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I made a car tracker (a GPS/LTE module connected to an arduino that communicate to my web server) to log gps positionning data. I’m planning to use this to track my new car in case it gets stolen.

My initial thought would be to hot-wire the tracker to the car’s 12V battery followed by a DC-DC converter to 5V.

For redundancy I was planning to also power the tracker from an 18650 battery in case the thief disconnect the car’s battery.

My issue is that I’d like to use the car’s battery as the primary source of power, then the integrated 18650 battery. Finally the car’s battery would be used to charge the 18650 when needed.

The tracker would be low power usage (eg sending location every 10min, in sleep mode other wise).

What would be the best way of powering such system without draining the car’s battery with multiple DC converters ?

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  • \$\begingroup\$ How are you thinking of connecting these together? What you describe seems odd... I would use a dc to dc converter to power the device from the car battery. \$\endgroup\$ – Solar Mike Dec 25 '18 at 20:26
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    \$\begingroup\$ Can you run the GPS/LTE module at raw Li-Ion (3.8-4.2V) battery voltage, and the Arduino (AVR) at 3.3V? That would make this design somewhat easier to work with... \$\endgroup\$ – ThreePhaseEel Dec 25 '18 at 20:33
  • \$\begingroup\$ It seems so. The GPS/LTE module I use is based on the SIM7000 chip (Supply voltage range: 3.0V~ 4.3V, Typ: 3.8V). The GPS antenna may need a bit more to provide correct satellite reception though ? \$\endgroup\$ – Fredovsky Dec 25 '18 at 20:43
  • \$\begingroup\$ @Fredovsky -- what do the specs on the antenna say? \$\endgroup\$ – ThreePhaseEel Dec 25 '18 at 20:51
  • \$\begingroup\$ @ThreePhaseEel I just checked and the antenna accepts 2.7V-5V, so indeed I'm not restricted to 5V. Thanks I'm getting on the right tracks already. \$\endgroup\$ – Fredovsky Dec 25 '18 at 21:09
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You need to solve two problems here: battery charging and undervoltage lockout

The good news is that the voltage range of your parts (the Arduino's AVR microcontroller and the SIM7000 module) is wide enough that once we step the voltage down for your 18650, we do not need to mess with it any further -- they will run just as well on 4.2V from a fully charged cell as they will from the 3.something volts from an empty one. However, we will still want an undervoltage/brownout protector to keep the cell's protection circuit from having to kick in on a regular basis, and we will also need a suitable charge controller that can take car "12V" and charge the battery.

While Li-Ion charging facilities are common, most of them run off of 5V, not directly off of the variable "12V" automotive supply voltage, and so it goes for the SIM7000 module's charger, rendering it not useful for our application. As a result of this, we need to look further afield for a battery charger IC, and Linear Technology (now part of Analog Devices) comes through with the LTC4002-4.2.

At under $4 in unit quantities, it is not particularly expensive, and with the addition of a P-channel MOSFET, two Schottky diodes, and a handful of passives, it provides a complete step-down switching battery charging solution. It also comes in an easy-to-solder SO-8 package, and is well-documented, with detailed design procedures and simulation models (using LTspice, of course). Paired with a low-loss PMOS in SO-8 or IPAK, a couple of 20V/3A Schottky diodes, and a suitable inductor with at least 2A of current handling capability, it will be able to both charge the battery and handle the current draw spikes of the SIM7000 module.

This leaves undervoltage protection, and this is easy -- just about any reset supervisor IC with a threshold in the 3.3-3.6V range can be used, as holding both the AVR and the SIM7000 in reset will put them into a low-power-draw state. Optionally, the supervisor can also provide manual-reset and/or watchdog timer functionality as well, as the watchdog may need to reset both the AVR and the SIM7000 to clear a hung condition.

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  • \$\begingroup\$ I really appreciate your input. Can you confirm that the PMOS would be connected to the GATE pin of the battery charging chip, and its function is to select power from the car's battery (clamped to 8V below according to the datasheet (=4V)), or, if the car's battery isn't available, to provide power from the Li-ion battery ? \$\endgroup\$ – Fredovsky Dec 25 '18 at 23:27
  • \$\begingroup\$ @Fredovsky you have the right idea with how the PMOS is wired, but it's actually doing something more sophisticated than what you think. The LTC4002 is actually stepping down the voltage from 12V to 4.2V using a buck converter topology, switching the PMOS on and off to regulate the charge voltage going to the Li-Ion cell (and thus, to the rest of the circuit). \$\endgroup\$ – ThreePhaseEel Dec 25 '18 at 23:29
  • \$\begingroup\$ I see, so when the Li-Ion cell is fully charged after a charging cycle, the rest of the circuit will be draining power from the Li-Ion until its charge drops below a limit at which point the LTC4002 kicks in again and tops-up the Li-Ion? My concern is that we'd be constantly draining/charging the Li-Ion (maybe causing premature wear), where we could use the 12V car battery (if available) instead, saving Li-Ion charge cycles. \$\endgroup\$ – Fredovsky Dec 25 '18 at 23:52
  • \$\begingroup\$ @Fredovsky -- the Li-Ion will be sitting at its 4.2V float voltage, sipping power to keep it from self-discharging, while the rest of the circuit runs. It's slightly naughty to do this with a Li-Ion (float charging is more of a lead-acid thing) but not as bad as one thinks, AIUI \$\endgroup\$ – ThreePhaseEel Dec 25 '18 at 23:57
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After being put on the right track by @ThreePhaseEel, I found pretty much a chip dedicated to exactly my application (except it isn't coming in an easy to solder by hand package, but not impossible) : Analog LTC4091.

The LTC®4091 is a 36V Li-Ion battery charger and power backup manager. The integrated step-down switching regulator charges a battery from a primary power source while providing power to the load. If primary power is lost, the load is seamlessly transitioned to the backup Li-ion/ polymer battery.

Applications :

  • Fleet and Asset Tracking
  • Automotive GPS Data Loggers
  • Automotive Telematics Systems
  • Battery Backup Systems

For those looking for a similar application, there are dedicated IC that deal with battery charging called Battery Manager IC.

The feature switching from main power supply to the battery supply if power is removed (ie. using the battery as backup power), is generally called Power Path Management.

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