I know that it's usually perfectly possible to connect two grounds, and that voltage is measured against it's ground, but this got me wondering about batteries. I'm assuming somewhere in my train of thoughts I'm missing something, but I'm having difficulty figuring out where. So, let's go with an example of what I think should happen.
- If I plug a battery (say my laptop) in at home, charge it and unplug, from what I get the ground of the battery remains at the level at which the ground in my house was.
- I take my laptop to work, whole different city where the ground is probably at a different level from at home. I would guess that this difference would lay anywhere between -10 and 10 volt, but might be enough to fry a laptop. I don't really know.
- I plug in my laptop. The moment I plug it in, the ground in my battery equalizes with the ground at the office. This means that a voltage of about 10V shortly gets short-circuited, and my laptop is busted.
I know this doesn't happen, so I clearly got something wrong, but what exactly? And if moving a battery from one ground to the other isn't a risk, then why is connecting two grounds ever a risk?