Okay, thanks. I didn't realize I couldn't do that. I was investigating a short in my system (not a car), and I thought I'd disconnect everything and work my way up from the battery to see where it was. I tried this test on another 12v battery from a cordless drill and it should infinit resistance.
What's happening when I'm measuring the resistance across terminals?
(Disclaimer: Newbie here).
From what I understand, a multimeter will have to use electricity in order to measure resistance.
E.g., if you have a piece of wire, and want to measure the resistance of the piece, a multimeter will have to put current through the wire in order to measure the resistance.
The more resistance there is, the more the voltage will drop. So, when the multimeter checks the voltage, the lower the measured voltage, the higher the resistance of the wire is (which caused the voltage to drop).
When you connect a multimeter to the terminals of a battery, the multimeter will try to put current through the battery in order to measure the resistance.
In this state, the multimeter will set itself in an "open to easily receive current" position (or low resistance position), which is fine when measuring the wire because the maximum the multimeter can receive is what it produced itself (the wire will not add any current).
But a battery can produce a lot of current when attached to a low resistance. For example, if I was to connect a short piece of wire (very low resistance) directly between the battery terminals, the wire would heat up extremely and possibly burn out (better not to do this or do it extremely carefully to avoid burns or anything catching on fire).
When you are connecting a multimeter in the "resistance measure" mode or "current measure mode", in both cases the resistance of the multimeter is very small. It will act almost like that short piece of wire (get very hot) and something inside may burn out (some multimeters have decent protection against this, but others do not). That is why you should never put the multimeter in resistance, continuity, diode check or current modes across a battery terminal (especially not mains, that could cause a multimeter to explode and injure you or worse). There may be additional modes that have low resistance on some multimeters that I didn't mention (I can't guarantee that there are not).
Measuring voltage on a small battery directly is safe (as long as you don't short the terminals with a single probe, then the same or similar thing would happen as with a single piece of wire), because the multimeter is then in a high resistance mode. High resistance mode means a very small amount of current will flow through the multimeter (if the voltage measured is within the range the multimeter is supposed to measure), so nothing should heat up or be damaged.