Is there a way to check the health of the Hybrid Battery?

there are many issues that can cause lower mpg's. cold or wet weather, and reformulated winter gas being the most common.
it is highly unlikely that a 2017 battery would be the cause. if a cell drops below the preset parameters, you will get the red triangle of death on the dash lights.
the only way to test it is with toyota tech stream software and a compatible laptop. there are many threads here on the purchase and use of mini vci.
but i would concentrate my efforts on easier more likely cuprits, such as tyres pressure, air and cabin filters, oil level, driving style and etc.

also, keep a tally of your manually computed mpg's at each fill up, for comparison.

 

There is a way, with Techstream. It's called "Hybrid Health Check"? Ask your service department. FWIW I did that a year or two back and they were kind of in the dark about it. But apparently exists, is offered in the United Kingdom, and there having it done extends your warranty, for the cost of the service.

Toyota Hybrid Health Check - Toyota

 

Before we start troubleshooting too far... can you define increased fuel consumption?

 

You can check the voltage of each individual cell using a Scangauge. The parameters for the Scangauge set up to do this are on the web try googling Prius Scangauge codes or similar. Most of the cell voltages can be read direct but with 2 cells you have to read high byte and low byte separately and then combine them. I have them set up as Custom Gauges on my Scangauge together with Inverter temperatures.

 

I can do that have scangauge and the spreadsheet with the custom xgauges. Good idea.

 

Still would like to know the specifics about the Hybrid Health Check done by UK dealers.
From the link...
"We investigate the status and condition of your Toyota “traction” battery."

 

I use TechStream to check the voltage and ohm resistance from time to time. The problem with High voltage battery packs is there isn't an aftermarket for "brand new" battery module replacements. Just by swapping out bad cell modules with used ones or installing a rebuilt pack that consists of used recondtioned battery cell modules is only a temporary solution. Before you know it, you will have to repeat the process all over again as its a real job. It only takes just one weak battery module to throw the entire pack out of balance. That can lead to the entire car come to a halt since the Hybrid Synergy Drive system is not fault tolerance.

Panasonic that supplies the battery cell modules to Toyota and Tesla doesn't sell to the open market as its a B2B relationship between the manufacturers. So if you plan on keeping a Hybrid car for long term past the warranty, you have no choice but get a new traction battery at the dealer as that's the only place to get a brand new one from. They don't swap out modules as they take your old core and swap the entire pack out.