How long do I charge a battery?

Discussion in 'General Electronics Chat' started by coolguy_ar, Jun 26, 2008.

  1. coolguy_ar

    Thread Starter New Member

    Jun 7, 2008
    To charge a battery 6V 4Ah, how much voltage do I have to supply to it so as to recharge it at a good rate? Is 12V alright, or 9V? Plus,I need to know where I can find a transformer for nearly 24Ah, or even greater if possible, since I need to build an inverter which will have backup for atleast 3 hours or so... The best transformer I found was that one in a UPS, which was for 7Ah!
  2. SgtWookie


    Jul 17, 2007
    You supply it with a constant current. The current you need to supply it depends upon the manufacturer's recommendations. Attempting to charge a battery too quickly may cause it to overheat and explode or leak.

    Batteries are rated in Ah, or mAh (Ampere-Hours) - that is how many hours they can supply a current of 1 Ampere, or one milliampere.

    Transformers are rated in voltage and current or VA or voltage and wattage. For transformers, the concept of time is not really relevant, since it is a given that they will be supplied with a specified voltage to produce the output.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    That battery in your UPS was 12 volts, 7 amp-hour. That is pretty much a standard size of battery in a UPS. The transformer will be rated by the power it can handle.
  4. coolguy_ar

    Thread Starter New Member

    Jun 7, 2008
    Well, I know transformers are rated in VA. SO i'd probably like to have a .5kVA or a 1kVA... But my question is, will it be able to step up the voltage from a car battery, 12V 25-75 Ah?? I haven't known of transformers being able to handle that much current in domestic appliances. Would a microwave transformer do the job. I'm in essence going to reverse a step down to make it a step up.. So consider this..
  5. coolguy_ar

    Thread Starter New Member

    Jun 7, 2008

    I understand it has to be with constant current, but I need to know how much.. The rating is 1A, but then I need to know when to chrage the battery.. In other words, when the charge in the battery is low and during charging, an indication to show it is full. Probably just using an op-amp to compare with a specific voltage is enough, but I'm not sure. If I could ge ta schematic or any help, would be nice.
  6. Audioguru

    New Member

    Dec 20, 2007
    Measuring the voltage of a charging battery is poor because you are measuring the charger's voltage, not the battery's voltage.

    The voltage of many different types of batteries is the same when they are half charged as when they are fully charged.