Wednesday, February 22, 2012

Progress on the CellLog8s Front

Some general tuning and tinkering over the last couple of days.

After watching the battery pack charge and float, I noticed that it was starting to discharge a bit more than I'd like when floating.  So I increased the float level by 0.1V to 26.90V and then observed the next charge day.  This time, rather than discharging at 3 Amps, it settled into a discharge of around 1 Amp.

Meanwhile, things are moving forward with the CellLog8s problem and development of an interface to my inverter for low Voltage disconnect.

Junsi, on the OEM RC Groups thread, managed to replicate the unstable alarm port output problem in his lab and set about looking for a remedy for it.

Not 24 hours later, I received a PM on the forum and then a new beta firmware code to test!  Now that's FAST.

Uploaded the firmware (v2.09) to the Cellog8s, now connected to all the cells in the pack, and played about with the battery until 3am to see how it was now.

Much better, is the answer.  But still a ways off being useful without bodging some external electronics to fix the remaining problem...


At least now the alarm triggers reliably when some way above / below the set points.  But there's still a lot of instability at the set point.  The alarm trigger has no hysteresis in it.  With a big battery you get VERY slow changes in Voltage and then the battery can spend a long time transitioning across the set point (and I mean a few minutes spent dropping the pack Voltage by 1-2mV at a 150W load!).

Anticipating that they would fix the software, I built a follower relay module (that just follows the sense of the alarm output of the CellLog8s).  The relay itself came from an old broken mains timer switch and was convenient as it had a 24V DC coil.


The instability of the CellLog8s alarm output made the relay chatter noisily near the set point, with the transition instability.

If I used it "as is", it would probably do what Jack Rickard's VDR (voltage dependent relay) did to his test load and A123 battery pack.  The load and pack cycled on and off furiously at the switching set point, and then both of them exploded with the stress of a few hundred Amps being pulsed rapidly.

The addition of a programmable variable for alarm set point hysteresis would eliminate this problem.  If you have a low Voltage alarm trigger point at 24.00V and hysteresis of 0.5V, then the alarm will trigger ONCE at 23.99V, and then stay triggered in the alarm state until the pack Voltage rises to 24.51V.

With no hysteresis, your alarm triggers multiple times as the Voltage floats around the 23.895 to 23.995 zone, in exactly the same way you see a DVM last digit toggle randomly between two values when it is close to the threshold of the next digit. Fine for a DVM display (even desirable as you can interpret the toggling as meaning the value is very close the the transition point)... VERY BAD for a load controller.

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