Overview

The power/charging system will provide 5V and the raw battery voltage to the other subsystems of the design (pi, display, and input). The rationale behind providing both of these voltages is that 5V will be the main power rail for the pi, external devices, and (likely) the input system, and the raw battery voltage will be used for boosting up or dropping down the voltage with greater efficiency. The raw battery voltage will be used for the step-up converter for the backlight , and will be locally regulated for (what will probably be) an FPGA to control the lcd.

The power into the charging circuit will be provided by a micro-USB cable. There will also be a USB-A port which can provide power to external devices.

Design Log

The most concrete development so far is the choice of battery for the design. I have chosen this battery for the device. It's dimensions seem fairly ideal for the design. It will be able to fit underneath the pcb and beside the pi, while providing some room for taller components if needed, and it stores quite a bit of power to boot! There are a couple options for the termination of the leads, but bare wires simplify the BOM, are cheaper, smaller, and can be replaced by a battery with a JST connector later if necessary.

Last night I spent quite a some time looking for battery charging IC's. Dave Jones has a video on li-po charging if you're interested. The battery I've chosen has a maximum charge rate of 2.5 A. Initially I was looking for a charger which would vary the current output based on the output of the load of the rest of the circuit. Most chargers, it seems, have the battery and load in parallel. If the charger is set to charge at 1.5 A in constant-current mode, and the load draws 1 A of current, the battery is only being charged at 0.5 A. Most, if not all, of the IC's with this feature are not hand-solderable (leadless or BGA packaged). I realized this morning that because the maximum charge current is so high (2.5 A), there are very few cases where the battery would discharge while a USB charging cable is attached.

I have not found an IC yet, but now I know what I want, and sort-of what's available. It should charge at least at 1.5 A, be a switching type (to not produce as much heat as a linear regulator), work with a single-celled battery, and be fairly integrated (not too many peripheral components). I feel like it shouldn't be too long until I've found a good match.

A bq2057 seems like it would work if I was willing to use a linear device rather than a switching device.