When opening the trunk, there's a definite sulfur type smell noticed. Its not the spare tire. This smell just recently appeared. The Prius is almost a year old. Has anyone else noted this with their lovely hybrid?
I have not experienced the smell you have noticed, but consider this: The 12 volt battery is a lead-acid type, and is located in the trunk, between the left wheel well and the left tail light. It is covered by a step-like cover.
The battery has a vent tube that comes down the rear side of the battery and through a rubber grommet in the floor of the trunk, to the underside of the Prius.
If the battery was being overcharged it could emit the sulfur smell. The battery may be damaged (cracked) or the tube is not through the grommet, or is plugged up.
Potassium Hydroxide and Sodium Hydroxide are the key ingredients in the HV battery or NiMH electrolyte. Both are odorless and so are not going to be the cause of sulfur smells in the trunk or (boot).
For the non-chemist these are some very common ingredients used to make many of our household soaps, detergents, facial creams and shampoos.
Now before I cause you to believe that there is no danger, be aware that the pH level of this electrolyte is (pH 13.5). This is a very caustic level that can indeed be very harmful to human tissues.
Toyota has declared that the chances of our HV battery or NiMH electrolyte leaking is very small, even in an accident. The HV or NiMH battery is enclosed in a metal housing and the liquid level of the electrolyte that could leak if both the metal container and the NiMH cells were ruptured is quite small.
However if such a thing ever happens you can neutralize the electrolyte with boric acid 5.5 ounces per gallon of water or simply use common household vinegar; a more familiar and available neutralizer for Sodium Hydroxide & Potassium Hydroxide.
Our small 12 Volt battery that sits on the left or North American drivers side of the Pruis and in the trunk, has an electrolyte of Sulfuric Acid. This battery is of a more typical automotive battery design, that in most autos sits under the hood or (bonnet) where the ventilation of any escaping sulfur or hydrogen gases is not in communication with the passenger cabin and poses little problem for ventilation and or odor control.
Because our battery sits in the trunk of the Prius, ours has a small tube attached to the top that is supposed to direct any gases, especially noxious sulfur based and hydrogen gases out of the bottom of the trunk or boot. If for some reason either side of this tube is loosened or not able to maintain its seal, we can smell the sulfur types of gases in our trunks/boots on occasion.
Keep in mind also, that the hybrid battery fan blows air across the small 12-volt battery in the trunk (after it has passed over the big hybrid battery). This air either blows out a vent below the 12-volt battery (if the main cabin air is not in recirculate mode), or blows into the trunk, then past the back seat back into the main cabin (if the main cabin air is in recirculate mode). So, if you do a lot of driving with air set to recirculate, and your 12-volt battery is releasing sulphury fumes that aren't making it through the battey vent tube, these can get into the trunk and main cabin. It only takes a tiny bit to make a noticable odor.