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Battery Charging Algorithm-Confusions

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Siddhat:
Hey guys!
I've been designing a battery charger for a 12 Volt 7.8 Ah sealed lead-acid battery following the CCCV algorithm. The algorithm states, charging the battery at a constant current while the battery voltage rises. The current starts to drop as the battery voltage rises. As the battery voltage rises and reaches the gassing voltage, the charger now goes into a intermediate mode which maintains the voltage of the battery at the gassing voltage. The current starts to drop and when the current reduces to a certain percentage of the Amp-hour rating (determined by the manufacturer), the battery needs to be charged in a float voltage, which is the Constant Voltage mode of the charger. Usually the float voltage is lower than the gassing voltage.
But there are some confusions! How can a battery whose voltage has reached the gassing voltage be maintained at a lower float voltage? This seems strange. Need some explanation.

Nusa:
Do an experiment. If you remove the charger, what's the battery voltage? Does that answer your question?

Also, you don't want to actually reach the gassing voltage on a sealed battery, just get close. If they start venting, you can't replace it. On a battery where you can check and top off the water levels it's less critical.

trobbins:
Siddhat, I recommend you google and read up on as many technical articles about VRLA batteries as you can, including from large battery manufacturers that include operating instructions for their batteries.

That way you will gain some experience about what variety of charge profiles may be being recommended by different manufacturers, and for what applications (eg. standby, UPS, cycling, intermittent charging, PV, traction, anything and everything in between).

Then read some operating manuals of a few commercial chargers to get a feel for how they approach the topic - some provide a list of different charge profiles with the ability to set parameters associated with various stages within a profile.

The information is all out there.

Also note that a charger must only allow output current to flow in one direction, and many chargers use a series diode to enforce that spec.  So if your charger changes its command level for output voltage from say 14V to 13V, then initially the charging current will fall to zero as the battery voltage relaxes back to its open-circuit voltage level.  Assuming 13V is still above the OCV then at some time the charger current will start to increase from zero again.  So you need to read up on OCV and how that changes, and look at charge discharge voltage curves, and discussion about the chemistry of that type of battery, and look for some experimental graphs of voltage and time.

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