Thursday, March 22, 2012

Bumping Off the Limiter

Just pushing the boundary of the Winston Battery and charging system.

The system has been in and running stable for a over a month now, so I thought I'd try to see if the end Voltage could be raised a bit from the conservative 3.50V per cell.

I have been increasing the time that the charger spends in the constant Voltage absorption phase from 10 minutes up to 30 minutes now.  But I wondered if it might be possible to increase the final Voltage and shorten the charge time.

So I increased the final Voltage to 3.525V per cell (28.00V to 28.20V) and decreased the absorption charge time from 30 minutes to 25 minutes.

Today was extremely sunny again and the battery reached the absorption Voltage by 11:20am.  The CellLog8s was set to slightly higher cell and pack Voltage upper alarm limits (3.57V and 28.60V).  But the alarm tripped all of a sudden at about 20 minutes of absorption.

A couple of cells seemed to be climbing up the steep part of the charge curve and the alarm tripped on the cell differential being greater than 60mV.  I increased the alarm points to 3.58V per cell and 28.70V pack Voltage and 70mV differential.  After that the alarm did not trip again, but the the charger soon flipped to the lower float mode so I couldn't observe if the high cells were going to continue to run away.

So it seems that the original 3.50V per cell Voltage is about as high as you can go and still see the cells track each other fairly closely (under 60mV difference).  A longer time at that constant Voltage at a tapering charge current seems to keep things in line while allowing the pack to get close to full. 

Pushing the pack to any higher Voltage just causes the cells to amplify their differences in the final phase of charging and could risk one cell getting too far ahead.  This would falsely increase the pack Voltage and ironically prevent the other cells from being more fully charged as the charger would cut back the current too sharply, too early.

So the original settings are back on the chargers and I'll watch them some more to see if the current 30 minute charge is long enough.

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