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Battery Charger using LM317

  1. Jul 2, 2010 #1
    I'm designing a battery charger consisting of an LM317. The regulator will charge three batteries successively, for example, when battery 1 reaches a terminal voltage of 13.8, a voltage comparator will activate a relay that will switch on the second battery while switching off the switch for battery 1. The same will happen to the third battery when the second battery attains a full charge.

    Also, all the switches are paralleled with a resistor so that a trickle charge will compensate for the self-discharge of the battery when the switch for that battery has been turned off.

    For a 12V 10Ah, with the configuration attached below, here is my expected current to be delivered to the battery that is currently charging,

    I = (Vs - Vb) / 1 ohm

    I have researched the state-of-charge voltage level of the battery, based on a 0.1C charge rate:

    80% 13.8V
    60% 13.4V
    40% 13.1V
    20% 12.8V

    (these are just approximations, the paper where i read this said that the average error is less than 5% )

    therefore, as the battery voltage increases, the charging current decreases:

    Imax = (13.85 - 12) / 1 = 1.85A
    Imin = (13.85 - 13.8) / 1 = 50mA

    so that means that from the moment the switch is turned on for the current battery, 1.85A is being delivered, and then when the 80% charge has been reach, only 50mA is being delivered. The remaining capacity will be fully charged by trickle charge.

    my question is, am I correct with everything I said?

    Attached Files:

  2. jcsd
  3. Jul 2, 2010 #2


    User Avatar

    Staff: Mentor

    Be sure to put good hysteresis in your comparator circuit. When a relay is opened, the voltage sensed across the battery will drop some. You might want to experiment some with one battery to see what that voltage drop is like, and then use that to help tune up your hysteresis value in the comparator circuits.
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