Fix manual-charge-energy reading

[torbennehmer]

System Health details

./.

Checklist

• I have enabled debug logging for my installation.
• I have filled out the issue template to the best of my ability.
• This issue only contains 1 issue (if you have multiple issues, open one issue for each issue).
• This issue is not a duplicate issue of currently previous issues..

Describe the issue

The reading manual-charge-energy seems to work in strange ways. It is given in kAh using the individual cell's voltage as a baseline, but somehow counted across all cells.

This is a f'up on #40.

Reproduction steps

Charge 100 Wh
Afterwards, the reading is approx. 0,027 at a indvidual cell voltage of approx. 3,65. Multiplying it, you get back to the 0,1 kWh.

Debug logs

Taken with the 100Wh sample
...
      "manual-charge-energy": 0.027804025694444467,
...
            "voltages": [
              3.65,
...

Diagnostics dump

./.