25V To 12V Car Battery Charger.

Discussion in 'General Electronics Chat' started by Kaiser Ed Bayawa, Apr 5, 2016.

  1. Kaiser Ed Bayawa

    Thread Starter Member

    Mar 19, 2015
    60
    6
    I have here a 3000 watts transformer from a voltage regulator. I want to make this into a car battery charger.
    Can anyone help me how to make this?
    Its output voltage is 25v
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
    1,667
    Well, ideally you need to get an output of around 11 -12 volts AC to work for 12 volt batteries.

    Does the 25 volt secondary have a center tap or the primary have two windings that can be configured in series to reduce the output by half?
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Unless its got a centre tap, you will need a voltage regulator circuit, or use a Bridge Rectifier and charge two batteries in series....
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    If the secondary is on the outside, you could remove winding to lower the voltage.You also might be able to find the center winding and break it out for a center tap. A center taped 25 Vac winding would give you 17 volts unloaded.

    What kind of battery charger do you want to build; smart or simple (dumb)?
     
    Last edited: Apr 5, 2016
  5. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
    1,667
    Wound a bit tight there aren't ya? o_O

    He wants to know how to make it work correctly with a 12 volt battery.

    As for high current battery charging I do it all the time.

    A large near dead 12 volt starting type battery (group 31 100 Ah 1100+ CCA) connected to my big MIG welder set for 15 VDC output will draw about 500 - 600 amps for maybe 20 seconds before dropping back to under 100 amps for a minute or two finally leveling off at a slow drop from ~50 amps to under 10 over the next hour or two.

    No boiling no explosions.

    To be honest, years ago I made a pair of high amp battery chargers that have either 2 KVA or 2.5 KVA transformers and multi-hundred amp welder diodes in them and they work super for fast charging yet being they top out at just under 15 volts work just fine for leaving on a battery over night. Even on small batteries like what's in my riding lawn mowers.

    To me a properly built 12 volt battery charger based around a 3 KVA transformer is totally realistic. Granted given its weight I would put it in an old role around charger body not a handheld one.

    As long as it's designed so that the at light load DC voltage stays under 15 volts it will work just fine.
     
    Kaiser Ed Bayawa likes this.
  6. Kaiser Ed Bayawa

    Thread Starter Member

    Mar 19, 2015
    60
    6
    it is not a center-tapped sir. only two wires of output.
    i like to make it simple charger sir. if i will buy diodes and capacitor can you please give me some codes and voltage ratings of it?
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    The transformer, without modification, will work very poorly as a battery charger. The transformer voltage is just too high to charge a 12 volt battery.

    Please post some clear and close up photos of the transformer. Perhaps we can assist you to modify the transformer.
     
  8. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
    1,667
    Okay? Someone disappeared from the thread between what is now post 4 and 5? :confused:

    Unfortunately as I and others have stated the present 25 volt output voltage is way too high to directly charge a 12 volt vehicle battery. You need to get it down to around 11 to 12 volts AC to be workable.

    To charge a vehicle battery you need around 14.5 volts DC which with AC current has a peek voltage 1.414 times higher than its RMS voltage we rate it at. To get a peak voltage of ~14.5 VDC from and AC supply you need to drop the AC voltage down below 12 volts so that the peak voltage and the included voltage drops in your diodes (~.6 -1 volt ) works out to being around 16 volts peak at no load.

    So, ((16 - 1.2)/1.414 = 10.5) VAC. Add in the input and output line losses and miss other small voltage losses and a AC input of around 11 volts works really well for a high power 12 volt lead acid battery charger.

    So there is what you need. You need to get that 25 volt AC output down to around 11 - 12 volts first.

    For diodes any large 100 volt or more rated diodes in the 300 - 500 amp range mounted to a big heatsink would work well.
     
    Kaiser Ed Bayawa likes this.
  9. MrAl

    Well-Known Member

    Jun 17, 2014
    2,439
    492
    Hi,

    How much current are you looking to charge with? This is very important. A 10 amp charger is easy to build, while a 50 amp will be harder to build and require more costly parts.
     
  10. Kaiser Ed Bayawa

    Thread Starter Member

    Mar 19, 2015
    60
    6
    I will reduce its winding sir so that I can get 12v.
    Here is the photo attached
     
  11. Kaiser Ed Bayawa

    Thread Starter Member

    Mar 19, 2015
    60
    6
    ok sir thank you. can i control its current sir?
    like a variable current controller
     
  12. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    Unless you can remove the windings to give you approx 15v output, you will have to use a Voltage regulator circuit, or better still a buck down converter like Lm2596.
     
    InspectorGadget likes this.
  13. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
    1,667
    If you keep the peak DC output voltage under 15 volts physics will take care of the rest. ;)
     
  14. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    To all: Looking at the photo, is a transformer of that size really > 100A capable? I've had 25V transformers half that size that only did about 1A. I'm wondering if there's something lost in translation here. I imagined a 3KVA transformer to be much bigger.

    And Kaiser Ed, what is your knowledge level about electronics? Do you know all about Ohm's law and Watt's law, to begin with? Diode and junction transistor operation?

    Yes, you will need a voltage regulator and a current limiter. Google "car battery charger circuit" and start learning about them. With a current limiter in the circuit, you can avoid the 200A inrush current of a dead battery and limit your charger to cheaper, more readily-available parts.
     
  15. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
    1,667
    At roughly 7.5" cubed I would put it at 1000 - 1500 watts continuous capacity myself which if used as a battery charger transformer it's not likely to ever see even that power draw level long enough to do any harm to it.

    When first connected to a near dead battery the peak amps can go really high but that spike only last for a few tens of seconds at best quickly falling to under a 100 amps within a minute or so which I would say that transformer could easily handle.
    Given realistic voltage drops associated with the power cords plus the transformers internal drops followed by the cables and clamps to the battery at high current loads I would still say that getting it to go much over 300 amps into a dead battery for more than a few seconds no matter how big of vehicle battery it was would be difficult.

    As for current limiting that just adds more cost and complexity to the design that is largely unneeded and if anything a hindrance when you are in a pinch and need to have maximum charging and cranking amps like when you go out to work and find you left your lights on overnight and your battery is now stone dead. :oops:

    That's where an overbuilt oversized high amp capable charger shines by being able to yank that dead battery up to a workable level in a minute or two and supply a massive current assist while the engine is cranking on top of that.:cool:

    If battery life and ideal charging rates and what not are a concern that's where you use a smart charger but if your stepping out to work and have no time to sit and wait while a smart charger decides what is the perfect charge rate and then takes an hour to get your battery up to a workable level that's where a big growling brute like this is worth having around. ;) :D

    Yea hitting your dead battery with 200 - 300+ amps might be hard on it but then again you already ran it down too far to start your engine anyway and you're going to be late for work.
    What's avoiding that speeding ticket and getting questioned by your boss worth to you?
    To most when they are in that position abusing a battery for another minute or two to get on the road is a no-brainer and cheap price to pay. :p
     
    InspectorGadget likes this.
  16. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    You need more than 12V to push charge current into a 12V battery.

    In most charging situations it shouldn't exceed 14.4V.

    Some "intelligent" chargers pulse at up to 30V to recover sulphated batteries - but that has to be current limited to avoid damage when the battery starts taking current.

    The "intelligent" chargers usually drop back to about 13.6V at full charge to minimise distilled water loss due to gassing.
     
  17. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
    1,667
    Yes, I know. 11 - 12 VAC RMS has a peak voltage of ~ 15.5 - 16.9 volts.

    So once you factor in rectifier forward voltage drops and the unavoidable line losses plus the very narrow duty cycle that the peak of the sine wave is above the voltage required to pass current through the system and 11 - 12 volts AC gives you a nice solid 14 - 15 volt DC average as needed for bulk charging a common LA battery without excessive overcharging. ;)

    I've been repairing and rebuilding battery charges since I was a kid. Believe me I know my as applied to reality numbers for basic battery charging circuitry by now. :rolleyes:
     
    InspectorGadget likes this.
  18. InspectorGadget

    Active Member

    Nov 5, 2010
    211
    42
    OK, I see where you're coming from. And that's probably more along the lines of what the TS wants. But it would seem that that particular transformer would have to be tapped at a 12V point in order to make a simple, passive charger as you describe.
     
  19. gerty

    AAC Fanatic!

    Aug 30, 2007
    1,153
    304
    You haven't told us about the primary voltage. If it's 220v, and you have 120v available you could connect the primary to 120v. Half voltage in equals half voltage out.
     
  20. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,039
    1,667
    Yea. Unless he has a double primary that can be put in series there is no easy way to get the output voltage down into the 11 - 12 volt range without doing a rewind or adding a lot of electronics to it.

    If a rewind is necessary I would pull all the original secondary winding off and double it up to handle more current. If he pays attention to his winding count recalculating the new number of wraps to get into the 11 - 12 volt range should be fairly easy.

    Either that or rewind it to be a 22 - 24 volt secondary with a center tap.

    If it was me I would either shoot for 12 volts and use a fullwave bridge or go for 22 volts with a center tap and run a dual diode half wave bridge like most battery chargers use.
     
Loading...