Bartimaeus

When we leave Bartimaeus unattended at this time of year, our main concern is long periods of freezing weather.  Frozen pipes on board is something to be avoided!  Instead of draining every drop of water, we rely on keeping the temperature inside above zero.  We set the diesel central heating to run continuously but with the thermostat set to 3°C.

To reduce our use of diesel, two years ago I bought a 100W electric greenhouse heater for use in the cabin when we are away.  With the thermostat turned to its lowest, it comes on at about 4°C and goes off again at 6°C. A constant drain of 100W would completely flatten the batteries in a fortnight, so it is only sensible to use this when we have shore power available.

This worked well most of the time that winter.  Unfortunately, one particularly cold night, the prepaid electric meter ran out.  I had chosen an inverter setting that saved power by leaving it off, so at that point the heating went off too.  The onboard temperature dropped below freezing before I’d noticed.  That was probably the point at which the shower valve froze and cracked.  We now make sure the shower valve is empty and open, but I don’t want to use that inverter setting any more either.

When we left the boat this time last year we were plugged in to shore power which was not on a pre-pay meter.  I chose a setting which meant the inverter was available continuously.  The boat stayed warm the entire time we were away, but it cost us about £1 per day for electricity which most of the time was doing nothing useful.

At the start of this season I had experimented with automatically turning the inverter off when it was not being used, and then turning it back on when the fridge got too warm.  I realised that I could do a similar trick with the cabin temperature – turn the inverter on when the cabin gets close to freezing.  The heater’s thermostat will turn it off once the cabin is a little warmer, so the inverter will go off then too.  (We turn the fridge off at the socket and leave the door open when we are away, so I have to disable the fridge monitoring.)

This bit of programming means the electric heater can do its thing, but the inverter can stay off when it isn’t needed.  This saves quite a lot of shore power, but I realised I could do even better.

The solar panels on the roof can usually generate enough power to keep the batteries topped up, though this is less reliable as the days get shorter and the sun gets lower.  Continuous shore power keeps the batteries fully charged at all times.  This means the solar panels have nowhere to send their power, so not only is this capacity wasted, but we are using shore power in its stead.

I have now programmed the system to monitor the level of the battery.  Shore power stays disconnected until the state of charge drops below a certain threshold.  At that point the power is reconnected until the battery level returns to 100% and then disconnected again.  If the average solar power can keep up with the rate of discharge, the shore power won’t be used at all.  If the sun isn’t strong enough, the batteries are protected from deep discharge.

It has been quite tricky to test some of this, and I was a little concerned that I might leave the system in some peculiar state.  I can turn the optimisations off remotely – but only if the programming is correct.  I have continued to watch what is happening – so far everything has been working much as intended, including the heating coming on when it has been very cold.

Cloudy weather and a weak November sun mean that the solar power is (just) insufficient to keep the batteries full, but in the two weeks since we’ve been away, we’ve used only a single unit (kWh) of shore power (and no diesel).  I got a fright when the power production dropped by a factor of ten for several days.  It turned out the panels were covered in snow – they’ve worked better again since the thaw.

Without the optimisations we’d have used (wasted?) about 40 units by now.  A further bonus is that I’ve enjoyed figuring out the details of the programming.