Charger as power supply?

Discussion in 'General Electronics Chat' started by axeman22, Feb 4, 2012.

  1. axeman22

    Thread Starter Active Member

    Jun 8, 2009
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    I was talking with a mate yesterday and the subject of using a car battery with a charger across it as a cheap 12volt supply with high current capability. As opposed to using my 12v 35amp supply.. Note load is not constant, just occasional peaks.

    So.. If I take my 12v 6amp charger.. And remove the 12v battery it is across... Why can't I just use that as a solid 12v, 6amp supply..?

    I'm guessing it is because the voltage will be all over the place, seeing the load as a wavering battery ... But, practically speaking, what is the 'problem' with this idea..? I've just never actually looked at this in detail.
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
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    It might work with a very large capacitor on the output. After all, the charger sees the car battery as a large capacitor anyway. This will lead to a question of economics vs. performance.
     
  3. gerty

    AAC Fanatic!

    Aug 30, 2007
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    A lot of the newer chargers won't turn on untill it's connected to a battery.
    It's for reverse polarity protection, if it's connected to the right polarity it'll turn on the primary(mains) voltage. Without a battery it will not turn on at all, that said, I would test for a output voltage first. And it'll probably be higher than what you anticipate.
     
  4. axeman22

    Thread Starter Active Member

    Jun 8, 2009
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    Has anyone looked at this in detail..looking for a somewhat authorative post. I'm guessing the control circuit is going to be moe interested in moving the carger voltage up and down in order to get current flow(charge) and will be chasing gassing voltage for lead acid, as opposed to trying to hold 13.7Vdc regardless of load..

    Interestingly enough, does anyone have the 'logic' used in a 12v lead acid charger..? JuSt the human logic, ad opposed to the circuit diagram etc..
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    A battery has a high momentary current capacity, a charger by itself has a low current capacity.
    A Lead-acid battery charger provides a medium current (not high current) for a long time.

    Why can't you use the battery to provide the high momentary current pulses occasionally and have it continuously charged with a low top-up current?
     
  6. axeman22

    Thread Starter Active Member

    Jun 8, 2009
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    I'm talking about a scenario where there is no actual battery connected.. just the charger, connected straight to the load, say a CB radio etc
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Like I said, .... read what I said.
     
  8. K7GUH

    Member

    Jan 28, 2011
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    By all means, hook it up to a CB radio, and see how long the radio lasts.
     
  9. axeman22

    Thread Starter Active Member

    Jun 8, 2009
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    guys..thanks, but no one is answering the question :) . perhaps i didn't word it correctly. I know its not a suitable solution to use a charger (without any other thing - like a battery or large cap etc) as a power supply as such. What I'm after is the technical reason why so - ie someone who really understands the differences between a power supply cct and a charging cct to explain the way the charger will interact with the load as opposed to the power supply.

    I'm not looking for advice on how to make the charger work as a power supply or on whether it will work solo - I know the answer to those questions :)
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    It is a big world.
    Maybe cars and their batteries in your country are tiny.

    But in North America most cars are pretty big and their engines and starting batteries are also pretty big and powerful. The alternator in the car keeps the very powerful battery fully charged so it can deliver a few hundred AMPs to its starter motor for a few seconds to start the engine. An ordinary charger cannot provide such a high current.
     
  11. axeman22

    Thread Starter Active Member

    Jun 8, 2009
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    I'm not talking about cars, starting cars.. or anything like that - havent i been clear? I'm talking about using a 6amp lead acid battery charger as a bench style power supply.. thats the use case.. yes we have big cars here, we also have running water :)

    so.. just to be clear, imagine you have something that needs 12Vdc @ say 4amp.. perhaps a car stereo or something like that.. not a dumb load.. but not overly sophisticated either.. imagine just taking your 12v 6 amp lead acid battery charger and using it to power the load. Yes, I know it wouldn't work.. is not ideal, I'm looking for someone who can explain why, in detail.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Why don't you buy a proper power supply for your application?
    A car battery charger does not have the big filter capacitor that an amplifier circuit needs. The charging doesn't care if the supply voltage jumps up and down (by rectified AC) but your amplifier and your ears will care very much.
     
  13. gerty

    AAC Fanatic!

    Aug 30, 2007
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    If it will turn on without being connected to a battery.

    If the voltage isn't too high

    It will make a lousy choice for a power supply for a radio for all the reasons already mentioned.


    Does that answer your question? If not, let us know what else it is you need to know.
     
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