I would like to know how to properly use a backup battery, or rather how to charge it and whether it is possible to prevent over-discharge.
I made a board with a Ublox zed-f9p GPS module and use a battery SEIKO MS621FE on it. When I designed the board, I did not think about the need to charge this battery.
On the SparkFun GPS-RTK2 Board (https://www.sparkfun.com/products/15136) , the same GPS module and the same battery are used. Sparkfan engineers thought about charging, and it is on the board.
The datasheet https://www.smd.ru/upload/medialibrary/ … _seiko.pdf says that the standard charging current is 0.015mA (15nA).
For a voltage of 3.3V, for a current of 0.015mA a 220kOm resistor is required.
On the schematic GPS-RTK2 Board (https://cdn.sparkfun.com/assets/9/a/0/0 … ED-F9P.pdf) charging is done through a resistor and a diode. There is no exact indication of the diode on the circuit, but the voltage drop across it is 420 mV. The resistor is at 1K ohm.
If I calculate correctly, then the battery charging current will be:
3.3V - 0.42V = 2.88V
2.88 V / 1000 Om = 0.00288 A = 2.88mA
The charge current is 192 times greater than that specified in the MS621FE datasheet.
In addition, I am still worried about such a question:
Battery Capacity 5.5mA*h
Backup battery current for ZED F9P is 60 uA (from datasheet https://www.u-blox.com/sites/default/fi … 259%29.pdf).
Time to full discharge of the battery: 5.5 / 0.06 = 92 hours, or less then 4 days.
After 4 days, the battery will be discharged completely, but will be discharged further, to 0V.
My questions:
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am I calculating the charge current correctly?
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why is there a resistor for charging current 192 times larger on the GPS-RTK2 board?
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Is it possible to somehow prevent the battery from discharging to 0V? The minimum voltage for f9z is 1.65V. There is no point in discharging the battery.
The datasheet on the battery says that you need to charge it with a current of 0.1mA 96 hours. Is this normal for such a small battery?