# Sense resistors in system-side Li-Ion charger/gauge circuit

I'm working on designing a charger and fuel gauge circuit for a premade 3S3P Li-Ion battery pack. The battery pack has basic protection circuits onborad (over/undervoltage and a PTC) and only the pack positive and negative leads are exposed to the outside. I'm planning on using a TI BQ24735 for the charger and a Maxim MAX17205 for the fuel gauge based on clear documentation showing connection to a pack without using the balance inputs.

Battery charger circuit:

Fuel gauge circuit:

I'm confused as to how the current sense resistors in these two circuits are going to interact. The charger current sense (RSR in the first circuit) is on the high side and then the fuel gauge RSENSE is on the low side. Both circuits measure voltage across their respective resistor so I'm not too worried about having to account for the values of both resistors in a given circuit. What I don't quite understand how a low-side current sense in the fuel gauge is going to understand that the charger is doing its thing in this configuration. Is that actually going to work, or do I need to go a different route with this?

Also, what do I need for the protection circuit mentioned in the fuel gauge schematic given what's already in the battery? I'm assuming just reverse polarity protection.

• Does the fuel gauge IC monitor both charging and discharging? – jonk Mar 25 at 2:36
• I believe so? It does some kind of algorithm that weighs coulomb counting and voltage measurements to correct for drift. It specifically mentions an end of charging cycle detection feature. I'm adding a datasheet link to the question. – Joe Baker Mar 25 at 3:13
• I think this subject is complex. Electric car companies (+ hybrid electric cars) have to keep track of a lot of stuff, I've been gathering lately (since I now own one.) Over time, the 100% battery level declines in terms of total Joules. They don't want them to decline below 20% (in my case), also, as the peak energy storage wear is greater/faster then. But on a charge to charge basis, it would seem that any error in charge vs discharge would accumulate as an uncalibrated offset and therefore an error in reported %, too.I imagine taking a year to work out details. Do these ICs handle all this? – jonk Mar 25 at 3:18
• Yes, the fuel gauge chip is supposed to handle all of that. You get that kind of drift if you use just a coulomb counter but this looks at voltage as well to compensate. – Joe Baker Mar 25 at 3:28
• @SunnyskyguyEE75 That's on the wall adapter though, not the battery pack. – Joe Baker Mar 25 at 4:40