I've read on the internet about topping up your battery level but no website really goes into any detail. I have a 1991 Nissan with a battery and the vent caps always overflow... I recently topped it up a few weeks ago, and when I checked it today it had spilled all over the side wall, a little bit on the headlamp wires, even a tiny bit on a rubber hose, and the plastic battery tray was soaked. Is this bad for all the surrounding parts? I wiped it up as best as I could with water/baking soda. A little while ago a mechanic said my alternator wasn't that great, so battery is doing too much work, something like that if I remember right... does it sound like I'm overfilling the battery or is it something else?

I think I need to look at a picture of the inside of a battery, because there's the plate, then there's a gap between the bottom of the hole and the plate... I'm still unsure of where the level should be.

Dont top it off. Just put enough distilled water in it to go about 1/4-1/2" over the cells. Use something you can stick down in the battery to measure how much of it you filled. Ex. Use a stick put it in the battery and with a tape measure, measure of much of the stick is wet.

The voltage regulator might be broken in your car, and your battery is being overcharged. This would cause the battery fluid to bubble excessively, as if its boiling. It is not boiling, but 'gassing' where hydrogen and oxygen are being separated from the water and venting. This will force fluid through the vents and go all over.

This situation is also an explosion hazard and is dangerous.

You can test it by placing an accurate, digital volt meter on the battery when the engine is running/ The voltage should read 13.7 to 14.5 volts. If its more, you have a over charge condition.
Also. ALWAYS wear eye protection when working around a battery, especially one which may be defective.

Okay thanks guys... the mechanic did do a voltage test, basically told me a need a new alternator, it was a real shady place though. I'd consider just getting a new, maintenance free battery, this battery was bought who knows when, but I've had the truck over 2.5 years now and batteries aren't that expensive... but if my charging system is messed up then I don't want to ruin another battery. I'll test it again on a voltmeter.

Dude, when you say the cells, you mean the metal plate inside the battery?

Also are the other engine parts doomed from having the battery spill on them?

but if my charging system is messed up then I don't want to ruin another battery. I'll test it again on a voltmeter.

yes, your voltage regulator usually is inside your alternator, so when it goes mechanics usually just replace the entire alternator. IMO I think the mechanic is correct.
If your voltage regulator has failed, you WILL eventually destroy any new battery that goes in there, maitenance-free or not. Check and fix the problem before you install a new battery.

Dude, when you say the cells, you mean the metal plate inside the battery??
Yes

Also are the other engine parts doomed from having the battery spill on them?
The worst it will do is strip off paint, and sometimes cause corrosion, or flakey-looking grey deposits.
Rinse off the areas with water.
If the battery fluid seems to have corroded some parts, mix some baking powder in some water and pour that on the corroded parts, and wipe off with a brush.

For the time being, is it okay to lay a cotton cloth over the vent caps so it will prevent splashing? After every few drives I mop up the acid on the vent caps, but there are little splashes all over the other parts under my hood.

You need to get your car fixed. Letting that acid spill over stuff is not good at all. If you miss a spot on cleaning it you can destroy anything plastic (Wiring,Sensors,Paint,Connectors etc) . Even though you cant see it eating stuff it is.

I got a new battery... hopefully the problem is solved. It just sucks having this crap on other engine parts.

Thanks for all the help guys.