There is no doubt that hybrid automobiles (electric/gasoline powered) have
great advantages in fuel economy and reduction of atmospheric pollutants.
What are the downsides, other than slow acceleration and reduced passenger
space? Are these problems factual? 1) 500 volt system in some hybrids could
lead to electrocution of inexperienced rescuers who cut open hybrid cars to
save passengers involved in accidents? 2) Rear-ending a hybrid car could
result in a spray of sulfuric acid because of their relatively huge battery
packs? 3) What is the warranty period and replacement cost of the battery
pack?

Hello monroe22, thanks for your interesting question. I am a
mechanical engineer and while I have see quite a bit in the news about
Hybrid cars I had not really looked at any of the information
available. In the first part of your question you ask about advantages
vs. disadvantages. After reading the many articles I found (links
shown later), in my opinion the Hybrid may not be the answer to all of
our problems. Apparently when operating accessories such as A/C and
other electrical equipment the gas mileage drops off to the point
where it is comprable to existing cars with fuel efficent gas/diesel
engines. However, the reduction in pollution might be a real
advantage. Here are links to articles which you will find of interest:

http://autos.msn.com/advice/article.aspx?contentid=4020586&src=News
Gas-Electric Hybrids: The New Wave

http://moneycentral.msn.com/content/Savinganddebt/Saveonacar/P37272.asp
Hybrid cars: Do they make sense for you? 

http://eartheasy.com/live_hybrid_cars.htm

The New Hybrid Cars

Now for the second part of your question.
1). http://www.motoring.co.za/index.php?fSectionId=751&fArticleId=427333
    Hybrid cars a danger to rescue workers
    The growing popularity of hybrid vehicles poses a new danger for
rescuers at accident scenes - a network of high-voltage circuitry that
may require some precise cutting to save a trapped victim.

Manufacturers have a list of safety checks that the car's computer
must go through for the electrical system to run and they've published
guides showing the location of the electric components.

On the Toyota Prius and other hybrids, the high-power cables are
colored bright orange to catch the eye of a rescue worker or a
mechanic.

Rescuers are taught to disconnect the battery and turn off the key
immediately before cutting into a car, but that's not always possible.
There are concerns over what happens if something goes wrong and the
battery, ignition and other points are inaccessible
 
Toyota service training instructor Chris Peterson said the Prius'
electric system should shut down if anything goes wrong.

"There should not be high voltage in those cables, but I'm not going
to stand up and say there isn't," he said.

After reading this I would say that there is some danger to rescuers
and special precautions will have to be taken to avoid injury.

2). This question was the toughest. I first had to find the specs for
the batteries that Toyota and Honda are using.
     http://www.evrental.com/prius.html

Toyota Prius Vehicle Specifications

Electric Motor/Generator/Power Storage
Motor type: Permanent magnet
Power output: 33 kW/44 hp @ 1,040 - 5,600 rpm
Torque: 350 N-m/258 lb.-ft. @ 0 - 400 rpm
Battery type: Sealed Nickel-Metal Hydride (Ni-MH)
Output: 273.6 V (228 1.2-V cells)

http://www.top100musclecarsites.com/Honda-Insight.html

The Hybrid Honda Insight new 2004, and for 2004, Integrated Motor
Assist powertrain is a combination of a gasoline engine an electric
motor and a five speed manual transmission. In the area behind the
driver and passenger are the electronic controls for the system, the
nickel metal hydride batteries and gasoline fuel tank. Honda Insight's
VTEC E system was designed to create a swirl effect in the cylinder.
This allows the air-fuel charge to burn more cleanly for enhanced fuel
efficiency and cleaner- emissions.

     So, we see that both Toyota and Honda are using the Sealed
Nickel-Metal Hydride battery. This battery does not contain sulphuric
acid, but it does have hazards associated with it.

     http://216.239.39.104/search?q=cache:THUofVN-xcAJ:www.sanyo.com/batteries/pdfs/nimh.pdf+Sealed+Nickel-Metal+Hydride+safety&hl=en

MATERIAL SAFETY DATA SHEET

Section V - Health Hazard DataThreshold Limit Values: See Section II
Effects of a Single (Acute) Overexposure:Inhalation: During normal use
inhalation is an unlikely route of exposure due to containment of
hazardous materials within the battery case. However, should the
batteries be exposed to extreme heat or pressures causing a breach in
the battery cell case, exposure to the constituents may occur.
Inhalation of cobalt dusts may result in pulmonary
conditions.Ingestion: If the battery case is breached in the digestive
tract, the electrolyte may cause localized burns. Skin Absorption: No
evidence of adverse effects from available data.Skin Contact: Exposure
to the electrolyte contained inside the battery may result in chemical
burns. Exposure to nickel may cause dermatitis in some sensitive
individuals.Eye Contact: Exposure to the electrolyte contained inside
the battery may result in severe irritation and chemical burns.

Section VII - Spill and Leak Procedures Spill and leaks are unlikely
because cells are contained in an hermetically-sealed case. If the
battery case is breached, don protective clothing that is impervious
to caustic materials and absorb or pack spill residues in inert
material. Dispose in accordance with applicable state and federal
regulations.

     If you are interested, this MATERIAL SAFETY DATA SHEET lists all
the materials used in battery construction. It appears that there is
some danger associated with this type of battery. As you will notice
as you read these articles the next generation battery will probably
be Lithium-Ion. I would guess that it will also have some hazard
associated with it.

3).  http://cars.about.com/cs/familysedans/a/hybrid_explain.htm

What You Should Know About Hybrid Cars

Those same critics have insisted that if you replace the batteries
after the warranty expires, it will most likely cost several thousand
dollars. Toyota claims prices will drop as hybrids become more
popular; battery packs currently run around $4900. However the pack
contains 38 modules that can be replaced individually for $138.00
each. Honda's Insight and Civic hybrids have an 8-yr./80,000-mi.
warranty on most of the powertrain, including batteries, and a
three-year/36,000-mile warranty on the rest of the car. The Prius has
an 8-yr./100,000-mi. warranty on the battery and hybrid systems, plus
a three-year/36,000-mile warranty on everything else. The electric
motors and batteries don't require maintenance over the life of the
vehicle.

I believe that this will answer all the questions you have about
Hybrids, it did mine. I am not ready for a Hybrid yet and will
continue to drive my 1977 Dodge van.

Redhoss

redhoss: Excellent answer,thanks.
monroe22-ga

The high voltage system will shut off in the event of an accident. The
cables are also sealed in a bright orange casing, and they do not lie
in the area that rescue personnel would cut. Also, an "Emergency
Response Guide" for Alternate Fuel vehicles has been distributed for
some time to appropriate rescue agencies throughout the country. The
important thing to remember is that there are many other vehicle parts
(e.g. airbags), not just Hybrid components that pose a risk to rescue
personnel. That has not stopped the American public from buying
vehicles equipped with those features.

"Apparently when operating accessories such as A/C and
other electrical equipment the gas mileage drops off to the point
where it is comprable to existing cars with fuel efficent gas/diesel
engines."

Unlike the previous generation, the 2004 Prius has a dedicated
electric motor to operate the A/C. This is a significant improvement
over the 2003 model, and fuel economy is no longer severely impaired
by running the A/C. By the way, the published MPG is a result of the
EPA mandated test, which does not simulate 2004 real world driving. As
with any vehicle, true mileage will probably not be up to the
published MPG. Unfortunately, that is even more true with a Hybrid
because the proper driving style to maximize fuel economy is not how
they perform the EPA test.

An interesting fact about the Prius is that it has a LOT of low end
torque (because of the electric motor), so it accelerates very well
from a stop. It is hard to make a quantifiable comparison to a
traditional combustion engine, but Toyota compares the acceleration to
a 4-cylinder Camry.

Some of the new "hybrid" vehicles (e.g. Ford Escape) are not *true*
hybrids like Toyota and Honda have developed. I have a lot of
confidence in the Prius, but if you ask my personal opinion, I would
be wary of the non-Japanese technology, at least for a couple of
years.

Hybrids are not for everyone, but they can be extremely fun to drive,
are great for the environment, and are always a conversation starter.
Also, the 2004 Prius also has a lot of features normally reserved for
luxury vehicles (e.g. SmartKey).

Hope you find this informative.

lordofcatan73-ga: Many thanks for your most informative comment. As
with many new concepts, extremist views get the most early publicity,
if only because they are provocative. Your balanced appraisal is
helpful.
Regards,
monroe22