Sunday, February 26, 2012

CellLog8s "One-Shot" LVD

With little prospect of the firmware being fully fixed, I decided to implement a work around to make the CellLog8s at least work as a "one shot" Low Voltage Disconnect (LVD) for the inverter.

The problem was that without proper hysteresis in the CellLog8s firmware, the alarm output would flip-flop in an unstable way near the alarm set point value.  So I had to devise a way to iron out this transition behaviour and make it trigger once only.

I found a little DPDT latching relay in Maplins that does the trick, but I had to rebuild the interface board that I'd made previously.  In the video you can see the new circuit.

In this new version, the inverter receives an "Enable" signal from the interface.  This just connects to the common pin on the Remote/Off/On select switch on the inverter front panel.  The new relay is stable in both positions of its double throw output and has two coils, one to select each output mode.  It only needs a single short pulse to cause the state change and then further pulses have no effect (as you have to energise the opposite coil to change the state).

So, you press a button to "Enable" the inverter (or reset it, if it had tripped).  This just flips the relay "on".
The 680 Ohm resistors in series are because the relay has 12V coils with a measured DC resistance of about 700 Ohms.  They weren't quite equal though and (by luck more than judgement) I happened to pick the coils in such a way that the alarm state coil is the "stronger" one, so that when the alarm state is "true", the "reset" button does not work... Useful that.  You can't force the inverter to start up when something is wrong.

The second pole on the relay is just used for the LED indicator.

The output of the CellLog8s alarm port (now set to Normally Open) sits and does nothing until the set point is reached, at which point it will trigger.  The alarm port goes to closed state and triggers the "Disable" coil on the relay.  The LED goes out and the inverter is forced to shut down.  It cannot restart until the alarm condition has cleared and the reset button is pressed on the CellLog8s interface (and of course after you've investigated why it tripped!).

As programmed in the CellLog8s now, either a pack LVD or a cell imbalance alarm can cause it.

Next, all I had to do was hack the inverter to accept the Enable signal...
Here's another video of me "hacking" the inverter to get at the switch on the front panel and wiring in the connection to the new interface.  A bit of testing, too.
Now the battery is fully protected from any low Voltage drain from the inverter (the main load).

The advantage the new system has is that the relay consumes no power to hold the inverter in the enabled state.  Just a pulse of current from the reset button and then nothing.

In the alarm state, the other coil consumes 20mA for as long as the alarm is triggered. In practice, the load from the inverter is usually such that the pack or cell Voltage sags to the limit and triggers the alarm.  Instantly, the load is disconnected and the pack/cell Voltage recovers enough to rise above the alarm set point, which cancels the alarm.  Now the relay consumes no power again but is latched in the "Off" state.

In theory, the charge controllers, the SmartGauge, and even the CellLog8s itself could cause the pack to drain down and be damaged. But as I've set the cut-off Voltages quite high (24.0V pack and 3.00V per cell), it would probably take several days with no solar charge (the PV disconnect breaker thrown) to drain the last few Ampere.hours from the pack and damage it.

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