Rally1 hybrid cars (2022-)

[WRCStan]

thanks for explain .I really misunderstand the kW and the kWh. if it's 80% of the total capacity always .then the really capacity is 3.12kWh right?

The battery capability goes down with energy use and back up with energy regeneration/recovery through braking.
Energy is measured in joules
Power is a rate of flow measured in Watts, 1 watt = 1 joule/second
(1kw = 1000w, 1kj = 1000j)
Capacity is useful when using time instead of maths with joules - so Kilowatt hours. As in, that battery will offer *kw for 1 hour at *kwh.

This battery:
3.9kwh rated capacity
3900wh
3900j/s for an hour
234,000j/s for a minute
14,040,000j/s for a second
14,040kj energy when full or at capacity

FIA rule of Launch Boost limit of 1000kj is known
100KW rated max power consumption of the propulsion motor
100kj/s
1000kj / 100kj/s = max 10 seconds (as seen noted in various places)
1000kj = max 7.1% battery capacity used at launch

I see some artical said the driver can make the regen to let the battery up to 80% during the road section .so that it can be ready for next stage.but if there have not enough length road section or not enough time for regen to back to 80%.what could the driver do

It's part of the fun. No juice, no boost. Have you got a source for that 80%? Has anybody seen the start line state of charge supposedly in the manufacturer's user guide?

and for the boost time. the 3.9kWh can boost for 2.5 minutes with 100kw .so if there have the long long straight during the stage.will the driver make the boost as long as possible or they will touch the brake to end boost to save the energy

There are limits set for each stage by the FIA for how much energy can be used in one boost before having to regen, the regen is also a minimum FIA set limit to recover. We don't know what they are yet to have a meaningful answer.

will the team reduce the each boost power less than 100kw(90kw 80kw ) for the long stage like 40km with more long straight stage in Finland

No. Energy will be recovered through braking. Have a mooch at this video https://www.youtube.com/watch?v=yhGDYDkrb8M. The torque maps and how the electric motor integrates with the engine is beyond me.