SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

The time settings might very well be retained, by way of non-volatile memory, but the usual desire of controller owners is to never have to reset the clock after a power failure. That is a job for battery backup, and not for controller memory. Controller memory can't keep time.

If a controller uses some of its non-volatile memory to store the displayed time, that still wouldn't change the need to correct the time after a power failure.

It happens that the Rainbird SST is a product not offered to the professional trade. Every controller a contractor might buy will have a battery backup that keeps time during power failures.

Quoted from "Amateur"

I assume, that it takes separate time-keeping circuitry (chips, etc.) powered by either line voltage or battery, to maintain clock and date settings during power failures. Why is this absent in so many controllers when it’s so essential to proper automatic program operation? Is it merely a cost-saving factor?

Name another controller on the market that doesn't have a back up battery.

Quoted from "Amateur"

Thank you. So that means, I assume, that it takes separate time-keeping circuitry (chips, etc.) powered by either line voltage or battery, to maintain clock and date settings during power failures. Why is this absent in so many controllers when it’s so essential to proper automatic program operation? Is it merely a cost-saving factor?

Yes, an RTC or Real Time Clock is needed to keep the time. An RTC circuit consists of a crystal to generate cycles which is then converts into time, and a memory chip to keeps track of the time. The memory cannot be non-volatile because non-volatile memory chips have limited number of write operations and the chip will be worn out soon if it is constantly written. This is why an RTC circuit typically has a battery and a volatile memory chip. The power to run an RTC isn't much. It can run on a 3.3v coin battery for years, but it too, eventually dies. Then you just replace the battery, tell it the current time and the battery will keep it running.

RTC circuit isn't cheap. That's why many low cost controllers avoid it. They would rather put a 9V battery in order to keep the whole controller alive during the temporary power outage.

Internet based controllers can get the time from the internet so they usually don't have RTC or backup battery. The time only needs to be updated once, right after the controller comes back from the outage, and it can just keeps on running until the power is interrupted.