Charging iPhone from MacBook decreasing macbook's battery health so fast

Hi YousufKalim,

Thanks for contacting Apple Support Communities!

We understand that your MacBook battery health has decreased.

Your MacBook battery is a consumable part which will eventually need to be replaced. It works on charging cycles. Check out Determine battery cycle count for Mac notebooks which explains how your battery works in detail.

About battery cycles

When you use your Mac notebook, its battery goes through charge cycles. A charge cycle happens when you use all of the battery’s power—but that doesn’t necessarily mean in a single charge.

For example, you could use half of your notebook's charge in one day, and then recharge it fully. If you did the same thing the next day, it would count as one charge cycle, not two. In this example, it might take several days to complete a cycle.

Batteries have a limited amount of charge cycles before their performance is expected to diminish. Once the cycle count is reached, a replacement battery is recommended to maintain performance. You can use your battery after it reaches its maximum cycle count, but you might notice a reduction in your battery life. 

Knowing how many charge cycles your battery has and how many are left can help you determine when a battery replacement is required. Your battery is designed to retain up to 80% of its original charge capacity at its maximum cycle count. For best performance, replace your battery when you reach its maximum cycle count. 

This is also a great resource which will help you to optimize your MacBook battery: Get help with your Mac notebook battery

Optimize your battery life

Your notebook battery life depends on your computer's configuration and how you're using the computer. Here are some settings and steps that you can take to get the most from your computer’s battery.

We hope this helps.

Take care!