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I'm designing a mobile device that uses a Li-Ion battery. It is required to indicate to the user the State-of-Charge of the battery, similar to how it's done in smartphones.

At first I considered simply measuring the voltage of the battery. It doesn't decrease linearly with the SoC, so I figured I'd use a look-up-table in order to convert the voltage to the correct SoC. This approach obviously fails when the device is being charged: an external voltage is forced on the battery, and so there's no point in measuring its voltage. After reading several articles on-line, I figured that Coloumb counting has to be the way they do it in smartphones, since charging doesn't interfere with it, and it doesn't require sensing of internal chemical properties of the battery.

My question is: how is it usually realized in smartphones? Do they use a serial current sensing resistor, whose voltage is amplified and read by an A/D? Does the smartphone processor do the integration, calibration and other calculation, or is there a specialized IC for that purpose? If there was, I'd love to use it!

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  • \$\begingroup\$ Depending on your budget / volumes, you might want to look into smart batteries packs, which do all this stuff internally and just provide a serial bus for you to query the charge state (and lots of other info). Like a laptop battery, really. \$\endgroup\$ – user1844 Apr 9 '14 at 8:52
  • \$\begingroup\$ My application is very size-constrained. I guess that these packs don't come in very small packages? \$\endgroup\$ – OBU Apr 9 '14 at 9:05
  • \$\begingroup\$ There are smart LiPo batteries for flying models, so I'd guess they go down pretty small/light, but I'm not sure what the intersection is between hobbiest smart packs and smart packs with documented serial interfaces for reading charge state. Depending on your volumes that might be worth looking at \$\endgroup\$ – user1844 Apr 9 '14 at 11:11
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To start with the smartphone question: Charge management, as well as almost all other power related functionality like dc/dc conversion for the SoC, display and power switching is done with usually a single monolithic integrated circuit, colloquially known as the PMIC (power management IC). These are seldomly if ever available in low quantities, extremely specialized and optimized for low footprint area. This is not a good option for your design question.

However, charge management ICs exist in the free market that do coulomb counting. Try looking at TI's bq series chips; these range from just simple coulomb counting pulse-output chips to fully integrated charge managers with the craziest functionality. They are the go-to design choice for almost every integrated battery solution for portable electronics - with Maxim's charge management chips taking care of most of the rest of the market.

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