Hello Arnold, the current drawn by the Motorola GPS from the backup bat is not linear versus voltage. On an M12+, the real time clock takes most of the current, and will stop operating after about a day or less on a Supercap. The SRAM memory keeps maintaining it's contents for much longer than that. After the RTC crystal stops, the power consumption goes down to extremely low discharge rates. I have seen our FireFox units (M12+ with supercap) still maintain position and almanac over more than 1.5 weeks! Down to 0.5V or so. The Date is off by that amount too of course since the RTC just stops ticking. This presents a serious problem due to a fault in the Motorola firmware (in my opinion): The M12+ sees a "valid" Almanac, but the time and date are off due to the fact that the RTC crystal just stopped a some point. The M12+ thinks it's a different time/date than it really is, and desperately tries to search the sky for a sat constellation that is not there. It doesn't even reset its internal RTC when it sees a couple of Sats. I have seen instances where it took a day or more for the firmware to catch on, and reset the time in the RTC. When using a small coin Lithium battery, the backup time is only 6 months to 1 year or so typically, also not a very good performance. Not a good situation, they could have solved that more elegantly for sure. bye, Said In a message dated 11/22/2008 16:06:57 Pacific Standard Time, [EMAIL PROTECTED] writes:
After your question I did want to know myself for what time these caps would serve. My result: For a backup current of 25 µA and allowing a voltage drop during the backup of 0.5 V, the data will be safe for around 5 to 6 hours, With only 5 µA backup current the time will be 5 times higher (abt. 27 h) not counting for leakage or other discharge factors. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
