The CellLog8s LVD is fine and works well at protecting the battery bank from over discharge but it doesn't give you any warning that the lights are about to go out.
So I started thinking about some kind of pre-alarm that would give me a few minutes warning (under heavy load) that the battery was nearly depleted. I could then turn off some big loads and buy some time.
The old SmartGauge Voltmeter has a programmable relay in it too. But it only works on pack Voltage. I've been using it as an obvious visual reminder of pack Voltage (it's no good at reading SoC for non lead acid batteries).
Then I started looking around the house for something to use as a buzzer or alarm sounder that the relay could activate so that you'd get the message...
I found a small toy sound effect thingy that you press the top and it makes a cool police car sound. I actually found another one that my wife had that made steam train noises and in fact a whole load of similar sound chip enabled things, like a Dr Who Darlek bottle opener that says "EXTERMINATE!" when you close a circuit (with the bottle top) and a Christmas card that plays George Michael's "Last Christmas"...
But I decided to go with the police car :D
Taking it to bits was very easy and then all I had to do was solder wires on to the existing switch contact and run these out to the alarm relay on the SmartGauge and program the chosen low Voltage alarm.
The toy still uses the same two 1.5V button cells to make the noise. The SmartGauge does not provide any power to the relay contacts so external power for the alarm or whatever is needed. If the batteries in the alarm go dead, it does not affect the safety of the battery bank as this alarm is just for information.
When the relay contacts close, the police siren only goes off once for a few seconds and then stops. This is a good thing! Saves the batteries in the alarm and prevents bricks being thrown at the thing for sounding too much once the message has gotten through to whoever is within ear-shot of it :D
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.
A bit more tinkering with charging settings, increasing the float Voltage again to 27.00V (3.375Vpc) seems to null out discharging with the long sunny afternoons and water heater running.
I also changed the timers a bit. If there was not much load on the battery for a few days, even charging for 10 minutes per day to the upper Voltage limit might start to cumulatively over charge the cells. So I'm now making use of the absorption extension timer.
If the pack Voltage never drops below 26.60V (3.325Vpc) during the night, the next days absorption timer is set for only 1 minute top charge. This effectively eliminates the possibility of cumulative over charging. If we went away for a long time, I'd shut the whole system down.
If "normal" amounts of charge are taken from the pack, the Voltage drops to under 26.60V and the next days absorption timer assumes an extended absorption timer setting of 20 minutes.
I had noticed that 10 minutes wasn't resulting in a very large fall-off in input power to the battery during the constant Voltage phase, so extending the time seemed appropriate (with the new safeguard of a much shorter default timer).
The small SunSaver 15 Amp charger has now had its absorption timer defaulted to 1 minute for any condition.
With the clear blue days we've been having, I've seen total charge rates as high as 72 Amps, and at the top of charge, the battery pack doesn't need the assistance of the small charger for long before the Tristar charger can hold the constant Voltage.
