Are you looking for an adapter for car charging cables?

I bought an older, very compact netbook (Asus Eee PC) for mobile use, which still has a good battery, but may be used longer on the go and may need to be recharged.

For my Lenovo Thinkpad notebook I have a car charging cable with various interchangeable end plugs on the go, but unfortunately for that

not such a tiny one that fits into the beech of the Eee (2.5 mm outside - not measurable inside, 1 mm?)

Therefore I'm only looking for a suitable adapter that fits the widespread dimension of these barrel connectors (5.5 / 2 mm).

Is good advice expensive?

You have to search the internet, e.g. With Pollin, Conrad etc. But you also have to pay attention to the correct polarity, plus outside or plus inside.

Look here:

https://www.ebay.de/...1108989109