So if we don’t use a a super capacitor or a small lithium rechargeable, then what is the alternative ?
We could of course use a non-recharegeable lithium battery such as a CR1216 but in the holders these have much the same issue as the super capacitor, they are physically large, see picture.
In addition even a battery as physically big as the CR1216 only has a capacity of circa 25mAhr, so that would only keep the GPS (@15uA backup current) going for 70 days. A tracker based on this setup would need it’s GPS battery replaced around every two months.
The answer to the problem is quite simple really. With the small lithium rechargeable or supercapacitor options, then the backup power for the GPS is actually coming from the projects main battery, so why not use the main battery as the backup power source for the GPS ?
All we need to do is take a 3.3V supply from the project and feed this voltage via a diode to the GPS Vbackup supply. We then need to ensure that the 3.3V supply is present when the project (using the GPS) is in sleep mode. This is not difficult even if the project is using something like a TPL5110 to switch off \on the battery supply to the project. A low drop out regulator such as the MCP1711 has a quiescent current of 0.6uA, so that can be used to go between the projects battery and the GPS Vbackup supply without having a significant impact on total GPS backup current consumption.
Whilst the solution to providing a long term GPS backup current is simple to implement, very few GPS breakout boards provide access to the GPS Vbackup pin, so you cannot feed in an external backup supply. For this reason I designed my own GPS breakout boards for the Quectel L70 and L80 GPS. These are low cost high performing GPSs with substantially lower power consumption than most other GPS modules, see a detailed report here;
GPS performance comparisons
The L80 GPS has a slightly worse power performance than the L70, but it has its own ceramic patch antenna and easy to include in PCB designs and solder in place, I like it. The L70 has a balloon mode, for use up to 80km.
If a small lithium battery is not suitable; what about a super capacitor that can at least be charged fairly quickly so it might accrue enough charge during a short hot fix power on to last for several hours ?
There is a web site which is an online super capacitor charge and discharge calculator;
Super Capacitor Calculator
First lets see what size of super capacitor we need. Assume a maximum backup time of one day, a fully charged voltage of 3.0V and a minimum volts for GPS backup of 1.5V, a GPSs backup current of 15uA which is typical for a Ublox GPS for instance. The calculator suggests a capacitor of 1Farad would last a couple of hours longer than one day.
If the hot fix gap was 10 minutes and the hot fix takes 5 seconds with a GPS backup current of 15uA, then assuming no charge\discharge efficiency losses the charge current needs to be 600/5 times the backup current or 1.8mA. That’s manageable.
So if we are to use a 1F super capacitor how big would it be ?
Checking on the Farnell web site a 1Farad 3.6V super capacitor would be around 20mm, perhaps a bit more, in diameter and 5mm high. That’s really quite big, see picture for how big the 5.5V version 1F capacitor is. The GPS shown is one of the larger 25mm ceramic patch antenna types.
There are some small super capacitors around the same size as the Seiko MS621, but these super capacitors are of very limited capacity and would only provide a couple of hours backup (based on practical tests).
So if a super capacitor is too big to be practical and we have allready dismissed small lithium rechargeables, then what is the alternative ?
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.