Landshark, you want a job as sales rep? LOL.
Dual batteries are pretty much a must for trucks equipped with winches, plows, or really good stereos. Unless you do as Landshark has said and buy a 350amps alternator. I have them for sale, real HD stuff used on EMS equipment all over the world. They start at just over $1200US. It is a little more expensive than a second battery. One of them is even really effecient and only requires 25hp to turn it at idle. But if you have a stock motor, I have a friend that can sell you a 500HP EFI monster small block. I can hear it now " But honey, I need the blower motor to turn the new alternator" But in all seriousness....
Dual batteries are useful because they help the alternator in a heavy load situation. We have done tests using our test benches several times. We have two batteries that power both our starter tester and old alternator test bench. When a starter with a 300amp draw is tested, while an alternator is being tested at the same time, the alternator's output increases to about 90amps while the starter is running. Which means the batteries are making up about 210amps of the load. Then when the staters is disengaged the output drops off to 60amps, then tapers to 10amps or the float load of two batteries. This takes about a minute.
Now if you consider that most 3g's put out about 90amps at idle (not mine, but most others), you still are going in the hole with the load of a winch and the vehicle's other loads. Depending greatly on how hard the winch is being worked. But this just means a voltage drop of 1 or 2 volts, something a half decent alternator can handle easily. Doing this for minutes will cause some minor damage though. Doing it for 10-20minutes will cause great amounts of damage.
Now if you go with just a single battery, the whole picture changes. Now the alternator will be required to make up more of the 300amp load. Likely being asked to put out more than it is capable of. The voltage will really drop, to the point you see lights dimming etc. This causes damage to the alternator, to the winch, and to anything running on electricity in your vehicle. Our big body 3g's can pretty much handle the load with the engine revved to over 1500rpm. you would still get some voltage drop as they only produce slightly over 225amps. (shameless self promotion)
You can bump up the alternator to a real high output unit, say something with a 125-150amps at idle. (more shameless promotion) But this is only going to help slightly, not really solve the problem. We do have HD EMS units that are putting out about 190amps at 1000 engine rpm, 260amps max. Which would pretty much solve the problem foresure. (They are only about $650US, to you guys) So again you get into cost and available engine power. You end up with dual batteries being the cheapest way of addressing the problem.
Now why don't daul batteries work? Usually because someone has been misinformed on how to set it up, or is using the wrong setup, etc. It is very important for the batteries to have only one stud used for the positive take off, and the negative takeoff should come from the opposite battery. This way the batteries are made into one large cell. Problems arise when power is taken off both batteries (or the batteries have seperate grounds), as it causes them to become unbalanced, which is the death of them. It costs a little more to wire this type of setup up, but it should give years of trouble free power.
There is my two cents worth.
Also here is link to what I can do with dual batteries, LOL (more shameless self promotion):
http://www.worldchat.com/~sva-sales/ And the answer is yes, she does ride the bike.
