Why A 12V Bulb Connected In Series When Charging

Discussion in 'Power Electronics' started by zakmuh, May 17, 2018.

  1. zakmuh

    Thread Starter Member

    Jul 25, 2016
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    Hi Guys,

    I need your help to understand certain things when doing a DIY charger. So please help me with ALL THREE questions below and then add more if you like.

    I've got this ATX power supply unit from the PC and intending to use yellow and black wires, which should give 12V 20A when connected.

    1. Should I solder all the yellow together? Can I not use just one yellow wire?
    2. Can I directly connect these wires to SLA 12V 7Ah battery? Then the charging time would be 35mins (7Ah/20A=0.35h)?
    3. Why some people connect a 12V indicator bulb in series? Is this to reduce/increase charging ampere and what would happen if I charge with a bulb in series?

    Any advice on this would be highly appreciated.

    Cheers
     
  2. jpanhalt

    Expert

    Jan 18, 2008
    7,671
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    1) In my experience, all of the yellow wires will be connected together on the PS PCB. I usually remove all but one or two (if I need more outputs) to clean up the mess.
    2) You need to control charge current or you may damage your battery. You do not want to over charge it either.
    3) I believe it serves 2 purposes : 1) Limits charge current; and 2) Provides a visual indicator as the battery becomes fully charged and the charge current decreases.
     
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  3. dendad

    Distinguished Member

    Feb 20, 2016
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    1. All the yellow wires are connected together in the power supply. Use a few for higher current.

    2. No. An ATX 12V power supply will not charge a 12V SLA battery fully. The voltage is not high enough. Google "charging SLA batteries".

    3. A 12V lamp is used as a simple current limiter. As the current increases, the filament heats up and increases resistance thereby limiting the current. When the current is low, the filament is cold and at a quite low resistance so it will allow the max charger volts to be applied to the battery to fully charge it. This is slower than a properly designed auto charger, but quite simple and works well.
     
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  4. zakmuh

    Thread Starter Member

    Jul 25, 2016
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    Thank a lot for your replies. There is -3.3V, so if I use that along with 12v, I get 15V and that voltage is high enough for SLA?

    Okay, so I connected a bulb in series and measured the ampere in multimeter-it reads only 0.5.
    Am I doing this right? I thought it should be more that 10A as the diagram on the PSU shows 20A for yellow wire. If I connect the multimeter before bulbs, would it give the actual current rate of that 12v?
     
    Last edited: May 17, 2018
  5. wayneh

    Expert

    Sep 9, 2010
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    The bulb is working. Think about it - if the bulb alone is connected to the power rails, how much current does the bulb draw? There's no way that adding the battery in series with the bulb can raise that current, it can only reduce it.

    Don't make the mistake of trying to measure current in a parallel connection directly to the power supply. This will drive the maximum available current through the meter and something will blow, either the supply or the meter.
     
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  6. zakmuh

    Thread Starter Member

    Jul 25, 2016
    52
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    Thanks mate, I'll keep that in mind - not to measure in parallel.

    How Can I increase that limiting current from 0.5 to 5.0A?
     
  7. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    No. First, that probably is +3.3 V, a standard ATX value. Since the +3.3 and +12 share a common ground, you cannot add them to get +15.3 V.

    1. The long form: Measure the diameter of the yellow wire conductor (not including the insulation), look up the wire gauge in a table, and then use a conservative estimate of the number of wires to combine in parallel. For example, if the wire measures to be #18, a conservative load would be 10 A per conductor. I would use 3-4 wires in parallel. Make sure you use an equal or greater number of ground wires - what goes in must come out.

    2. If you connect the battery directly to the supply and the battery is discharged, the supply *will* charge the battery but there will be no current limiting. The full 20 A could pour into the battery. The battery datasheet will list the max charging current. My guess is that 20 A (probably more like 25 A) is too high, and you risk damaging the battery plates or rupturing the case.

    3. The light bulb serves as a current limiter to prevent damaging the battery. If you want the peak charging current to be C/10, a common charging specification, C is 7 Ah so C/10 is 0.7 A. If the discharged battery voltage is 9 V (as a starting point), then the light bulb has to have a resistance such that 0.7 A flows through it when 3 V is across it. Note that light bulbs are weird in that they have a very large temperature coefficient. A cold bulb has a much lower resistance than a hot one, so be careful in your selection.

    ak
     
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  8. wayneh

    Expert

    Sep 9, 2010
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    You'd have to eliminate the bulb, or use a different one. A headlight bulb draws less than 5A on a 12V battery. I forget the exact value but it's close to 4-5A. Might be worth a try if you have one in the garage.
     
  9. zakmuh

    Thread Starter Member

    Jul 25, 2016
    52
    4
    Nice one both of you, thanks!

    Let me get this right....

    PSU ampere rate is 20A. So when I use a full beam headlight (12V/60W) in series, that bulb will reduce that 20A to 5A (headlight 60w/12v = 5A) and pass that rate through, taking 4 hours (20Ah/5A = 4h) to charge?

    Please correct me if I'm wrong.

    Cheers
     
  10. MrChips

    Moderator

    Oct 2, 2009
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    You cannot reduce the charge time by forcing a higher current into the battery. You will shorten the life expectancy of the battery.

    A 12V 20A power supply is rated to deliver 20A when the load is 12V/20A = 0.6Ω
    If the load resistance is higher than 0.6Ω the current will be lower.

    A 12V PSU will not properly charge a 12V SLAB. You need about 15V to charge the SLAB.

    Connecting a 12V 20A PSU to a 12V SLAB could damage the battery or the power supply. The lamp in series is added to limit the charge current. However, as stated before, the battery will not receive the proper charge required to ensure longevity.
     
  11. dendad

    Distinguished Member

    Feb 20, 2016
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    Have a look at..
    http://batteryuniversity.com/learn/
    A common misconception I've seen on this forum time and time again appears to be that a power supply, rated at 20 Amps, will force the 20 Amps into the load. No. A 20 Amp power supply is capable of supplying any load UP TO the 20Amp if so required. What determines the current running into the load is the voltage of the power supply and the load resistance. If you have a load that draws 1 Amp at 12Volts, it can be supplied from a 12V supply capable of 1Amp, or 10000Amps. It will still draw just 1 Amp. But a 12V power supply rated at 0.5Amps cannot supply the 1 Amp current, and if it is not protected by current limiting circuitry or just a fuse, stands a good chance of damaging the power supply.
    So your 12V 20Amp ATX supply will not force the battery to charge at 20 Amps. It will charge, but only very partially because the voltage is too low, and at only the current the battery "needs", until the battery voltage reaches 12V and than almost no current will flow. And a "12V" SLA is not fully charged at 12V. A number of 13.8V to 14V is a starting point, and it varies with temperature too just to make it interesting. The smart chargers vary the volts depending on the battery charge condition.
     
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  12. crutschow

    Expert

    Mar 14, 2008
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    The bottom line is that an ATX supply won't work to properly charge a 12V SLA battery.
    You need at least 14V.

    Edit: There are websites that show how to increase the voltage of an ATX power supply.
    That might work if you are up to doing that.
     
    Last edited: May 17, 2018
  13. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Your 12V SLA battery needs at least 13.6V to charge at all - tweaking an ATX PSU to do that might vent a few electrolyics on the other rails.

    In most cases - only the 5 & 3.3V rails are regulated. Once they get their filter caps up to voltage - the SMPSU chip throttles back to stop them going any higher. If you don't load those rails - you won't get much current from the others on an idling PSU.
     
  14. wayneh

    Expert

    Sep 9, 2010
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    That would be true if the battery stayed at 0V and had no internal resistance. The current will drop as the battery voltage increases. Once the battery is charged to 12V, there’s no voltage across the bulb anymore and thus no current. So the charge curve is not a straight line and the time is difficult to predict. The bulb’s resistance drops as it dims.
     
  15. zakmuh

    Thread Starter Member

    Jul 25, 2016
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    All, thank you very much for your help and advice on this topic. It has given a huge knowledge to me to understand how all these work.

    I dropped the idea of using that PSU conversion thing. I spent whole day in the garage and managed to find its own charger. All is well now :)

    Cheers
     
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  16. ian field

    AAC Fanatic!

    Oct 27, 2012
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    AFAIK: the old battery charger transformers were; "impedance wound" - its another way of saying they have terrible regulation.

    The sulphuric acid is at its weakest when the battery is flat and needs more voltage to get any charge current moving (a sulphated battery takes a *LOT* more voltage to get anything moving). a transformer with lousy regulation does that. Charge current increases as the battery picks up - it loads down the secondary voltage and delays wrecking the battery if you leave it charging too long.
     
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  17. zakmuh

    Thread Starter Member

    Jul 25, 2016
    52
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    Thanks Ian.

    I topped it up with battery water and when I charge it, say for 30 mins, it starts leaking through the seal. Am I charging it wrong way?
     
  18. ian field

    AAC Fanatic!

    Oct 27, 2012
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    Probably overfilling it - nearing full charge, a lead acid battery begins gassing as water is split into hydrogen and oxygen. The gasses eventuall float to the surface and leave via the vents, but it temporarily "expands" the electrolyte. You should fill to just over covering the plates. car batteries usually have filler holes big enough to see in and most motorcycle batteries have translucent cases with a level mark.

    Never use a naked flame to provide light - the explosive gas will detonate and shower you with bits of battery.
     
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  19. zakmuh

    Thread Starter Member

    Jul 25, 2016
    52
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    Cheers
     
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