# Requesting the maximum available power from source

I’m trying to design my own battery pack. I want to use usb-c as one of the inputs. The goal is to draw as much power from the usb-c port as the source supports and to feed that into a boost converter to charge the cells.

I’m thinking about using the STUSB4500 (datasheet). The IC has 3 PDOs, PDO1 is fixed to 5v, PDO2 and 3 can be set to any voltage. A minimum required current can also be set for each PDO. Upon plugging into a charger, the IC will check whether any of the set voltages match the available voltages of the source, highest priority to PDO3, lowest to PDO1. If, for a common voltage, the source’s available current is greater or equal to the set current, it will agree on that voltage and allow power output. There’s also a parameter you can set over i2c REQ_SRC_CURRENT. When set to 1, the IC will automatically select the highest available current, instead of the one set in the PDO.

So I was thinking, maybe I could enable this parameter and set the PDOs like this:

• PDO3: 20v, 0.5A
• PDO2: 12v, 0.5A
• PDO1: 5v, 0.5A

Every charger will be able to output at least 0.5A, so by doing this, the IC will pick the highest voltage and then set the current to the highest available. I think I could then get this maximum current using i2c, and limit the cell charging current to not exceed this.

Do you think this is a viable solution? One problem I’m thinking of is that if the charger would output for example 15v, this wouldn’t be a match and they would settle on 5v. Because there’s a limited amount of PDOs, I can’t set one for every common voltage.

• Why a boost? all cell chemistries I can think of have a cell voltage < 20 V, so if anything, you'd need a buck converter. By the way, that'd solve your 15V issue: simply always ask for the higher voltage and use your buck converter to go down to 15V. Jan 2 '20 at 13:21
• I’m planning to use 8 li-ion cells in series, discharging to 3v and charging to 4.1v (pack voltage of 24-32.8v). I’m using this many cells because I want a 19.5v output (using buck converter) to charge my laptop. Jan 2 '20 at 13:30
• Hm, wouldn't a natural approach to that being charging in parallel, discharging in series? Jan 2 '20 at 13:32
• Also, the 15v was just an example. Ideally I want it to pick the highest voltage, whether that’s 5v, 15v, 18.2v, ... to maximise charging speed. But I also want it to accept a 5v input if that’s the only thing available, for example when plugging it into a normal 5v usb charger using a usb-a to usb-c lead. Jan 2 '20 at 13:36
• That would have the disadvantage of not being able to charge and discharge at the same time, and I think the circuitry for switching 8 cells between series and parallel could get quite complicated (floating gate drivers, ...) and possibly expensive. Jan 2 '20 at 13:42