I have a battery recharger that shows no indication when the batteries are fully charged. I'm supposed to assume they are fully charged after 8 hours.
If I leave the batteries to charge while I'm out during the day and there is a power cut while I'm out, they will not be fully charged after 8 hours, but I will not know this if the power is back on before I return.
Therefore it would be useful if I had a way to know whether there was a power cut while I was out.
The only method I can think of is to have a mains-powered alarm clock permanently on, so that a power cut will cause it to reset. But is there a better method?