Smart meter backup

Big Clive's break down of a smart meter shows a capacitor for back up. So not much backup potential. I suspect those with smart meters will be switched off as and when. Those without will carry on as before.

https://youtu.be/G32NYQpvy8Q

There is a 3VDC battery in the meter for something??

I would expect that smart meters would function correctly even after a prolonged power failure, whether by internal battery or by a capacitor.

Large numbers of customers were cut off for a week or more due to storm Arwen, I do not recall any reports of significant problems with smart meters. I would presume that after a relatively short time without power, that a smart meter would enter a low power mode in order to preserve data in the memory, turning off the backlight for the display for example.

So no concerns about the odd 3 hour rota power cut.

Let's start with an ordinary meter. They must draw some power, albeit a tiny amount, to power the LED display. What happens when the power goes off? Is there a battery, even a small one like those which used to be (still are?) in PCs; or is the reading stored in non-volatile memory (similar to a USB "stick")?

If the power is off, does a smart meter need to do anything more? If there is no leccy, there is nothing to meter!

The electronics shuts off everything, include the display, and the current needed to back up the status and keep the minimal functions ticking over is about the same level as a digital watch - some hundreds of nano-amps to few microamps, As a consequence  the 'hold over' period compares to a watch, it is at least 30 days, probably more like a year with a brand new meter.

As an aside note that even 'non volatile' memory like flash and EEPROM rely on capacitor-like charge storage, it is just that the capacitors are made with sub-micron volumes of silica inside the chip itself, and these have self-leakage times of many years, and store so little charge it is quite sensible to talk in terms of numbers of electrons.

Mike

mapj1 said:

As an aside note that even 'non volatile' memory like flash and EEPROM rely on capacitor-like charge storage, it is just that the capacitors are made with sub-micron volumes of silica inside the chip itself, and these have self-leakage times of many years, and store so little charge it is quite sensible to talk in terms of numbers of electrons.

I have heard that memory sticks should not just be put in a drawer for decades, but used once in a while.

They use FLASH, and the stored charge does leak with time. However, some devices now quote storage times of getting on for over 50 years at elevated temperatures when the device is new (these are typically microcontrollers intended for harsh environments, such as automotive ECUs). The storage time does decrease as the device is used, as each erase cycle puts stress on the device.

Personally, I would be inclined to use FRAM for something like a smart meter, as this does not suffer from ageing in the same way and is easier to use.

I suspect the battery/capacitor is there not so much to preserve memory, but to allow the meter to communicate for a while into a power cut - i.e. to enable it to 'phone home' to report a supply loss to the DNO (who can then sometimes send someone out pronto to see who's been pulling their cut-out fuse without authorization..).

   - Andy.

If the mobile base station is on the same substation and is also off,  there  may not be a lot of point in phoning home either. It makes sense for one house where the incoming fuse has popped but during a ROTA cut I cannot see that feature being so useful.
I do not know but if I was doing it the 'phone part' would be high on the list of things to put to sleep early on in the shutdown sequence perhaps in the first minute or two.  Perhaps one SMS outbound  and then off. Mind you not all designers think in terms of battery life - look at BT and the amazingly thirsty fibre to the premises kit that has to be always on.
Mike

If the mobile base station is on the same substation and is also off,  there  may not be a lot of point in phoning home

Down south perhaps but as I understand it for the rest of us smart meters don't use the mobile network. In any even I'd hope the base stations would have a UPS or generators that could keep them going for a bit. Even a few minutes should be enough to provide the DNO with enough information to work out if it's a singe consumer's cut-out, or a substation fuse, or roughly where in between a cable's gone bang.

   - Andy.