Can an 18V-5W Solar Panel Provide an Equalizing Charge for a Battery?

(Assuming here the panel can provide 18 volts at a significant current; I'm not sure how quickly the voltage goes down with increasing current.)

The voltage is likely too high. You will experience water electrolysis, i.e. water is split to oxygen and hydrogen. Hydrogen is flammable, so do this only in an area where ventilation is good, and avoid any sparks near the terminals of the battery. Disconnecting the solar panel needs to happen far away from the battery terminals.

Because of the electrolysis, you need to add distilled water to the battery back to keep the water level at the same level. If your battery is one of the types where you cannot add distilled water, it will be destroyed by the high 18 volts (example: AGM and gel cell will be destroyed as will some flooded batteries where there is no way to add distilled water -- well, gel cell isn't used in cars typically).

I wouldn't bother. Instead of this, use a continuous trickle charger with proper voltage control to keep the battery healthy. You may find some solution where a charger controller uses a solar panel as it power source, which you may find useful if the car is stored far away from an electrical outlet.

However, if your battery is already destroyed, you may try to resurrect it with the high 18 volts if there's a way to add distilled water. Be careful with the explosive hydrogen and with the corrosive battery acid, though.

If you want to expand your set of tools, consider purchasing an adjustable laboratory power source with current and voltage limits. It allows setting those limits precisesly, so that it will find many users, along with charging and resurrecting batteries.

A solar panel that is rated 18v 5w will trickle charge your battery and keep it topped up,but only if the battery is less than 90AH if the battery is larger you need a larger solar panel.

The solar panel will only suply 5wh for about 6hrs so about 30watts each day,If the solar panel is mounted inside your car it will get crazy hot and it might fail so if possible mount the solar panel on the outside.

An 18v solar panel will only deliver the 18v when not under load so as soon as you connect it the voltage will drop to a couple of volts more than the battery voltage,be sure the solar panel is protected by a diode to avoid drain on your battery during the night.

Lead acid batteries do not need equalizing only li-po and simmilar batteries need this,lead acid batteries need to be at a 60-100% charge for them to not degrade in performance(or to slow the degradation to a minimum).

Yes. But:

1. Only if it has chance to charge the battery to the top. Hard to happen in a car. Radio standby, remote unlocks, etc... will probably draw enough not only to fail the equalizing overcharge, but to gradually drain the battery, esp. if the panel is inside and shaded most of the time.

2. Only if the battery is in good enough state (its self-discharge is low enough).

3. Only if there is no something in the car that activates at 13.5-13.8V (some DRL modules use the voltage as a signal of a working engine)

etc, etc...

And what's more, modern lead-acid batteries generally don't need equalizing that much. Once per year, well, possibly, for some low-end battery. And you will be losing water from the battery in the process. Not a great deal, esp. for maintenance-free batteries.

Even considering it is mostly useless, some cars do it for you, given chance.

Equalizing charge as a practice is better left to the diesel-electric submarine operators. They have both the need and the experise available onboard. A typical car owner is less literate in battery chemistry and has way less a stake in the battery maintenance.

A solar panel can help topping the battery if the car is left alone for extended periods of time (3-6 months). 5W panel is probably way too little for a modern car. I tried to use 20W panel mounted on the top and it was okay-ish. For a car that is used at least weekly, it is pointless.