Limiting current to 12v battery?

Discussion in 'General Electronics Chat' started by majhi, Jun 21, 2015.

  1. majhi

    Thread Starter Member

    Jul 2, 2014
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    Simple yes-no question (or so I assume):

    I have a 12v 5Ah SLA battery (UB1250). I currently charge it using a 12v wall wart that's rated at a max output of 500mA. I am receiving a 12v 8A power supply in the mail soon. I know that input current is automatically regulated when charging a device such as a phone or laptop, but I don't know about a "raw" battery. In order to charge the battery with the 8A power supply, do I need to limit the current? My assumption is yes, but you know what happens when you assume... (and better safe than sorry)
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Each battery maker specifies the max charging current for each model battery. Find the data sheet specific to the battery you are charging. I'm guessing it will no more than about 1.5A

    Does your power supply have an adjustable current limiter?

    Here are some ideas for you to think about...
     
  3. majhi

    Thread Starter Member

    Jul 2, 2014
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    For standby use, it says 13.6-13.8V at 0.75A and for cyclic use, 14.5-14.9V at 1.5A. My project is somewhere in between, I think. It will act as the primary power source for a few hours every week or two, and be recharged before each time. Thus, I'm not sure which one to go by for a current limiter.

    As for the current limiter on the power supply, I can't do that. I'm putting together a power distribution box that provides me a few 12v ports and a couple USB ports (hacking apart a car USB adapter for that voltage switcharoo). Thus the current that I will require will be completely variable. One 12v port, however, will be specifically to charge this battery.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Cyclic use means what your charger is supposed to do during a recharge.

    Standby use is what your charger is supposed to do after the battery is charged.

    Read about the proper charging steps, here.
     
  5. majhi

    Thread Starter Member

    Jul 2, 2014
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    The way I charge it now, I just hook it to the wall wart for about an hour or two before I use it. I'm not hooking it up to charge overnight or anything (because I understand that it should not be left charging after it's already full). But then again, my wall wart only puts out 500mA...
     
  6. cornishlad

    Member

    Jul 31, 2013
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    Have you tried measuring the voltage on the battery terminals when it's on charge by the 12v wall wart ? If it's a transformer type it's output may rise towards 12x1.4 volts and hence be able to trickle charge the battery.
    A STABILISED 12 volt PSU probably won't charge your battery at all as it takes an absolute minimum of 13.2 volts to put any charge at all into a lead acid battery.
    Your figures tell you 13.6 to 13.8 for a standby charge and 14.5 to 14.9 (current limited to 1.5 amps) for proper charging when the battery is discharged. Unless you can up the output voltage of the PSU - and provide some current limiting - the multi distibution system won't work unless all the other devices which will be plugged in are OK at 14.5 volts plus.
    Fortunately many devices used in cars are rated at those voltages even if marked 12v.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If it really is 12V then it won't be enough to charge your battery.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The capacity of a rechargeable battery is sometimes called C. As a rule of thumb, a safe charging current for an SLA is C/10. So for a 5 A-h battery, a safe charging current is 0.5 A. The datasheet might give you other number that allow for a shorter charging time, but C/10 is a good starting point. So, yes, you will need a current regulator between your 8 A source (assuming it can be adjusted up high enough to work in this application) and the battery. The current limiting was being done with the limited size of the transformer core in you wall wart. Now you will have to do it some other way.

    ak
     
  9. majhi

    Thread Starter Member

    Jul 2, 2014
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    That makes sense. The 500mA charger that I have right now outputs at 13.4-13.6v so that's why it works. I will test the new PSU when I get it in the mail. For the moment, however, I will still need to consider making a current limiter.

    By the way, thanks for all your help, guys. I don't know too much about the intricacies of electronics, but I'm definitely trying to learn more.
     
    Last edited: Jun 22, 2015
  10. majhi

    Thread Starter Member

    Jul 2, 2014
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    Also it looks like it may not make a difference. I'm seeing that using an LM317T (I have a spare one laying around), when you reduce the current, it also reduces the voltage. While I did make an error (the PSU that I'm receiving is 5A, not 8A), to drop 5A to 500mA is no small amount. That means my voltage would drop to be too low to charge the battery. Is that correct?

    EDIT: Wait, could I put together a voltage booster to bring the 12v up to 13-14, and THEN run it through the current limiter?
    EDIT #2: It appears that I would need to use an LM338 to handle the higher amperage of the PSU, but that it would still drop the voltage too much.
     
    Last edited: Jun 22, 2015
  11. cornishlad

    Member

    Jul 31, 2013
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    You don't need to handle the "higher amperage" of the PSU. The battery shouldn't be sent more than 1.5 amp. You could be buying a 1000amp power supply and it would make no difference to the charging arrangement of you small battery.
    Think of this. Maybe your main house/breaker fuse is 100 amps so consider your house has a 100amp feed capacity. Doesn't mean everything you plug in will have 100amps pushed through it. At least I think that's where your confusion lies.
     
  12. majhi

    Thread Starter Member

    Jul 2, 2014
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    Right, I know that devices don't draw more current than they can handle because they have circuits that handle that, but I get the feeling that hooking a raw battery (with no circuit attached) to an 8A power source would be a bad idea. Not to mention:

     
  13. crutschow

    Expert

    Mar 14, 2008
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    Why not just continuing using the wall wart to charge the battery?
     
  14. majhi

    Thread Starter Member

    Jul 2, 2014
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    I certainly could, but that would just be too easy! Besides, the power distribution block I'm putting together was intended to eliminate the need for so many warts/cables.
     
  15. ian field

    Distinguished Member

    Oct 27, 2012
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    Its generally better to use a charger designed for charging the type of battery you have, rather than improvising with a PSU designed for something else!

    Intelligent chargers such as the Optimate have a microprocessor that monitors the state of charge, initially it charges a nominal 12V battery to about 14.4V to ensure its fully charged, then it throttles back to about 13.6V so the fully charged battery doesn't gas all the water out of its electrolyte.

    An SLA battery is not designed for adding distilled water - so you really need a charger that doesn't gas it all out.
     
  16. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If your new supply is only adjustable to say, 14V, that is just barely enough to charge your SLA, but nowhere near high enough to insert a LM317 current limiter between the power supply and the battery. The minimum voltage drop across a LM317 and its current setting resistor is almost 4V, so you would have to start with a 18V supply.

    There are current limiters that operate with a much smaller voltage drop; I can come up with one that would have a minimum drop of ~100mV.
     
  17. majhi

    Thread Starter Member

    Jul 2, 2014
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    Well it sounds like I should just use the wall wart I already have then. Bummer.
     
  18. ian field

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    Oct 27, 2012
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    If your wall-wart is the old type with an iron cored transformer and no regulation, its probably among the least bad of the inappropriate devices. As the battery reaches fully charged and the current draw tails off, the charge voltage increases ensuring the battery eventually reaches full charge.

    A well regulated SMPSU type wall wart is highly unlikely to be just exactly the right voltage - it'll either undercharge or overcharge.
     
  19. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Remember that the link I sent you to in post #2 started with a ~19V open-circuit wall-wart...
     
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