I'm in the same boat - I have a Volvo PHEV with an 11.6kWh battery, and Volvo upgraded that battery in newer models to 18.8kWh. And the thing is....the battery takes the exact same space as before - it was just an improvement in battery packaging which allowed them to do this. But....I can't fit this newer battery to my car, even though it would physically fit - the systems inside the car will not allow it to work. If there was a company that offered this as a paid upgrade for my car I'd gladly pay for it.