Battery charging

Discussion in 'General Electronics Chat' started by brandongordon2002, Dec 28, 2009.

  1. brandongordon2002

    Thread Starter New Member

    Dec 28, 2009
    Hey guys i am new to this forum, i have a question and would like some help. I have 2 sealed lead acid batteries each are 6V and 7.0amp hour. This weekend i bought a 12V 1.0amp trickle charger from Canadian tire. Ive got both the batteries hooked up in series so they are 12V at 7.0amp h, my question is how long will they take to charge and is it safe to be charging 2 6v batteries in series? Thanks guys
  2. beenthere

    Retired Moderator

    Apr 20, 2004
  3. SgtWookie


    Jul 17, 2007
    If the batteries were discharged the same amount, then charging them in series would be OK.

    But if one battery is heavily discharged and the other is nearly charged, the nearly charged battery will get overcharged. This won't be good for it.

    You're really better off to have two separate 6v chargers.
  4. thyristor

    Active Member

    Dec 27, 2009
    Charging 2 x 6v batteries in series is fine. Afterall, internally, each of your 6v batteries is actually 3 x 2v cells in series.

    As regards charging time, it all depends how discharged the batteries are when you start out. Firstly a charged 6v battery will have an open-circuit voltage (after resting) of about 6.35v. Each 0.05v (50mv) drop in open-circuit terminal voltage represents approximately a 10% drop in battery charge state. So a nominal 6v battery reading around 6.1v is actually about 50% discharged.

    To prolong the life of lead-acid batteries, never let them drop below this 6v value before recharging. Also never leave them discharged even at 50% for even a few days as the plates will start to sulphate and it will become increasingly difficult to remove the sulphation through charging. The result is diminished capacity. They will still show 100% charge on a hydrometer but the 100% will no longer be 7AH but rather a smaller figure.

    The actual current available for charging from a charger will depend on the applied voltage, the charger's internal resistance, the battery's terminal voltage and the battery's internal resistance.

    At this very low charging current, second order effects (the Peukert Effect) can be ignored and charge time will be:

    (capacity to be replaced in AH/current) x 1.4 hours

    The 1.4 figure is to account for the fact that the physics of charging mean that charging is only about 70% efficient so some energy is lost in warming the batteries.

    As the battery terminal voltage rises, the cuurent will drop as the battery voltage opposes the charger voltage. For the batteries you have, I would guesstimate about 5 to 6 hours, with your charger, assuming the batteries are at about 50% charge when you start charging.