Charging battery from another battery ?

Discussion in 'General Electronics Chat' started by hmx66, May 22, 2015.

  1. hmx66

    Thread Starter New Member

    May 22, 2015
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    I want to charge 5V battery from 12V battery. I attached 7805 voltage regulator with 12V battery. How many 5v batteries can i charge with this 12V battery ?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    What is the capacity (Ah) of the 12V battery?
    What are the capacities of the 5V batteries?
    What is the chemistry of the each of the batteries?
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    chargng a battery requires a higher voltage ( lok up battery specs) than the battery is rated for. a 7805 will give you 5 volts, your battery needs a little more to charge.
     
  4. hmx66

    Thread Starter New Member

    May 22, 2015
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    5V battery is not actually of 5V its a cell phone Li Ion battery of 3.8V which needs 5V and 2 ampere to charge.
    But i dont know about capacity of 12V battery. Its dry cell battery.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Charging one battery from another is certainly possible but it's very inefficient. Powering 5V devices directly from the 12V battery, using the 7805, is inefficient as well but would be better.
     
  6. hmx66

    Thread Starter New Member

    May 22, 2015
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    What can I do to give it voltage little higher than 5V ?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    In your application, you need 5V.
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I would use the automotive cigar-lighter plug charger designed for your cell phone. It is designed to make the right charging voltage/charging current starting from a 12Vdc input. Much more efficient than using a 7805, which will waste 14W in heat to deliver 2A at 5V (10W) to the cell phone.

    Does the 12V battery look like this?

    [​IMG]
     
  9. hmx66

    Thread Starter New Member

    May 22, 2015
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    Yeah 12V battery is like this.
    Also suggest me a circuit to charge this 12V battery from 220 AC input.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
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    Be sure to use the phone's built in charger - overcharging lithium batteries can cause "venting with flaming gas" or possibly a small explosion.

    Batteries of any kind tend to be among the least economic power source, so it would probably be better to make arrangements to source the 5V other than from another battery.

    The 2A you mention is too much for a 7805 type regulator, but there are ways round that. Linear regulators introduce yet more losses, your 5V load at 2A needs to drop 7V at 2A - that's a little more energy dissipated as heat than gets to the load.

    Some discount stores carry cigar socket USB chargers with much more efficient SMPSU step down - shop around, some are rated for the 2A you're asking for.
     
    hmx66 likes this.
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Buy one...
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    Lead acid chargers can be tricky - I believe the transformers are specially wound to produce a particular current curve - the alternative is some circuitry that isn't trivial.

    Going straight to 5V is probably the best option.

    Actually, basic USB chargers seem a little pricey unless you shop around in the discount stores - and a lot more for one with various plug tips for various phones. One local supermarket wanted £12 for just the basic charger.

    A while back I bought a cigar socket type just because I wanted to see what chip it used, it came with about 10 changeable plug tips which are now kept in a tin any time I need them for a cheap wall-wart USB charger.
     
    Last edited: May 22, 2015
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