Wednesday, March 14, 2012

Balance Tracking Data

Well, the pack has been in for a little over 3 weeks now and has been cycled to various depths from full to nearly empty.  It's been bottom balanced at 3.000V once, by hand with nothing more advanced than a test meter and a big light bulb.

I set the CellLog8s doing what it does... Logging data at 15 second intervals from all 8 cells plus the pack Voltage.  And here's the trace from the night of the 7th March to the night of 13th March.  Again, you can click on the graph to open a bigger view.
You'll notice that on the first night, the pack almost bottomed out before starting to charge on the 8th.  Just briefly it got down to about 3.1V.  Below you can see the zoomed in view of that discharge "spike".
The cells show good tracking with a spread that is just 15mV from the highest to the lowest cell in the pack.  This differential shrinks to about 7mV when under lower load.
The chart above shows the opposite end of state of charge at the 12th March.  Here you can see the pack reaching just shy of 27.80V and the spread of cell Voltages from 3.465V to 3.505V, some 40mV.

Remember that the pack is bottom balanced, so there will be more variation at the top of charge.  As long as we always undercharge the pack, this isn't a problem and requires no active balancing or Voltage limiting.  If we tried to do this, we'd be top balancing the pack and then would mess up the bottom balance.

As the charging current at the 28.00V target has been pretty massive (over 70A), I think the maximum regulation Voltage on the chargers was a bit too low.  They always seem to stop at 27.85V, measured on the CellLog8s and my DVM.  So I've tweaked the settings a bit.  The SSMPPT-15 and TSMPPT-60 have had their maximum regulation limit raised from 28.40V (3.55Vpc) to 28.80V (3.60Vpc).  Hopefully this will allow the terminal Voltage (at the charger end of the cables) to go high enough to raise the battery terminal Voltage to the desired 28.00V level.

I also tweaked the timer on the SSMPPT-15 so that it charges for 10 minutes (rather than 1 minute) before cutting out on the extended absorption timer.  You can see from the high charge chart that the SSMPPT-15 quit assisting the charge early and the TSMPPT-60 wasn't quite able to hold up the Voltage.  The big charger is still aiming to charge for 20 minutes, but now the small charger will support it for half the time.  Of course, the 10 minutes is not concurrent with the 20 minutes of the big charger, as the Voltage set point on the SSMPPT-15 is 0.1V lower at 27.90V.  It reaches this point while the TSMPPT-60 is still in bulk charge mode, trying to get to 28.00V.

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