As you are travelling on Lufthansa, Google's answer for "lufthansa battery policy" points to:
Portable electronic devices with lithium
Description: portable electronic devices containing lithium-metal or
lithium-ion cells or batteries, e.g. watches, calculators, cameras,
mobile phones, laptop computers, camcorders, etc., when these are
intended for personal use.
- Checked baggage: Yes
- Carry-on baggage: Yes
- Carrier’s approval required: No
Rules for spare batteries:
No mention of a requirement to turn on the laptop. But don't forget to either wrap each space battery individually, or add a bit of tape on the contact to avoid any unwanted short-circuit.
Nowadays, the problem with batteries is more that they tend to burn/explode than the risk of someone placing explosives instead (I believe this would be detected by current generation luggage scanners).
Note also that:
- even though you may encounter random secondary security checks at the gate for some flights (including to the US), security checks are usually common to all destinations
- most airlines apply the IATA rules about Lithium batteries
- in my experience, I've never been asked to turn on a laptop in at least 10 years
- I've carried quite a lot of spare batteries in hand luggage, and nobody ever raised an eyebrow