MacBook long life battery

That computer is a battery-CAPABLE device. It is not optimized as a battery-operated device. (It is NOT an iPhone.)

Your computer performs best when connected to AC power. It can use the full output of the Power Adapter AND when doing especially challenging work will also freely "borrow" power from the battery. In some cases, even with the power adapter connected, the charged state may decline during stressful work.

When used only on battery, your computer has no extra cushion of power, and may perform more slowly. However, for ordinary non-stressful tasks this may not be objectionable (possibly not even noticeable.)

In general, you should ALWAYS connect AC power when it is possible to do so, and only run on batteries (which could be somewhat slower) when no AC sources are at hand. Your Mac will NEVER over-charge.

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Catalina software 10.15.5 and later for MacBook Pro with T2 chip (2018 models and later) includes a new feature called Battery Health Management. Based on your usage patterns, this widens the hysteresis to initiate a charge cycle at a lower level, and stop before 99 percent.

About battery health management in Mac notebooks - Apple Support

About battery health management in Mac laptops - Apple Support

This relaxes the set points around re-charging (based on your usage patterns) and can improve long term battery lifetimes. When active, recharging may stop short of 100 percent charged. Recharging may only begin at a lower level than the previous "normal" threshold of 92 percent or less.

NSalmee wrote:

but I feel on ac power, my laptop heats up?

Then it is doing a lot of work at full speed (possibly including charging if the battery has been run low). In that case it will get warm. The fans in a Mac are run at full power in the fail-safe condition. The computer measures internal temperatures several times a second and reduces fan speeds to optimal levels to provide the needed cooling.

if the internal temperatures approach 100 degrees C, the processor will initiate an emergency uncontrolled pow-down to save itself well before any damage occurs.

Your computer can be run at 60 degrees C (140 F) all day every day with no bad impact.

That's about 10 degrees C hotter than 'burn your skin' hot.