Optimisation to speed up first pass levelling, fix bug in low power charge mode

What's Changed

• Update car-charge-planning.md by @springfall2008 in #642
• Optimisation to speed up first pass levelling by @springfall2008 in #651
• Fix issue with charge rate being reset when it shouldn't be in low power mode by @springfall2008 in #652

Full Changelog: v7.15.1...v7.15.2