My mac book pro shuts down when there is plenty of battry left usaly over 50

Maybe your 'gas gauge' is not working very well any more.

for recent Macs like yours, the guidance Apple has supplied for such problems is to power off your Mac, charge it all night long, and in the morning do an SMC reset.

Reset the SMC of your Mac - Apple Support

In general, you should ALWAYS provide AC power when it is reasonably available and use battery ONLY when no other power source can be obtained. If power is supplied, your Mac will operate on the supplied AC power and be faster and more responsive. It will only charge when needed. Your Mac will NEVER over-charge.

Batteries are a wear item, like the soles of your shoes and the tires on your car. Their expected lifetime depends on use (as reflected in Charge cycles) and also in the passage of time, used or not. After a while, you should expect to have to replace the battery.

Battery life also depends on two factors you may be able to control a bit better. Lithium polymer batteries deteriorate when they get too hot, and when they are subject to very deep discharge. These should be avoided when possible.

Despite using these batteries in millions of Apple devices, there is still a lot of of randomness in exactly which battery will fail when.

Apple SUGGESTS (but does not Warrant) that your battery MAY last as long as 1000 charge cycles, provided all other factors are well-controlled.

Apple uses the criterion that a battery that doesn't to hold 80 percent of its original charge capacity should be serviced, and likely replaced.

When designing systems, that 80 percent rule is intended so that, over a Very large sample of devices, almost all will be able to achieve their 'mission' -- still have at least 80 percent capacity at the three-year mark.

When your battery can no longer can perform as expected, you will need to replace it. Batteries wear out. Their levels decrease with time.

Batteries - Maximizing Performance - Apple