basics about a battery charger....

Discussion in 'General Electronics Chat' started by onlyvinod56, Nov 14, 2010.

  1. onlyvinod56

    Thread Starter Senior Member

    Oct 14, 2008
    i have a lead-acid battery. I would like to design a charger for that. It is 12v, 7AH.

    I want to know the basic needs for designing a charger.
    How can i know that the battery is fully charged.
    what is "constant current charging". what is its importance?
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    SgtWookie explains this well in that post/thread.
  3. Tahmid

    Active Member

    Jul 2, 2008
    You'd first decide if you want a crude charger or a smart one.
    For simple manual one, you can just have a voltage connected to the battery and disconnect the power once you think battery is fully charged.

    A 2-stage smart charger would first bulk-charge with constant-current or even with constant-current (I'll explain later), but usually constant-current.

    A slow safe charging current is taken to be 1/10th capacity of battery = 0.7A in your case.
    So if you apply about 2A constant until battery is 70-80% charged, ie, it reaches a specific voltage, eg around 14.2v, it will be 70-80% charged in 3-4 hours. The problem with continuing charging at 2A is 2A appied even when battery is >90% charged or fully charged will damage the battery. So after a safe 80% it is switched to constant voltage, where you charge at a trickle voltage, around 13-13.5v, where the current is low such that even when connected to the battery, the battery is not damaged.

    The other thing I mentioned:
    Charging current is proportional to charging voltage. So, during bulk you can constant charge at around 15-16v. Practically, this results in a charging current that starts off at around 2.3A and then decreases as battery voltage increases, to a final of around 1A. However, this 1A would damage the battery, so you could disconnect the charger or switch to trickle voltage. I've found this approach very efficient and makes the battery last long. What I usually do is charge at approx. 15-16v and periodically turn off charging using a Triac/SCR at primary, check battery voltage and then continue charging if battery voltage < 14.1v else stop charging.

    So the bottom line is that it is OK to charge a sealed lead acid battery fast as long as you don't overcharge it.
    Many may say my method is risky for battery and oxidation may occur, damaging the battery at such high voltage. However, you have to understand oxygen only forms when the battery is being overcharged, not during bulk charging.
    Refer to this great article:

    Hope this helps.
  4. Tahmid

    Active Member

    Jul 2, 2008
    You can have a 13.5v applied to the battery at all times without damaging it, but charge time will be slow. This 13.5v is, I mention, a theoretical value. The right voltage should be mentioned on the battery or in the battery manual. Most of the SLA batteries that I used stated a tricle voltage of 14-14.5v.

    Hope this helps.