How do I safely clean and tighten battery connections?

[adt2]

Okay I know this is a pretty elementary question,but I just shot sparks all over myself and cut out all the lights, so I'm going to swallow my pride and just ask before I blow myself up.

I suspect I've got a loose connection or three, and I can see corrosion at a couple of terminals, so I'd like to clean and nut & bolt everything. I googled some stuff and watched a few videos and no one really laid down any safety guidelines...they all just started wrenching stuff.

I turned off the battery charger and the (only) battery switch (I could find) thinking I'd eliminate incoming power and outgoing power. Very gingerly put a wrench on the negative terminal and started to loosen it. Suddenly got a shower of sparks (at least that's what it seemed like at the time) and the lights went out in both engine rooms. White I was standing there wondering if I was about to die by hydrogen explosion the lights came back on with an audible pop at the now-loosened terminal.

I worked up the nerve to tighten it back up and decided maybe I'd better call an electrician...but then I remembered you guys 😎. This seems like a pretty basic maintenance task and I need to learn how to do as many of those as I can or this is going to be too expensive a hobby for me.

So please...like I'm a 3rd-grader...please walk me through the proper way to do this.

[SKYCHENEY]

You still have a load on those batteries. Turn off all battery disconnects and then you should be okay to remove the negative terminal.

[adt2]

These are the only three things I can find that look like battery disconnects…the one in the middle photo is the one I turned off, since the other two have labels that say “don’t turn these off.”

[Pascal]

If you re going to work on the battery terminals you need to turn off all the batt switches and charger breakers.

Not necessary if you re just topping off the water.

[Captain Ralph]

I would also recommend removing the bottom (most negative) battery strap(s) also,

[Eddieclemons]

My path is usually, battery chargers off, disconnects off, neg battery cable off of battery and then I work on terminals and etc.

[adt2]

I thought that's what I had done, but apparently not. I suspect the previous owner re-jiggered the battery switches and I just haven't figured out how to turn them off yet.

[adt2]

It's hard to see in those photos above, but the blue labels read, "Batteries paralleled with switch on; leave switch on."

And one side looks original (with "Engine" and "Ships Service" knobs) while the other side looks modified ("Engine" and "Generator 2" plus an on/off switch above).

[Captain Ralph]

I would also recommend removing the bottom (most negative) battery strap(s) also,

When removing the bottom negative strap on all batteries, Kind of makes all battery switch positions moot,, uh?

[Pascal]

You re going to have to trace wires and see what these batt switches do. Compare with the factory diagrams. It s amazing the stuff people find a way to butcher

I ve seen many boats where owners or previous captains permanently left the parallel switch on. Same folks who usually run dual Racors on both. Makes no sense as you will not get any warning when one bank (or one filter) goes bad.

If you turn off all the batt switches and the charger, you will get any arcing when disconnecting the terminal. The sparks are caused by disconnecting the wires under load.

Same reason you NEVER disconnect shore power without turning off the breakers first