May 12, 2016

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 has the tendency to drop its voltage and attempts to settle down to a lower voltage position.

This behavior prompts the opamp to again switch ON the charging instantly, which creates a rapid switching of the opamp output at the over charge thresholds. This condition is usually not recommended in any opamp controlled comparator system

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