Couple of questions about designing an NiMH battery charger

[Marmotta]

I have a Neo Geo Pocket that I wanted to modify with a USB-C port and NiMH battery charger using a TI BQ25172. It uses two AA batteries in series and I wanted to be able to charge it while using it at the same time, but this is the first time I've tried to make a NiMH charger, so have a few questions if anyone can help out? It would be great appreciated

- I'm assuming first and foremost that it's suitable for the job - the datasheet mentions "when the system load is placed in parallel
with the battery, the charge current is shared between the system and the battery" so I'm assuming I can just connect my circuit to the AA batteries inside the console and it will work as desired? I don't really intend on playing it much while it's charging, but it would be nice to have.

- What is VREF in the datasheet connected to? There's no mention about what value it's supposed to be other than it being shown in example circuits and that it's a pullup line. I'm assuming it's an input for a refence voltage from the system? The Neo Geo does have a 4V reference voltage which seems to be pretty much constantly spot on 4V during use which I could use?

- I'm pretty unsure about the values of the capacitors I've used and most of them are based on semi-educated guesses. I have a 375 ohm resistor connected to ISET, as this defines the current charge value at the max 800mA, which I believe should be fine? (assuming two 2000mAh Eneloop batteries) I then have a 27K resistor connected to TMR, which would equal a safety charge time of 6 hours - I believe more than enough to charge the two batteries with 800mA, but would it affect anything if I increased the time? Finally, I've chosen a 27K resistor on VSET to select 2-cell + intermittent charge.

I've attached an image of my schematic so far and a link to the BQ25172 datasheet below.



https://www.ti.com/lit/ds/symlink/bq25172.pdf?ts=1706058046887&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FBQ25172