A modern GPS will normally get a fix from cold in 30 – 60 seconds when it has a good view of the sky. Once it has downloaded enough information on the GPS satellites the main GPS power can be turned off and as long as a voltage is supplied to the GPS backup pin it will retain the satellite information in GPS memory. When the GPS power is then turned back on the GPS can acquire an updated fix in as little as 2 – 5 seconds, this is called ‘hot fixing’.
In backup mode the GPS may consume only 5uA to 20uA, so clearly there are significant power savings to be had if we only want intermittent fixes from the GPS. A GPS that only needs to run for a few seconds to get an updated fix every 10 minutes or longer will use a lot less power that an GPS that is left running all the time.
A very common way to supply the backup supply to a GPS is to use a small Lithium battery, typically a Seiko MS621. However this is not without problems, the MS621 has a capacity of only 5.5mAhr and a max charge current of 100uA. So when first used with the GPS you need to leave it powered for 55 hours, just over two days, to fully charge the battery which is not very convenient. In addition you need to consider the cycle time. A typical GPS will consume 15uA in backup mode, so the MS621 will only last 15 days or so when fully charged. If the tracker is to use hot fixing for longer than 15 days then the backup battery needs to be charged, or it will just run out and hot fixing will fail.
If we are taking a GPS fix every 10 minutes and the hot fix takes 5 seconds, there is only 100uA x 5 seconds = 500uA seconds going into the battery. But in 10 minutes there is 10min x 60seconds x 15uA = 9000uA seconds going out. Thus the battery is draining 18 times faster than it is charging and will inevitably discharge quite quickly.
Although fitting an MS621 battery is common with GPS modules, its not suitable as a backup method if the GPS is to be used for a couple of weeks or more. A different backup strategy is needed.