What tool should I use to unbolt battery terminals?

so is this a trick question?

unbolt the battery hold-down clamp with the necessary socket/ wrenches

remove the black cable first with the appropriate wrench or socket

the main thing is this:

make sure you don't complete the circuit & cause a short by touching the positive cable / terminal with your tool(s). that's why some models have a big red plastic cover or rubber cap over the positive... if you don't have it, use a rag or something to make sure you don't cause a spark when your tool touches the terminal.

clean the cable terminals w/ a commercial battery cleaner (foam type) or be cheap and use baking soda/water to clean them w/ a wire brush.

if you're too timid to follow through, most auto parts stores & walmart does the install for FREE

A fully charged battery does not mean that its amperage output is adequate, have it tested at an auto parts store. While your problem could be battery, wiring, key switch, solenoid, starter or maybe really cold outside, best to start with simplest first, clean the battery terminals and cable connectors with a terminal brush tool and install anti-corrosion impregnated felt rings on each terminal. With the battery terminals disconnected, also unbolt and clean the solenoid and starter terminals too and re-tighten. Then find the "ground" connections from the negative battery cable to the metal chassis and from the engine to the chassis, unbolt, clean, sand clean each if necessary and reinstall. If when using a voltmeter there is any reading between the engine block and the negative terminal of the battery, installing an extra ground cable directly between the engine block and the negative battery terminal connector. When reinstalling battery terminals, install positive terminal first, place socket slightly larger than battery terminal on top of it and lightly tap to fully seat the cable connector on the battery terminal before tightening. Tighten snug enough that cable connector does not move on the battery terminal, do not over tighten. Beware of battery acid and accumulated corrosion, wear eye protection and protective gloves too and be aware that battery acid eats holes in clothing and cloth gloves. If problem persists, a DC voltmeter most useful to determine if when key turned to start, there is voltage applied to the solenoid. If yes, then to check if solenoid operates upon key turn to "start" applying voltage through solenoid to starter. If yes, and if battery amperage output tested "ok" then starter may need to be replaced.

They sell a variety of replacement clamps or terminals, unbolt it,cut it off, expose a 1/2" or so wire and connect it to the replacement terminal, and install back to battery. Like 5 bucks and a couple tools.

unbolt the terminals starting with the earth wire first.

Pop open the hood and use an open-end wrench to disconnect the negative terminal on the battery. Allow the vehicle to sit for 20 minutes before you start to take out the seats. Unbolt the seat from the chassis of the car, using a 3/8-inch etc
a good diagnostic tools

Use the jack to lift up the front of the Elantra and place the car on the jack stands. Double-check that the Elantra is secure on the jack stands prior to working underneath the car.

2 Disconnect the negative and positive terminals from the battery using the open-end wrench, removing the negative first, then the positive.

3 Slide under the Elantra and unbolt the engine cover on the underside of the vehicle using the 3/8-inch ratchet and socket. Remove it and place it out of the way.

4 Locate the starter motor, which is on the passenger side of the engine, connected to the transmission. Remove all wiring from the starter using the an open-end wrench or the ratchet, depending on clearance for tools.

5 Unbolt the starter using the 3/8-inch ratchet, extension and socket, being careful to hold it steady with one hand while undoing the last bolt. The starter is quite heavy.

It's on the passenger side, you lift up the bench (bottom) after you push back on the bottom front of the seat-do one side at a time, unhook it from the bottom front first, after you get both sides unhooked it just lifts up and off. then it is just a matter of unbolting the hold down and the battery clamps-do the positive post first, then the negative side. when installing, hook up the positive side first, then the negative side-clean those battery terminals inside and out with a wire brush or emory cloth, modern cars need really good electrical connections.

Remove all guards in the way, unbolt the baterry hold down clamp, remove,then unbolt each battery terminal. replace battery with a fully charged battery of the same type. Clean the terminals good before installing back on the battery. Use thw terminal washers, usually green and red in color, red for the positive post and so on. Rough the posts, smear some battery grease on each post install washers, then battery posts themselves. Tighten both posts just to the point that they will not turn, don't overtighten them they may break, then you'll need new post ends.

1. Remove negative battery terminal
2. Loosen serpentine belt by rotating the tensioner and remove from alternator pulley
3. Unplug the alternator connections.
4. Unbolt the alternator.
5. replacement is reverse of removal.

The negative terminal on the battery is the one with the minus, usually the positive is red.
Disconnect the negative cable from the battery for safety. Then follow the cable to where it attaches to either the engine block or the body. They unbolt it and replace it.
You shouldn't have to remove the battery for this. Just don't mess with the positive terminal. You will have to reset the radio stations etc since battery is disconnected.
If the connection at the negative side is corroded, just cut it and reconnect it with a new battery terminal. A parts store can hook you up with that.