Toyota Estima VSC-Hybrid system check message during drive

[Jozzy]

Hi all. This is my first post.  I’m a happy owner of Toyota Previa / Estima 2.4L Hybrid E - Fours S 2007.  Start driving it around mid October 2023. After the 2 days of driving the - VSC check - hybrid system check - ( translated from Japanese) came up. Brought the car to the mechanic, he checked it & said hybrid Battery is dead. He knocked out, saying I’ll need a new hybrid Battery. Since I’m driving the car the issue came up around 5-6 time, but by time did observe the hybrid Battery is working much better the previously & the fuel consumption is going down. At the very beginning the fuel consumption was 12km / Liter average speed 100km / h & now is 13.6 km / Liter same, average speed 100km / h .( some time is going over 13.9km/L , 14.7kn/L). Car been sitting at least for 5 months. Do you believe/ think the error on the hybrid side is because the car was sitting / wasn’t on the road for at least 5months and the hybrid battery is drained/ completely dead & by driving the car now will improve the hybrid battery performance? Or do I have yo but a brand new hybrid battery ? Does anybody have any experience ? Any suggestion is welcome.

Thank you in advance. 
Joseph

[Jozzy]