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.

 Discussing Battery Charging Parameters

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.


Full Charge Cut-off and Hysteresis

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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