open". The likelyhood is that his charger is faulty. The most probable cause is some shorted turns in the secondary of the transformer, caused by attempting to charge a very flat battery without monitoring the current.

Chris

In the context of Usenet, I came in halfway through the thread and was responding to your 'parallel universe' comment about the shape of the output waveform..

FWIW I agree with your diagnosis, in the context of the OPs question ;-)

I'm sorry you've had to endure the techno-babble this thread has generated, yet still not had an answer!

Your method of checking your charger by measuring the open circuit voltages is valid. The readings you are getting are incorrect however, and indicate a possible fault with the charger.

As you correctly assume, the voltage needs to be higher than the nominal battery voltage. As your ammeter is showing a 3A charge initially, it seems likely the battery you are charging is completely flat. Your charger will not bring the battery up to a fully charged state.

I have just gone out to the garage armed with my calibrated Fluke multimeter. I have two chargers; a trickle charger, and one with a 5A rating. The trickle charger has an open circuit voltage of 12.86, and the

HTH

Chris