Dear Tom and Ray:

Please help me settle a bet with my father-in-law ... the winner will buy the loser his next battery. My father-in-law claims that even though car batteries may be marked "maintenance-free," if the battery has vent caps you can pry off, you should do so, and check and -- when necessary -- refill the electrolyte levels as needed. I say no, that modern car batteries do not require checking electrolyte levels, and that by prying off the vent caps, you actually could cause some harm to a maintenance-free battery. Can you help settle our bet? -- Michael

RAY: We can, but you're not going to like the settlement, Michael.

TOM: Back in the 1970s, battery manufacturers switched over to calcium lead plates, because they seriously reduced outgassing.

RAY: Outgassing, aside from being a byproduct of eating a beef-cheese-chorizo burrito, is when the fluid inside the battery heats up and "boils off." It's similar to what happens when water boils and gives off vapor -- eventually, you run out of water.

TOM: Because calcium lead grids reduced outgassing significantly, and all but eliminated the need to add water to the battery cells, these batteries were called "maintenance-free."

RAY: But they don't eliminate outgassing completely. And if the engine compartment is subject to extremely high temperatures, even a maintenance-free battery can lose fluid.

TOM: With maintenance-free batteries, outgassing is most likely to happen in areas of the country that get extremely hot in the summer, or where cars are in constant stop-and-go conditions and their engine compartments get up to very high temperatures.

RAY: Most people will never need to add water to their maintenance-free batteries. In fact, the vast majority of batteries don't even allow you to check the fluid levels anymore. But if your maintenance-free battery does have removable caps, there certainly is no harm in removing them and adding water if the fluid happens to be low.

TOM: It's not a regular maintenance item, like it was 20 or 30 years ago, Michael. But you owe your pop-in-law his next battery. If you're really lucky, his next battery will be for his flashlight.