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I have an STM32 based vehicle tracking circuit with an SIM808. I want to use it in vehicle with an LM2596 regulator and a 3.7V li-ion backup battery.

I searched and found out that there are two topologies as attached picture. I need to know which topology is better according to li-ion li-polymer cell internals and increase its lifetime.

Question 2: What is the recommended way to implement that topology?

Our load must continue its working during power switch without interruption.

enter image description here

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  • \$\begingroup\$ You'll need to be more specific about the connections, such as, is the circuit attached to a live (continuous supply) car battery connection ? or is the external input power cut-off when the car keys are removed/switched-off ? If it is a continuous +12V car battery connection why even use a battery back-up ? Is it for redundancy in case the car battery is ever disconnected ? \$\endgroup\$
    – citizen
    Jun 21 at 11:40
  • \$\begingroup\$ Note to readers: For anyone interested in the subject, the original version of the diagram in the question came from "Texas Instruments Application Report SLUA376 - Implementations of Battery Charger and Power-Path Management System Using bq2410x/11x/12x (bqSWITCHER)", which has more discussion about power-path management. \$\endgroup\$
    – SamGibson
    Jun 21 at 13:42
  • \$\begingroup\$ @citizen The main power supply is car's battery , the backup battery is for main power cut-off for any reason. I want this device continue sending position even with disconnected main power. I also want to include a car battery supervisor to cut-off the main power when car's battery is low. \$\endgroup\$
    – Behdad
    Jun 21 at 14:53
  • \$\begingroup\$ @Behdad Sounds like its starting to get complicated ... I suggest reading up on it in the TI (or other) application notes as already noted. Are you sure there aren't already such devices readily available ? For instance the such as the "Power Magic Pro" car battery supervisor... \$\endgroup\$
    – citizen
    Jun 21 at 15:02
  • \$\begingroup\$ @citizen Thank you, my base is mainly software and I'd see its better to hear experienced designers opinion and recommendations. Maybe there is a ready to go pattern or solution in market like PMIC with esd protections for vehicles. \$\endgroup\$
    – Behdad
    Jun 21 at 15:13
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In brief, first topology is simpler but not recommended because the Li-Pol charger will never know when charging is ever terminated since there is a continual current drain (Isys) and Li-Pol chargers usually detect charge termination by the drawn current level of the charging battery (although if the normal current drain of the end equipment is very small then this may be ok in some applications). Second topology is preferred since it "multiplexes" and essentially separates the charging side, from the power supplying side, assuming the +12V is continuously available. If it is not, then the 3.7V battery would take over supplying the end-equipment... (I assume this is you requirement overall ?!)

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