Forecasting run time

“Battery 40 %” sounds reassuring. Whether it is enough, however, depends on how fast it runs empty. The real question is: how many minutes left?

Charge and consumption#

The station battery delivers Charge (stored energy in J) and Maximum (capacity in J). Over all batteries Sum is right – energy adds up.

You do not measure consumption via watt figures but directly at the battery: how much charge has vanished in the last minute?

sub loss old now
move old now
div minutes now loss

If 20,000 J are missing after one minute and 580,000 J are left, the battery lasts 29 minutes. That is a simple rule of three, and it is always right, whatever hangs on the network, because it measures the real loss.

When the battery is charging#

If loss is zero or negative, the battery is charging or idle. Then there is no remaining run time, and the division would be meaningless. The chip then shows −1:

sgtz tmp loss
select minutes tmp minutes -1

The chip reads the starting value for old before the start label. Otherwise old would be 0 the first time, and the first minute would look like a huge charge.

Bridging the night#

NIGHT is the time the battery has to hold out without sun. Set the value to your night, plus some reserve. If the remaining run time is below it, the warning light goes on – while there is still time to act: fire up the generator, switch off the furnace, dim the lights.

When the batteries are missing#

Sum over no batteries gives 0, not NaN. 0 J of charge would look like an empty battery. That is why the chip checks the total capacity: if Maximum sums to 0, there is no battery – display −1 and warning on.