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  1. #1
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    Default bulb for battery charger

    I have a 12v battery charger that I would like to replace the light in. apparently the power light was blown and the previous owner removed the bulb and twisted the wires together ..Having said that, I assume it was a light !
    Q.. when it is on, I get 18V DC at the wires, but for some reason it wont light my 12V test lamp ?? can someone please explain..

    Ken.
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  2. #2
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    It's going to be difficult to help Ken, without knowing the circuit for this charger. The older battery chargers were often fairly primitive designs which were ok for giving a quick charge to a flat battery, maybe not that quick but overnight say for a full charge, but they would boil a battery dry if left on for months at a time. They consisted of a small transformer, a rectifier bridge, some form of current limiting, like a fuse but sometimes a light bulb, and finally the connecting leads and clips. It was necessary to limit current because if the output leads were short circuited, either the rectifier or the transformer would be destroyed and the magic smoke let out.
    The light bulb was a good cheap and effective current regulator, because all you had to do was choose a bulb which would consume just less than the maximum current that your transformer and rectifier could safely supply. If the leads became shorted, the bulb would limit the current to a safe level for the transformer /rectifier combination, but in use as a charger on a flat battery, the bulb would limit the current to say 5 Amps for your particular charger, but as the battery charged, the current input would drop and the bulb would dim in brilliance. Incandescent light bulbs have an unusual resistance characteristic in that they have a higher resistance the hotter the filament is, so at maximum current which is also maximum filament temperature, the electrical resistance is also at a maximum. This limits the maximum current but as the current falls, the resistance also falls, and the battery continues to charge at a higher rate than it would if the charger had a fixed resistor to protect the transformer and rectifier combination. An easier to grasp analogy would be an engine with a governor compared to one with just a fixed throttle setting. The light bulb regulator is like the governed engine, where the throttle adjusts for minor load changes, while having a fixed resistor value to protect the circuit would be like an engine with a set throttle, which would be unable to adjust for small variations of load.
    Without knowing the internal circuitry of your charger, it is hard to give you much useful info. I have tried Google but could get no info on your charger, so if you can photograph the internals, or a circuit diagram if you have one, someone may be able to assist, especially if it is old school, but if it is like looking at the insides of a computer motherboard, it will be very difficult I imagine.

  3. #3
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by kenny_10 View Post
    I have a 12v battery charger that I would like to replace the light in. apparently the power light was blown and the previous owner removed the bulb and twisted the wires together ..Having said that, I assume it was a light !
    Q.. when it is on, I get 18V DC at the wires, but for some reason it wont light my 12V test lamp ?? can someone please explain..

    Ken.
    First question is - is the test lamp an LED, if so try the wires the other way around.

    What voltage do you get on the wires when the test lamp is attached? If its 18V something inside the test lamp is making it open circuit.
    If the V is zero the either something inside the test lamp is making it short or the volts behind the circuit that drives the original lamp cannot provide enough current.

  4. #4
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    Quote Originally Posted by BobL View Post
    First question is - is the test lamp an LED, if so try the wires the other way around.
    Test lamp is the old screwdriver type... i use chargers for my electrolysis .. have always used a vintage gayrad 6/12 volt charger.. lately it has only been working on the 6 volt setting !!!.
    Hence I thought of using the pictured one, and despite putting out 15V, will not cause a single bubble in my solution... maybe someone more familiar with electrolysis can explain why.

  5. #5
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    Quote Originally Posted by kenny_10 View Post
    Test lamp is the old screwdriver type... i use chargers for my electrolysis .. have always used a vintage gayrad 6/12 volt charger.. lately it has only been working on the 6 volt setting !!!.
    Hence I thought of using the pictured one, and despite putting out 15V, will not cause a single bubble in my solution... maybe someone more familiar with electrolysis can explain why.
    It sounds to me that your charger is one of the first generation of smart chargers. They have circuitry which often won't start charging unless the charger is connected right way round to a battery with at least some charge. In other words if the battery has zero volts or close to zero, the charger will not start charging. The reason for this is possibly to prevent a battery being reverse charged if the leads are not connected correctly. This first generation often went into "float" mode when the charger detected that the battery was fully charged. This cut the voltage from around 17V to just a bit over 12 volts and the charge current dropped to just a few mA, and so did not boil dry. I should just clarify, that by boiling dry I do not mean that the temperature rose and so boiled in the conventional sense, but rather the water in the battery acid solution is electrolysed and broken down into Hydrogen and Oxygen. This could be topped up again in the case of a non sealed battery by adding distilled water to keep the top of the plates covered.
    Try connecting your charger to your car battery and see if the Ammeter indicates a charge when you turn the headlights on. This will indicate if it is working or not and if it is a charger with just enough smarts to get you into trouble. If it is of that type, I would have thought that it was too modern to be still using an incandescent globe for current regulation. A picture of the board or even better a circuit diagram might assist some of our members less challenged with electronics than myself to clarify things for you.
    Finally 6volts is enough to de-rust stuff, depending on size of course, but your old charger will most likely do the job you want if you can't get the Peerless to work.

  6. #6
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by kenny_10 View Post
    Test lamp is the old screwdriver type... i use chargers for my electrolysis .. have always used a vintage gayrad 6/12 volt charger.. lately it has only been working on the 6 volt setting !!!.
    Hence I thought of using the pictured one, and despite putting out 15V, will not cause a single bubble in my solution... maybe someone more familiar with electrolysis can explain why.
    Newer battery chargers only release current to charge batteries if they have a certain resistance load on them. If you want to use it for electrolysis you need to put a half flattened battery in parallel while it is charging. If you start with a fully charged battery, run the electrolysis off just the battery and then when it needs to be charged, charge it up and keep going with the electrolysis. Because of this I gave away using battery chargers some time ago.

    For a while I used the 12V output of an old computer power supply but then I found out that 12V was too high a Voltage (produces large amounts of H and O which are not good in my crowded enclosed shed) so I used the 5V output. Then I picked up a neat little DC power supply that delivers 5A up to 6V and run it on about 3A.
    I also use carbon arc welding rods as the anode (+ve) electrode. Much less mess.

  7. #7
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    Quote Originally Posted by kenny_10 View Post
    Test lamp is the old screwdriver type... i use chargers for my electrolysis .. have always used a vintage gayrad 6/12 volt charger.. lately it has only been working on the 6 volt setting !!!.
    Hence I thought of using the pictured one, and despite putting out 15V, will not cause a single bubble in my solution... maybe someone more familiar with electrolysis can explain why.
    From the picture of the charger, the word "AUTOMATIC" across the front might be a clue. Most automatic chargers will only output a charging current if connected to a battery which has a voltage level of at least 8 volts. This is to protect the charger from operating into a short circuit. The 15V you are measuring is possibly an internal biassing current.

    The light you describe is almost certainly a LED, and the current available on the wires from the 18V source you are measuring cannot light your 12V incandescent test due to the heavy current required. Hopefully the twisting together of these wires has not resulted in damage.

    Chas.

  8. #8
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    Thanks guys... put it on my car battery and ammeter showed 3 Amps.. no change when i put on the lights... it looks like the old one is back on the table... ATM only runs on the 6V setting...like to get the 12V side working again.
    will send some pics soon...
    Ken.

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