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My GPS receiver has a small MS621FE battery:

GPS receiver PCB

Any ideas what might be the purpose of a battery on a GPS receiver?

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    \$\begingroup\$ Fast restart due to retention of ephemeris & almanac. Might be an RTC on there as well. \$\endgroup\$ – markt Jun 24 '14 at 0:12
  • \$\begingroup\$ Might also have been cheaper/easier to use the battery to store persistent data (user settings, etc) in ram rather than adding an additional rom chip. \$\endgroup\$ – Ponkadoodle Jun 24 '14 at 0:14
  • \$\begingroup\$ This has been discussed in this thread (although from a different angle). \$\endgroup\$ – Nick Alexeev Jun 24 '14 at 5:34
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According to the datasheet:

VBAT

This is the battery backup power for the SRAM and RTC when main power is off. Without the external backup battery, ET-662 will always execute a cold star[t] after turning on. To achieve the faster start-up offered by a hot or warm start, a backup battery must be connected. The battery voltage should be between 2.0V and 3.5V

As mentioned in the comments, the main reason this saves time is that it avoids having to wait for the "almanac" of GPS satellite positions to be downloaded.

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    \$\begingroup\$ +1 for RTFM - a skill that many people are sadly lacking. \$\endgroup\$ – markt Jun 24 '14 at 0:39

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