Fix #145 charging logics

[JakobWeg]

Fixes #145

Changes proposed in this pull request:

• Including efficiency eta_cp

The following steps were realized (required):

• [X ] Correct formatting with black src
• [X ] Add the description of your changes to the CHANGELOG.md in subsection with release name [Unreleased] and state the PR number with #

Also the following steps were realized if applicable:

• Write docstrings to your code
• [X ] Write test(s) for new patch of code