Current limiting vs current regulating in 12V SLA battery charger

Discussion in 'The Projects Forum' started by Bosparra, Jul 2, 2010.

  1. Bosparra

    Bosparra Thread Starter Member

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    I built a simple LM317 based SLA charger, schematic can be found here: http://www.vt52.com/diy/myprojects/other/charger/charger.htm

    This circuit limits the current drawn, by the battery, to 500mA. My question is: am I correct in saying this circuit limits current and does not qualify for a constant current charger?

    The way it limits current is by lowering the voltage, which gives a much lower than required voltage on the battery terminals, in the region of 12.4V. This voltage slowly increases as the battery draws less current, until it reaches its preset voltage of 14V.

    How is active current limiting possible, without lowering the voltage?
  2. SgtWookie

    SgtWookie Expert

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    It is a voltage limited constant current supply.

    The output current is determined by the Vbe of the BC547 transistor, which is caused by current flow through the resistor labeled "See text".

    Your mileage will vary depending upon the transistor used.

    Very roughly, the output current will be:
    Iout=0.5v/Rseetext
    where 0.5v is the cutoff threshold Vbe of the transistor. See the transistor's datasheet.
    Conversely,
    Rseetext = 0.5v/Desired_Current_Out

    Current through Rseetext will cause a voltage drop across Rseetext. When that voltage exceeds the transistors' cutoff voltage, the collector will start conducting current, which will decrease the voltage on the ADJ terminal, causing the output of the regulator to source less current.

    Note that the more discharged the battery is, the more power will be dissipated in the regulator. You will need a large heat sink on it.

    Note also that Vbe(threshold) will vary over temperature. Very roughly, the hotter a transistor is, the lower Vbe(thresh) will be.

    Please review the manufacturer's datasheet for your battery, and observe the charging/maintenance specifications. If you conform to those specifications, you will maximize service life.
  3. Bosparra

    Bosparra Thread Starter Member

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    Thanks Sarge, the circuit works no problem. My question relates to how I can limit the current, without lowering the effective charge voltage as seen by the battery, ideally by just modifying the existing circuit. One idea that comes to mind, is to put a transisitor in line to limit the current, but the varying Vce will cause the charge voltage to lower?
  4. SgtWookie

    SgtWookie Expert

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    The current is limited by dropping more voltage across the regulator, thus dissipating more power in the regulator. That's just the way it works.

    In order to limit the current, you have to reduce the voltage applied across the battery.

    On a basic level, Ohm's Law says:
    I=E/R, or Current(Amperes) = Voltage(Volts) / Resistance(Ohms)
    To decrease the current, you can either increase the resistance or decrease the voltage applied. The regulator actually increases it's own resistance (by reducing it's conductance) to decrease the voltage across the battery, thus reducing the charging current.

    It is not an efficient way to do it, but it works.

    If you want an efficient charger, look at using a UC2906/UC3906 or UC2909/UC3909 by Unitrode/Texas Instruments. It's a switching regulator designed specifically to maintain SLA and lead-acid batteries. Four phases of battery maintenance are implemented in the devices; bulk charge, absorption charge, overcharge (for equalization) and float charge.
  5. Bosparra

    Bosparra Thread Starter Member

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    I was trying to get my head around regulating current without affecting the voltage. As you stated, Ohm's law does not allow for the voltage to stay constant as well.

    I guess what I was after is that there is no linear way to achieve current regulation and constant voltage. So, the only way to achieve that is by switching and using PWM to regulate current by adjusting the duty cycle?
  6. SgtWookie

    SgtWookie Expert

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    If you want to have the output voltage constant yet regulate the current, about the only way you could achieve that is via PWM, or pulse width modulation. You turn on the voltage supply and wait until the desired amount of current for the time period has been achieved, and then you turn off the supply. Repeat. If the PRF (pulse repetition time, or 1/frequency) is low, you may wind up with damagingly high current flow if the battery in question is defective or in a discharged state.

    Switching regulators are all the rage nowadays. That's because they are either ON or OFF; very little "resistive" state where they dissipate power as heat. An inductor is used to keep the current flow relatively constant. If something is getting hot, then it's causing power loss as waste heat.
    Bosparra likes this.
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