EV Charging dropped from 33 to 27 miles

It is fairly linear, perhaps except at 99–100%. I was able to get up to 1.0 mi before it dropped to 99%, especially if the battery was freshly charged.

Also note that the HVAC is detrimental on the EV range, as well as going uphill, driving or accelerating fast, cold temperatures, humidity, rain, etc.

Set the HVAC at 68 ℉ for colder ambient, 74 ℉ for warmer ambient, and the same as the ambient for anything in between.

 

I have never plotted hard numbers but I don't think you can expect a linear GOM EV range vs SOC if you are driving the route I usually drive the way I drive. Yes, on a flat road at a constant speed, the GOM EV range vs SOC would be fairly linear, but on my routine route, I have uphill where the SOC decreases precipitously. Then down hill where the SOC stays the same or even increases. Also, the speed changes from below 15mph to above 50mph during the trip based on the segment speed limit.

I can try to get some numbers recorded next time.

 

Example: I start with battery fully charged and the estimated ev mileage (GOM, I guess you call it) is, say, 40mi. I start out with Trip A meter at zero miles. After I have driven 10miles as indicated on Trip A meter, the GOM ideally would then show 30miles left. In the past, this was approximately what happened assuming no use of AC or heating and normal weather conditions. Lately, under such a trip, I am often showing, when I have travelled 10 miles shown on the Trip A meter that the GOM is down to around 25 or 26 miles instead of 30 miles. So, for some reason, I have lost some of the original estimated mileage, which hadn't usually happened before.

 

Typical lithium-ion battery loses 20% of capacity after 800 charge–discharge cycles. If you are driving only in the EV mode, this would translate into 800 × 30 mi = 24,000 mi. So, only in about two and a half years, you would experience a 20% capacity/range loss. The useful life for an EV battery is said to end at 30% capacity loss, which is about 36,000 mi of EV driving for Prius Prime. Prius Prime is certainly not meant to be driven solely in the EV mode.

Other than that, cold temperatures will temporarily reduce the battery capacity. HVAC usage also reduces the range.

 

It is 800 full cycles but the number of cycles has been steadily increasing.
The car is not using 100% of the capacity though.
In reality batteries last a lot longer if the BMS is good.
As an example, when Toyota started with the Prius the average NiMH cell had 200 to 300 cycles but by using a good BMS and only using a smaller part of the capacity they managed to have them work insanely long. For the NiMH cells they only use the part between 30 and 80% SoC.

 

That sound like the GOM is just not predicting accurate estimation. If your driving habits (route and way you drive) and environmental factors such as temperature, road condition, winds, etc. have not changed from the past, then it may be due to changes in the traction battery capacity. But it is more likely the combination of changes in driving habits and external conditions. In any case, I would not trust the EV range remaining displayed on the dash. As I have shown before, my 2020 PP had the highest GOM EV range of 48.6miles. during the middle of winter when the actual EV range was less than 20 miles.

 

If you are worried about battery health, the only thing that matters is the actual EV range you get, not what the 'distance to empty' display says.