Question regarding increase in terminal voltage of battery in every new cycle of profile while same current is applied.?

[abkakar]

I am currently working on Flooded VRLA battery. I applied a current profile on the battery in which its SOC moves from 50 % to 80 % SOC range in every cycle. Its a constant profile for 440 hours. But in every new cycle terminal voltage is increasing ( or we can say expanding in both upper and lower side ). Also if we notice the OCV after 30 mints of resting is increasing. Now i need to estimate the SOC of this profile. As the current is same in every cycle but OCV and terminal voltage is increasing , so SOC estimation is different for every next cycle while it should be same as current is same. I would be really thankful if someone help me by telling the reason of this phenomenon.
P.S. Battery is placed in a chamber where temperature is maintained at 25 degree centigrade. I am using a BiPolar power supply controlled by computer to provide current to the battery. Voltage is measured by a very accurate sensing circuit.440 hours profile of current and voltage is shown. Also zoomed profiles of first five cycles are shown. In the current graph , positive current means discharging and negative current means charging. Capacity of battery is 90 Ah and 4.5A current pulses are used for 1 hour with 30 mints resting between each pulse.

Edited November 26, 2015 by abkakar

[Sensei]

Q=I*t= 90 Ah = 90A * 3600s = 324000 C

With I=4.5 A

battery will last 20h. 324000 C / 4.5A = 72000 s = 20h.

But there is also another parameter, voltage,

when it's multiplied by Q, we have total energy stored:

E=Q*U=P*t=U*I*t

your discharging cycles when voltage is increased, look like they shrink accordingly, less time was spend on discharging battery.

Zoom-in your graph, and measure minutes between full load, and full discharge, of different cycles. Differences should be pretty small, barely visible.

 

I can only speculate. You're passing quite significant current, with 55-60 Watts, for hours, perhaps there is made somewhere layer of metal oxide, that require higher voltage to be passed by electrons?

Can you place your battery, and set up in container with Nitrogen only gas or other Oxygen-free gas.. ?