Battery Health vs Usage Time

Planning on buying a MBP soon...

I've read a bit around here about battery health. I was wondering how the health of the battery relates to the amount of time the battery will work for.

Is it simple arithmetic?
Max battery usage is 4 hours (just picking a number) so if your battery health is at 75% will it last for 3 hours and if at 50% it will last for two hours?

It seems that people get very concerned that their battery life dropped to 92-96% after 6-12 months, so that leads me to believe that the drop off in usage time may be more drastic. I would assume that after a year a battery lasting for over 90% of its initial full amount (in my example 92% of 4 hours would be 3 hours and 41 minutes) is still very good. This makes me wonder if a 90% battery health will only get you 50% of the original charge.

Also when do people typically buy a new battery? When battery health gets under 90%, 75%, 50%???

Thanks for the help. I'm just trying to get an idea of what I can expect when I get the MBP. I don't think it is unreasonable to have to spend $130 for a new battery after 18-24 months of use if I want to match the original full charge usage time. I'm wondering if people here start freaking out once their battery health goes under 98-100% despite the fact that it is a pretty minor deal in terms of how it will affect the real world performance.

Thanks again.

 

It did, thanks. While not an exact formula it does sound like a 10% loss in battery health will cost you 10% of usage time (far from a tragedy after 6-12 months).

Also when people say they've had 100 cycles, how is that measured? Is that anytime you unplug the laptop and then it recharges to full, or is that only for a full discharge and charge?

If you unplug your laptop every day and use it for 30 mins and then recharge it to full, after a year would you have 365 cycles?

Thanks.