The question is: how would I modify the below circuit to make it work with an 18650 3.7V 2500mah battery which I want to charge at 1C during constant current phase? Note, 1C should be fine as datasheet says battery can be charged in rapid-charge mode with 4A, 4.2V.

I want to build a battery charging circuit with an Arduino Uno, IRF510 mosfet, temperature sensor, and some resistors, based on this article. The original setup is to charge a AA battery. Here is the design, the battery goes between 5V and analog pin 1 reader:


The cool thing is you can use the Arduino to measure current by measuring the voltage on both sides of a resistor (R2 in the schematic). Depending on the battery voltage, you then adjust the current up to some maximum.However, I want to charge an 18650 battery, which should be charged to 4.2V, not 1.6V. The problem is the resistor would drop the voltage from 5v to below 4.2V, so it won't be able to charge the battery. Does it make sense to add a step up voltage regulator somewhere in the circuit to circumvent the issue?

What should I add to the circuit to make this work? Sorry, I'm an electrical noob. Note, in terms of safety, I will follow the multi-phase charging profile as specified by the battery manufacturer (i.e. trickle, pre-charge, constant current, constant voltage, cutoff at max time or min current or max temperature).

  • \$\begingroup\$ Hi @Vega, I think what you want to do will be easy and we'll be able to help you, but you're going to want to re-draw your schematic in a much more human-readable form., Right now it looks a lot like a rats nest and makes it more complicated to understand. \$\endgroup\$ – Jim Feb 20 '18 at 18:48
  • \$\begingroup\$ You can remove a lot of clutter by using ground symbols at each component connected to ground. Make sure they're the right way up. \$\endgroup\$ – Transistor Feb 20 '18 at 18:53
  • \$\begingroup\$ @Jim, I swapped the image for the simplified schematic. Not shown, but battery goes between 5V and the first voltage reader (analog pin 1) \$\endgroup\$ – vega Feb 20 '18 at 19:14
  • \$\begingroup\$ in your schematics the Pin1 is shorted to 5V supply. The battery, when connected as you described, will be shorted. \$\endgroup\$ – Ale..chenski Feb 20 '18 at 20:52
  • \$\begingroup\$ A 3.7V battery probably is lithium based. Lithium is more critic than nickel. The full-charge sensing alghorithm is more difficult. I'm not a noob, I'm an experienced hobbyst, but, if I would build a circuit where an Arduino would charge a lithium batt, I would use Arduino to control a relay, and the relay would feed a dedicated chemistry specific charger module. \$\endgroup\$ – mguima Feb 21 '18 at 23:35

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