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Full Charge Voltage for Lead-Acid Battery and Hysteresis

The discussion tells us regarding what's full charge voltage for lead acid batteries and hysteresis significance in battery charging systems. The questions were asked by Mr. Girish Radhakrishnan.


Hi sir, Good day.

I have couple of questions which makes me scratch my head:

1) What is the full battery voltage for a standard Lead-Acid battery, at what voltage the battery need to cut-off from the charger. What must be the float charge voltage for a lead acid battery.

2) Is hysteresis resistor is crucial in comparator circuit? without it will it work properly?

I've googled and found many confusing answers. I hope you can answer.

Projects are on the way.


My Reply:

Hi Girish,

1) For a 12V lead acid battery the full charge from the power supply is 14.3V (cut-off limit), float charge can be the lowest amount of current at this voltage which prevents the battery from self-discharging, and also prevents the battery from over-charging.

As a rule of thumb this current could be around AH/70, that is 50 to 100 times less than the AH rating of the battery.

Hysteresis is required in opamps to prevent them from producing a fluctuating output (ON/OFF) in response to a fluctuating input which is being monitored by the opamp.

For example if an opamp without a hysteresis feature is configured to monitor an over charge situation in a battery charging system, then at full charge level as soon as it cuts off the charging supply to the battery, the battery will show the tendency to drop its voltage and attempt to settle down to some lower voltage position. You can compare it to  pumping air inside a tube, as long as pumping pressure is there the air inside the tube holds, but as soon the pumping is stopped the tube begins slowly deflating...same happens with the battery.

When this happens the opamp input reference reverts, and its output is prompted to switch ON the charging again, which yet again pushes the battery voltage towards the higher cut off threshold, and the cycle keeps repeating....... this action creates a rapid switching of the opamp output at the full charge threshold. This condition is usually not recommended in any opamp controlled comparator system and this might gives rise to relay chattering.

To prevent this, we add a hysteresis resistor across the output pin and the sensing pin of the opamp, so that at the cut-off limit the opamp shuts off its output and latches on in that position, and unless and until the sensing feed input has truly dropped to an unsafe lower limit (wherein the oamp hysteresis is unable to hold the latch), the opamp then switches ON again.

If you have more doubts regarding full charge voltage for lead acid batteries and hysteresis significance in battery charging systems, do not hesitate to put them out through comments.

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    1. The submersible pump has two capacitors connected to the starting coil. One is permanently connected while the other is connected at starting time for a few seconds only and then disconnected. This procedure is done presently using a relay. As the current(about 30Amps) is very high, the contacts get corroded or stuck. So I would be very thankful to you if you could give me a circuit using triacs,optocouplers and timers(555). Thank you.

      1. you can try the last circuit from this article:

    2. Hi sir,
      Thank you for your valuable reply.

      what can be the Hysteresis resistor value in op-amp for 12v battery?


      1. Hi Girish,

        The formula is given at the end of this article:

        or the easiest way is to check it practically by using a high value pot for the hysteresis resistor and then changing it gradually to see at what point the latch just breaks at the applied lower voltage threshold. Measure and note this resistance in the pot and replace it with a fixed resistor of the same value

    3. Dear Sir,
      I have one question related to battery charging.
      I am confused about Constant Current Battery charging method.
      My basic question is what should be the supply voltage in constant current mode.
      And what should be the supply voltage in constant voltage mode.

      What i could find on the internet is that in constant current mode charging voltage could be anything above 14.2v (for a 12v lead acide battery).
      Now i want to know whether i can supply 20V to battery while fixing the current at 10A??

      And when battery is about 13.8v can i provide supply voltage about 14.2v?? To my opinion this charging mode will go to constant voltage mode and battery will automatically consume less current.

      Right now i had made 12v lead acid battery.
      By using arduino i am checking battery terminal voltage. At the start i apply about 15v at fixed 10-A current. When battery reach 13.8v arduino switch relay to 14.2v i have checked that after reaching 13.8v battery consumes about 5 amp or less and decrease with time..
      Is this the safe method of battery charging.

      My most important question is that can i use 20v at constant current mode of charging?

      1. Dear Waqar, first of all you must ensure that if your battery is a lead acid battery then the charging current must be preferably around 1/10th of its AH rating, so if your referring to 10amps that means your battery is rated at 100AH??
        If it is, then the fixing the input current at 10amps is fine, and the input voltage measure will be immaterial in that case because the limited current will safely pull down the 20V to the battery's instantaneous charge level.
        However using a high voltage is never recommended because it is neither important nor relevant, and therefore using the specified higher threshold voltage makes more sense.

        and when the battery reaches the full charge threshold you must switch OFF the high current at any cost, and shift it to a low current float mode, this is the right method....using high voltage initially and then reducing it is not the right method.

      2. ...when the battery reaches at 14.2V, the 10amp must be switched OFF, and replaced with 1 amp

      3. hello sir , my name is vivek
        please help me with charging time for battery of 4 V 3Ah using solar charger of 10 V 375mA ?
        is it a good charger for battery of 4v, 3ah?

      4. Hello Vivek, the charging time cannot be judged...because it will depend on many factors such as the varying solar power, the battery health and age conditions, etc

        the best way is to connect it practically and check the response for a day.

      5. yes the charging specification of the panel looks OK...


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