On desktop Macs, there is a very small long-life non-recharging coin cell battery that backs up the stored date&time for most convenient startup. On a MacBook Pro, there is already a battery available, so the main battery is used for this purpose.
It is presumed that you will NEVER run the main battery down so low that the parameters will be lost. Doing so could reduce battery longevity substantially.
STOP running the battery so low!
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.
