HV battery drops quickly despite good gas mileage

I don't think this is unusual, if you are making frequent stops at traffic lights. It also depends on what accessories you have running, air conditioning especially draws down the HV battery quickly. Once you are on the highway, though, the HV State of Charge should stabilize between 4 and 6 bars, if everything is working properly.

If you don't have a load tester, the best way to measure the strength of the battery is to put a multimeter on it and test its voltage after it has been sitting overnight with the hood unlatched*. (You want to be able to measure the voltage without disturbing it by approaching the car with your smart key or by opening any of the doors). If it shows something like 12.6 V under these conditions, it is probably alright. To do a load test, you can see what voltage you get with the car in Accessory mode (2 presses of the Power button without your foot on the brake) when you turn the headlights on. You should expect the voltage to drop somewhere below 12 V, but after leaving the lights on for a minute or 2 and then turning them off, you should see it recover close to its previous value.

A weak 12V will negatively affect your MPG, but I would not expect to see it change the behavior of the SOC display, since each bar represents HV battery voltage that is probably an order of magnitude (i.e. 10 X) more than the difference between a strong and weak 12V battery.

*I'm talking about measuring the voltage of the 12V at the under hood jump points, of course.

 

It's not an absolute but that is an early sign of a weak battery.

Remove it and get it load tested (for free) at any major auto parts store that offers this service.

GOOD LUCK!