RTC not keeping accurate time & forgetting time

Hello all, I'm hoping this is as simple as a flat battery. The RTC on my Adafruit Datalogger shield lost 15 minutes overnight. The shield is logging data from two thermocouple breakout boards saving the data to SD and sending it out over serial to the PC where the GoBetwino beta is logging the feed to a text file. If the Arduino is left unplugged for a day the RTC forgets the time and has to be reset. I left the datalogger running overnight as the experiment it is logging from needs to run for 30hrs and so this was a test to see if it performed. This morning it was still logging but the timestamps on the logs were 15 minutes slow.

Can anyone tell me what the cause might be & how to remedy it?

Many thanks,

If the Arduino is left unplugged for a day the RTC forgets the time and has to be reset

That does suggest that the RTC backup battery is flat. Check the voltage on the battery.


Is this something that previously worked correctly? My understanding is that the RTC should keep good time even with a dead battery, as long as power is supplied.

I have an RTC module here that's always slow by exactly 30 minutes when the battery is low, no matter what time I set it to. So I would say check the battery too.

Quick follow-up: The logger ran flawlessly for 60 hours over the weekend and the RTC didnt lose any time. I forgot to take a voltmeter over to that lab so I couldn't check the battery. Doh^10

Can the serial connection override the processes of the Arduino? InBetwino is really laggy and after 24 hrs it is 3hrs behind in writing to text file. Its still in beta though. I just wondered if the lag from InBetwino can feed back into the Arduino's processes. I've moved to HyperTerminal for PC-based logging to text file instead of InBetwino. No lag encountered. Splendid.

BTW, isn't HyperTerminal the most inappropriate name for such a gods-awful piece of crud program? It sounds like it should be the name of the next cutting edge mobile handset.

Thanks for all the replies, kids! Le Scare

postscript: just checked the voltage: 3.2. Probably not a flat battery then. Odd as I thought I did a pretty good job of soldering it together. Will have a look to reassess.