# Mysterious Trace in Battery Fuel Gauge Datasheet

I'm using a lipo battery in a project, and I want to include a sophisticated battery fuel gauge in my design. I have a battery input stage with a TI BQ24072 charge management IC and a linear down-regulator to 3.3v.

For the fuel gauge, I've chosen the TI BQ27421-g1 as I've had good luck with their battery charging IC. The typical application circuit is shown below:

I'm a little confused about the function of the SRX pin. The datasheet only has about 2 sentences about it. To make things more confusing, this diagram on page 20 says that it should be shorted to the BAT pin (sorry if the pin names are tough to read):

Here are my questions:

1. Am I mis-reading the diagram on page 20?
2. If I am reading the diagram on page 20 correctly, then how does the coulomb counter even work?
3. If I don't understand the diagram on page 20, then how should the SRX pin be hooked up?
• That has to be the world's most confusing datasheet. Look at 10.2.2.3, which discusses the selection of an external sense resistor (unnecessary) connected to two pins that don't exist. I think the layout picture's completely misleading and you should not connect those pins. Just work from all the other schematics, etc. in the DS. – user1844 Nov 3 '15 at 18:58
• @WillDean I'm glad I'm not the only one confused by this thing. Usually TI does better than this with their datasheets. If this chip wasn't so nice and small, I would probably jump ship and get a different one. – johnny_boy Nov 3 '15 at 19:00
• @WillDean I would go off the schematic in 10.2, but that doesn't really make sense either. – johnny_boy Nov 3 '15 at 19:01
• Fuel gauge: Ah it isn't just me: en.wikipedia.org/wiki/Fuel_gauge - is it a UK only confusion? – Andy aka Nov 3 '15 at 19:20
• It might be worth asking in the appropriate forum at the TI E2E Community. IME, TI can be very good about providing support on their forums. – Tut Nov 3 '15 at 21:08

The resistance inside the chip between SRX and BAT is typically 7m$\Omega$ which produces a voltage for the ADC to digitise when battery current flows in or out. You do not add any extra resistance externally between these pins, or you will upset the calibration.