Concerning the GPS-15193 boards that have the ZOE-M8Q, I was interested in the circuit for recharging the backup coin battery. On the GPS-15193 this battery is the Sieko ML414H. When I looked at the schematics, the first thing I noticed was that the text mentions the initial battery capacity is “800mAh” even though the ML414H is only a 1mAh part. The next problem is that the battery is connected to 3.3V even though it is only rated for 3.1V max. I checked with the product datasheet found at https://www.mouser.com/datasheet/2/360/ … 066937.pdf. According to this datasheet, connecting the battery as done in the schematics is “prohibited”.
This seems to be a problem and I woud appreciate a second set of eyes on it. Either the user cannot apply more that 3.1V to the board or the ML414H has to be replaced with MS421R.
These are great questions. As to your first point I’m not sure where the initial battery capacity figure of 800mAh figure came from. The most logical conclusion is that I mistyped it or made some sort of illogical mathematical error that landed upon this magical number? Especially because I list the initial battery capacity charge right above it (???). Nonetheless, it’s clear from the datasheet that it has a total capacity of 1mAh and that’s a figure I clearly remember. I’ll remove that erroneous bit of information from the schematic.
As to your second point. You’ll see that the the supply voltage, 3.3 volts, runs through a diode before hitting the battery. The diode serves two purposes, the first is to keep the battery from back feeding the main supply voltage and the second is to drop the voltage going into the battery. The diode drops 420mV, meaning that the battery is charging at roughly 2.9V. Of course, when no power is being supplied the battery provides whatever the capacity of the battery has been charged to at that time. So the battery won’t ever be providing 3.3V. The 3.3V net name just reflects what is typically provided through the Qwiic connectors.
The ZOE-M8Q can operate at a voltage as low as 1.71V which is actually below the battery’s fall off voltage of 2.0V.
I hope this helps, let me know if you have any more questions.
The schottky diode will forward conduct with a voltage far smaller than the Forward Voltage, albeit at less than the rated current. Check the IF = ƒ (VF) plots in the device datasheet. Then take one of your GPS-15193 boards, power it to 3.3V and leave it powered overnight. In the morning, put on some safety goggles and measure the voltage across the battery. It will be 3.29V. I know because I ran that experiment last night.