I'm using two of the following rechargeable batteries http://www.renata.com/fileadmin/downloads/productsheets/lithium_polymer/ICP501233PA.pdf in a parallel configuration to power up a small development board that reads data from different sensors and saves it on an sd card.
The device works for around 10 hours and then turns off (battery drained). Recharged for a couple of hours and again measurement.
I've measured the voltage (on the board, so with both batteries connected in parallel) and usually it shows 4.1V (fully charged) and above 3V after discharge. Still, I've also noticed that some of the batteries show voltages between 1.7V and 2.1V after working ok for 10 or more hours (also measured in parallel).
I assumed that the protection circuit is defect and tried to replace the batteries.
My issue is that if I measure the voltage on the disconnected battery, the value is again 4.x V. (measured together in parallel OR each separately)
Can anyone explain me this ?
Are the batteries broken or the protection circuit is broken or it just needs a hard reset ?
Note 1: The batteries are soldered on the board and never disconnected.
Note 2: The batteries have already more than 300 charge/discharge cycles.
(Pretty specific question but I need to understand what's happening)
Note 3: The firmware installed on the boards on which I've measured the unusually low voltages doesn't start anymore. I've disconnected the battery and soldered back, the voltage is again 4.0x volts and everything seems fine. Having several boards in use, I am still concerned that it will fail again.