Float charge a lead acid battery

Discussion in 'General Electronics Chat' started by qitara, Apr 8, 2014.

  1. qitara

    Thread Starter Member

    Jan 18, 2013
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    Hi

    Is it Possible to use a normal battery charger (the once that just got a transformer and a rectifier inside of them), to float charge a Lead Acid battery ?
    The charger got a voltage selection switch on the front so i am planning to set it to 13.4V and connect it to the battery and keep it floating, will this do the job ?
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    As long as it has blocking diode, it should work. Since the rectifier bridge is made using diodes, they sort of qualify as blocking diode. I think. I would wait for more weighty opinions.
     
  3. #12

    Expert

    Nov 30, 2010
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    The exact voltage is important in the long term. batteryuniversity.com has some information, but a label on the battery would be very helpful. Variations like, Sealed Lead Acid, Absorbent Glass Mat, are different from common car batteries (flooded plate).

    I did this once by watching for the bubbles. I turned the voltage down until the bubbles stopped and saved a battery in a car for about a year (until the owner got back).

    Problems include the power line changing its voltage and temperature changes in the battery. Even if the power line is stable for months and you use a voltage regulator, the temperature of the battery changes its float voltage.
     
  4. qitara

    Thread Starter Member

    Jan 18, 2013
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    So technically it is possible with respect to the problems you meantioned ?
     
  5. #12

    Expert

    Nov 30, 2010
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    Yes.......
     
  6. qitara

    Thread Starter Member

    Jan 18, 2013
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    What makes a float charger so special, does it have an inbuilt voltage regulator ?
     
  7. #12

    Expert

    Nov 30, 2010
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    See post #5
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Can you show a photo of the charger and/or the circuit board inside the charger?

    I have a plugpack with voltage selector switches, it uses a LM317 adjustable voltage regulator inside which gives a stable regulated voltage (to deal with the issues mentioned by #12). Yours may be that type.
     
  9. qitara

    Thread Starter Member

    Jan 18, 2013
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    Well i don't think it's worth a picture because all there is inside the charger is a multi tap transformer feeding a 80A rectifier bridge, there is no special circuit inside it that you may be thinking of.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    You need a voltage regulator in there somewhere. For a few hours, you could get away with whatever "natural" regulation you get out of the transformer and your mains voltage.

    But if you really want a proper float that you can set and forget, it needs to control itself to a narrow range of voltage, and ideally adjust to temperature.

    You otherwise risk over-charging your battery and ruining it. You'd be better off to charge once a month and disconnect, than to leave it connected to an overcharging charger.
     
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    And it sounds like a big charger too with an 80A bridge in it.

    That thing will probably chew up a lot of power even if the battery charge current is very low. In other words, it's a poor choice for a permanently connected float charger.
     
  12. #12

    Expert

    Nov 30, 2010
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    Yes. I make float chargers out of old wall warts. Use a 317L chip to limit current to 1/10th of an amp and let it float for years.:p
     
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  13. wayneh

    Expert

    Sep 9, 2010
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    Me too. Tough to beat for simplicity and reliability, and cost.
     
  14. Little Ghostman

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    Jan 1, 2014
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    That saves me buying one those fancy linear tech chips with built in temp sensor :D
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    The microprocessor controlled Optimate battery maintainer settles at 13.6V once its bulk charged the battery.

    I have a spare motorcycle battery that's been hooked up to an Optimate for a couple of years - lead acid batteries are like muscles, they get lazy without a regular workout, to keep the battery keen I have to take the battery off the charger and make it do some work every few months.
     
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  16. THE_RB

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    I've done the same but use the LM317 to make a 13.8v voltage regulator. That's how it's done in most commercial stuff like alarm systems.

    For peak current limiting the LM317 limits internally at about 1.8A, and usually the internal resistance of the battery is enough that it doesn't reach 1.8A anyway (unless the battery is large and very flat).
     
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    For charging 12V lead-acid batteries on a float basis, I sometimes crack open a 6V wall-wart and put capacitors across 2 of the bridge diodes to make a voltage doubling rectifier.

    Depending on the size of the capacitors, the output has very limited current capability at 12V.

    Noise on the mains can bump up the "doubled" voltage - it can also recover sulphated batteries if it isn't too far gone.
     
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  18. profbuxton

    Member

    Feb 21, 2014
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    I have three vehicle batteries which I keep "topped up" by connecting them to a charger ( nothing fancy) and controlling it by a simple 24hr timer on the mains.
    Timer switches on for about a hour (or as you want) each day and batteries stay charged.
    A blocking diode in each lead to batteries prevents batteries from discharging to each other and just need to check water level every month or so.
    Never had a problem with overcharging or flat battery.
     
  19. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Funny thing happened last night.

    A friend brought around three large flashlights, all used a 6v 4Ah sealed lead acid battery.

    Two of them were charged through a 10 ohm resistor, 3W and 5W sizes. That was it! A DC plugpack and a resistor inside the flashlight. I don't even think 10 ohms was optimal, it's just such a common value it could be sourced cheaper for manufacture?

    The third one used an LM317 and it was all working so I could test the operation. The LM317 voltage regulator output 7.20v, through a 2W 3.6 ohm (orange blue gold!) resistor to limit charge current into the battery. That "smart charger" also had a two transistor latch circuit that lit up a LED when the battery was "full".

    Of course the main problems were 3 dead batteries, a dry joint, and many unmarked DC plugpacks of various voltages and polarities. :)
     
  20. ian field

    Distinguished Member

    Oct 27, 2012
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    The one I took apart had a 7805 with the ground pin tied to a voltage divider between the output pin and the "real" ground.
     
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