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Other Vintage Household Electrical or Electromechanical Items For discussions about other vintage (over 25 years old) electrical and electromechanical household items. See the sticky thread for details.

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Old 2nd Jan 2015, 4:47 pm   #41
Pete_kaye
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Default Re: 12V Charger

Now the season is over , I can return to this item. I connected the rectifier in with a 12v 55W car bulb as output. There was 7.3v drop across the bulb on my DMM and ammeter swung to max at 5A. Bulb glowed brightly . I am surprised by this or am I missing something? Should I fit a capacotor across the output to stabilise it?
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Old 2nd Jan 2015, 5:29 pm   #42
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Default Re: 12V Charger

Hi,

For battery charging purposes no smoothing capacitor is required. Your charger is working OK. Your DC meter is indicating the average value of a series of half sine waves at 100HZ.
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Old 2nd Jan 2015, 10:42 pm   #43
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Default Re: 12V Charger

That certainly sounds healthy to me. The battery itself acts as smoothing.


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Old 3rd Jan 2015, 4:13 pm   #44
Pete_kaye
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Default Re: 12V Charger

Good . I will put it back together again.
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Old 4th Jan 2015, 9:03 pm   #45
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Default Re: 12V Charger

Quote:
Originally Posted by Pete_kaye View Post
Should I fit a capacotor across the output to stabilise it?
No, the unsmoothed pulsed dc is desirable for lead/acid battery charging.

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Old 28th Feb 2015, 4:12 pm   #46
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Default Re: 12V Charger

Modern battery chargers seem to be fitted with moving coil meters,which are only correct on pure DC, older and more proffesional chargers are fitted with a moving iron meter which reads RMS,which I feel is more appropriate to work with.
If you measure the voltage with a MC meter it will read low ie 11.5v yet still charge a 12v battery. The limiting factor will be the peak voltage,and a large cap across the output will enable this to be read with a multimeter (set to DC) you dont want more than 14.7v for a fast charge or 13.8v for prolonged connection,(float condition.)
Another point, older battery chargers were often in excess of 16V,maybe to get around the extra drop with the metal rectifier,however these can ruin a battery if left connected after full charge.
The waveform makes little difference when the RMS figures are used.I have also found that lead acid cells have longer life when charged from a smooth DC supply,although some claims are made about pulse charging,(desulphaters) extensive testing does not confirm much useful results.

Last edited by happytiger; 28th Feb 2015 at 4:18 pm. Reason: added
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Old 1st Mar 2015, 2:24 am   #47
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Default Re: 12V Charger

I have an early 1970's 6V/12V battery charger that I still use occasionally in winter to keep the battery of my wife's car in good condition, as it spends most of its time doing short journeys. The charger is wall mounted and I made up a lamp board with a variety of 6V and 12V bulbs to select and limit the charging current, useful when charging smaller-capacity non-car batteries. For trickle charging car batteries I usually stick a 12V 21W bulb in series, which limits the current to about 1A and ensures that any excess voltage gets dropped across the bulb. It normally charges at about 3.5A when connected directly to a 12V car battery as measured on its moving iron ammeter. I have never tried checking its waveform.
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Old 1st Mar 2015, 4:15 am   #48
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Default Re: 12V Charger

The waveform does not matter since the moving iron meter is reading RMS.
The old chargers produced ecessive voltage,and will overcharge the battery if left connected .Like the older cars that had dynamo charging, batteries had a short life due to being overcharged. You need to limit the voltage to 13.8 to keep your battery in good order, with this voltage the current will reduce to a very small value. You can make a regulator with a 2n3055 transistor,and zener diode/s.
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Old 1st Mar 2015, 11:17 am   #49
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Default Re: 12V Charger

Hi Peter, those fixings were originally called Frig Rivets, as they first appeared to hold in place the plastic mouldings inside refrigerator doors etc.

Just push the centre pin in, until it falls out inside the charger. (when the charger is open, retrieve for reuse). The rivet body should then gently ease out upwards. When you reuse the pin, one end may be tapered for insertion.

In the unlikely event any rivets break, they should be easily obtainable as they tend to be standard sizes.

Regards - Mike

Last edited by mike_newcomb; 1st Mar 2015 at 11:18 am. Reason: Spelling correction
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Old 1st Mar 2015, 11:28 am   #50
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Default Re: 12V Charger

I think the short life of dynamo charged batteries was down to the battery technology not the (voltage regulated) dynamo. The charging voltage of the dynamo could be adjusted (or fiddled with, of course) and could drift off, though.

Give or take the ambient temperature, about 14.4V is right for a one off charge, terminated when the battery is full; 13.8V is float voltage which can be applied indefinitely to keep it full.
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Old 5th Mar 2015, 9:25 am   #51
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Default Re: 12V Charger

I used to work in laboratory testing all types of accumulators for a well known manufacturer, we took in failures tested and strip, automotive failures at the time were around 80% overcharge, causing disintegration of the positive plates.
As you say the reg could be adjusted though but these were set to 16V.
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Old 9th Jan 2016, 3:48 pm   #52
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Default Re: 12V Charger

I am using this now but would like to fit a thermal cut out, just in case... I have the one shown below but wonder where it should go in the circuit . It came from a Halfords unit and cuts out at about 65 C. Does it go after the rectifier or in the transformer leads?
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Old 9th Jan 2016, 7:44 pm   #53
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Default Re: 12V Charger

As others have posted, a moving iron meter will read true RMS whereas a moving coil mete may be inaccurate on anything but flat DC or sine wave AC.

What is not always appreciated however is that the degree of charge put into a battery is related to the AVERAGE current and NOT to the RMS current.

In most circumstances the RMS current of AC or of rectified DC is the relevant metric, it is what determines the safe capacity of wires etc. or the current at which a fuse will blow.

For charging batteries and for other electrochemical processes such as copper plating or hydrogen production, then it is the average current that is relevant.

There is no accurate ratio between the RMS current and the average current, but with most charging circuits, the average current (that determines the charging time) is about two thirds of the indicated RMS current.

The light output of a fluorescent lamp is also in proportion to the AVERAGE lamp current, but the heating of the supply wires is in proportion to the RMS current.
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Old 9th Jan 2016, 11:49 pm   #54
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Default Re: 12V Charger

Which is interesting, since an MC meter reads average presumably the previous statement about MI being a better indicator of true charging current must be wrong.
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Old 9th Jan 2016, 11:59 pm   #55
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Default Re: 12V Charger

It doesn't really matter where you mount the thermal cut-out, so long as it will interrupt the charging current if it gets too hot. Though my personal preference would be to insert it upstream of the rectifier, so it would protect the transformer in case the rectifier failed short-circuit. However likely that may be
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Old 10th Jan 2016, 12:32 am   #56
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Default Re: 12V Charger

Re MC v MI meters, my recollection is that most of the MI meters fitted to the old chargers like the OP's were very cheap and cheerful components and had 'ball-park' accuracy. But of course, we are talking about an era where you charged until you could see a few bubbles forming and then you knew you had finished, irrespective of meter readings.

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Old 10th Jan 2016, 2:46 am   #57
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Default Re: 12V Charger

I think you push the centre pin right through the "cap".
When you open the box ( the prongs will close so the diameter is the same as the holes) collect the "blanks" and reinsert them when you put it back together.
I have forgotten what they are called, but lets say re usable pop rivets.
Go check the grill on yours.

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Old 10th Jan 2016, 11:52 am   #58
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Default Re: 12V Charger

I am afraid that my experience has caused me to dislike and not trust modern battery chargers, give me a good old fashioned heavy one any time, you know exactly where you are with those.

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Old 10th Jan 2016, 12:22 pm   #59
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Default Re: 12V Charger

I have to agree. Modern "intelligent" battery chargers often refuse to charge totally discharged batteries. You have to resort to the good old transformer and rectifier arrangement. I discovered this the hard way when I tried to recharge a "leisure" battery. No charge current showed after several hours. At first I thought it was a hard fault with the charger, but actually it was just stupid rather than intelligent.
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Old 10th Jan 2016, 1:22 pm   #60
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Default Re: 12V Charger

I have the same basic model, bought new in 1972ish. On mine, you can push the centre of the white rivets through - it is a separate pin! Use a pin punch or terminal screwdriver. Then after dismantling, retrieve the pins and use again.
Mine is the 6V and 12V version, I have had to replace the switch and the fuse is now on the front panel rather than in the lead.
Have fun!
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