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Pete_kaye 5th Dec 2014 6:29 pm

12V Charger
I was given an old 12v car battery charger recently ,described as 'not showing any charge on the meter' and found it only gave 11.5 v on open circuit. However, there was a slight kick on the ammeter when connected across a large 10R resistor. It looks difficult to open as it uses 6 plastic breakable expanding plugs to hold the case on.
Before I start, what is likely cause of fault and will it be easy to fix?

Nicklyons2 5th Dec 2014 6:42 pm

Re: 12V Charger
Assuming it is a mains powered charger for 12V batteries, what are you measuring the 12V with? Are you measuring it whilst feeding the 10Ohm load?

If you're using a moving coil meter and the charger uses a simple rectified, non smoothed supply it may well still only show 11.5V when working correctly.

If it is very old 20yrs +, it may have a metal oxide or selenium rectifier (likely to be fairly big with 'fins' on) which has gone high resistance.

Dont forget to look for the obvious though - like it has 6V on as well as 12 and it's switched to the wrong setting, or it has a 'trickle charge' setting and that is switched in etc.

Nickthedentist 5th Dec 2014 6:47 pm

Re: 12V Charger
This sounds like a modern car battery charger.

If so, expect a SMPS and all the "fun" they bring!

Who knows whether it's fixable... you need to be brave and look inside ;)


merlinmaxwell 5th Dec 2014 8:40 pm

Re: 12V Charger
If it is an 'old' one mains these days is a bit lower than it was, these chargers don't do a full job now.

emeritus 5th Dec 2014 10:29 pm

Re: 12V Charger
1 Attachment(s)
I have an early 1970's vintage battery charger that I still use occasionally. I think it must have an old type rectifier, but haven't had the back off as it is rivetted on. On its 12V setting, it gives 13.5V open circuit when measured with a moving coil multimeter, and will charge a 12V lead-acid battery up to 15V if left for long enough. Its specified mains voltage is 240VAC, and our mains voltage these days has consistently been 235V whenever I have checked. I use it with a lamp board to regulate the charging current to suit my various 12V lead-acid batteries when I periodically cycle them as it has a higher end voltage than the other more modern electronic chargers that I have.

BGmidsUK 6th Dec 2014 12:08 am

Re: 12V Charger
When I worked for a transformer and charger manufacturer (J M Clarke) from around 1995 to 2004 all of our chargers for lead-acid batteries (except the tiny ones) were thyristor-based, including the later software-controlled units with PIC 16C711(?) ICs. I know there was at least one other company making similar products as we had some sort of trading arrangement with them.

The "fun" occurred when the assemblers would sometimes accidentally(?) transpose the driver transistors with the thyristors (same package), resulting in a very loud bang and flying shrapnel when I plugged it in to calibrate/test it. Safety glasses would have been advisable, including for the visitors who happened to be walking past on one occasion.

I'll open up my modern 4/8 amp unit as this thread has sparked my interest to see what lurks inside. I'll be surprised if the design is significantly different from those I worked on as it does bear quite a resemblance to a Clarke charger.

My old Halfords unit is simply a transformer with thermal switch and a bridge rectifier screwed to a (partially melted) plastic post! I should measure the output with various meters to see what readings this gives off- and on-load.


AC/HL 6th Dec 2014 12:53 am

Re: 12V Charger
As Brian says, it's likely to be a simple transformer/rectifier arrangement with a moving iron meter. Chances are the rectifier is faulty.
The only way to know more is to open it, or at least quote make/model numbers.

winston_1 6th Dec 2014 1:24 am

Re: 12V Charger

Originally Posted by merlinmaxwell (Post 726069)
If it is an 'old' one mains these days is a bit lower than it was, these chargers don't do a full job now.

I don't think that is true. My mains has always been around 242v as long as I can remember. It is called 230v these days, but it actually has never changed, nor do I think there is any intention to do so.

Alistair D 6th Dec 2014 8:53 am

Re: 12V Charger
Your meter is measuring the mean value of the rectified waveform. Connect an electrolytic capacitor across the output and measure again to get the peak value. Assuming that there are no high resistances in the path anything above 13V should provide some charge.

100R is too high as a load. A car indicator bulb or some other 12V device that consumes an amp or two would be better.


broadgage 6th Dec 2014 7:35 pm

Re: 12V Charger
A modern battery charger can contain some fairly complex electronics. Some older large or industrial battery chargers are also quite complex. I have a couple of "Lucas Marinapower" 12 volt 10 amp battery chargers that are now vintage, they are fairly complex and use a thyristor controlled output, and detect the presence of a battery before producing any output.
Such sophistication was unusual and expensive, they cost 100 each IIRC over 30 years ago.

An old domestic battery charger is usually much simpler consisting of a step down transformer, a full wave bridge rectifier, and a moving iron meter.
In theory a regulating resistance is also required in order in order to limit the charge current, but in most designs the resistance of the transformer windings, meter, rectifier and connecting leads is sufficient to avoid the need for a resistance.

emeritus 6th Dec 2014 8:11 pm

Re: 12V Charger
Indeed, my old charger has a 5A fuse on its secondary, but it has never blown, despite various inadvertent shortings of the croc clips.

Radio Wrangler 6th Dec 2014 8:25 pm

Re: 12V Charger
The old transformer-rectifier-meter-crocclips jobs were a bit more sophisticated in the transformer department than might be apparent.

They were designed with a lot of leakage flux and a high-is open circuit output voltage so that they tended to look like current sources.

They were often connected to dead flat batteries that would take hours for cell voltages to come up... much longer than the thermal response time of the transformer in the charger. THe fuse was mostly to protect against reversed battery connection.


G6Tanuki 6th Dec 2014 8:42 pm

Re: 12V Charger
Yes - and the use of well-finned Selenium or Copper-Oxide rectifier-stacks with their high forward-resistance also provided a degree of safe current-limiting when connected to a 'stalled' not-showing-any-volts battery.

Indeed, I remember once hooking up a Davenset charger to the dead battery of a Bedford "J" truck [a seriously-fun tool with a 3.3-litre 6-pot petrol engine and overdrive transmission that could easily manage 80MPH on the flat and quite a bit more given a downhill]. It was only some hours later when the battery boiled-over that I realised the charger was set to 24V and the battery was 12V.

Steve_Bell 6th Dec 2014 10:59 pm

Re: 12V Charger
#9, the OP stated a 10R load in post #1.

Pete_kaye 7th Dec 2014 3:13 pm

Re: 12V Charger
2 Attachment(s)
Here are some photos .I haven't got back to it yet. It is the 6 white plastic plugs I am more concerned about as they have to be broken to open it . Anybody got some?

AC/HL 7th Dec 2014 3:45 pm

Re: 12V Charger
A quick Google came up with this:

Battery charger
The Crypton model HC3 battery charger.
To meet the demand for a reliable battery charger at a reasonable price, Crypton Equipment (Thorn Group) has introduced the model HC3 which has an output of 3amp and charges both 6 and 12V batteries.
The specification includes a slide-action change-over switch, strain-relieving cable bushes, permanently identified charging clips and an easily replaceable fuse. The transformer is double wound; selenium full wave rectification is used and a reliable clear scale meter is fitted.
Made by: Crypton Equipment. Bridgwater, Somerset.

Doesn't sound anything special, yours is presumably a fixed voltage variant.

Pete_kaye 7th Dec 2014 3:47 pm

Re: 12V Charger
Yes that is similar except mine is 12v only so presumably later model. No fuse except in plug.

Nickthedentist 7th Dec 2014 4:04 pm

Re: 12V Charger
Ah, definitely a nice, traditional transformer in there.

Are those like those plastic rivets where you can push out the centre pin, then remove the rest? It would hardly be the end of the world if you broke them, as you could no doubt find another means of securing the case afterwards. They may have been "one shot" fasteners, i.e. designed to be drilled out and replaced should the need arise.


Pete_kaye 7th Dec 2014 4:10 pm

Re: 12V Charger
:)They are conical shaped and spread out once inserted to stop me opening it .

Nickthedentist 7th Dec 2014 4:43 pm

Re: 12V Charger
You may have to chop off their heads with a sharp chisel or Stanley knife blade then. That will show them!

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