There have been many excellent appreciations of the issue but perhaps another will not hurt?
Connecting a second battery introduces many potential problems such as: wiring, isolation, mounting, venting, etc.
A few thoughts as have installed and serviced thousands of vehicle/marine battery installations: the ST1100 alternator delivers about 45 or 60 amps according to measurement from many bikes. Later installations are the higher output but investigation has not revealed the timing when Honda (actually ND) changed. The 1300 delivers about 60 amps although have only measured a few bikes so some may be higher. This output can occur at just above idle speed so someone concerned (real, measured concern but not fear without evidence) with output may be advised to raise idle speed by 500 RPM.
A second battery is only required or even justified by unusual conditions such as emergency vehicle service. Period.
As several have stated, the alternator output must exceed load under all protracted operation or the battery will drain. Period.
Adding a second or additional battery capacity will only delay the draining of the battery to critical levels. Period.
If an additional battery is installed, it requires sufficient wiring capacity to support charging, as someone related earlier, because small wires introduce voltage drop which reduces charging rate. Installing an additional battery obviously requires that the battery charge from the bike's system unless one simply carries a spare.
Charging of a second battery can be by simply connecting the two batteries into parallel but this will require cabling sufficient that the second battery will not damage its wiring during heavy starting/cranking. This also introduces the risk that this battery will be drained by the same mechanism which has drained the main battery. For this reason, RV and marine (second) batteries are typically isolated from the main (engine starting) battery. Isolation adds another problem, how to do it?
Battery isolation by means of a diode is to be avoided because it introduces voltage drop of one volt or more which reduces recharge significantly and often leads to battery sulphation unless both batteries are similarly wired. In that event, the alternator's voltage sense wire must be placed with the diode voltage drop in mind.
This all becomes a major PITA in installation. Use of an isolation relay is the better solution but then the question arises: how to operate the relay? If the relay is switched off the ignition then leaving the ignition on for a period will simply drain from both batteries which simply extends the drain time but does not address longer term problems such as rider stupidity- I define stupidity as being something I could have easily avoided but did....again!
Someone mentioned installation of a relay which kills headlights unless the bike is running which can be accomplished from the fuel pump relay or possibly from the oil pressure light switch. I caution in this regards, however, since loss of engine oil pressure or engine ignition loss would shut off headlights.....possibly inconvenient under some riding conditions depending on how long it takes to get stopped. The time may be longer than the rest of your life so I avoid that.
I presume that your extra lights and accessories are wired as I usually see, directly to the battery? With all respect due to this practice (none), it is dumb! don't do that!
Driving lights, bar heaters and all that stuff must be wired so that it is automatically switched off. No negotiation as otherwise is simply looking for problems/dead battery.
One solution used by some is to connect the auxiliary load relay to ground through the netural light bulb so that these are off unless in gear. Possible.
Driving lights are required (in many jurisdictions) to be wired such that they cannot operate excepting when high beam headlamps are on. This is really a no-brainer for most applications as having to operate the dimmer plus driving light switch repeatedly is a nuisance and also encourages on-coming transport trucks to go back to high beams as punishment when one is slow to dim. The fact that they have experienced fail to dim, many times that night justifies their lack of patience.
I don't carry a spare battery excepting under very unusual conditions and then the battery is sealed as luggage.
It sounds as though you have other issues with your system which require correction rather than to add more potential problems to a bad mix. Happy to try to help if that is useful- hope this isn't misinterpreted as being critical?
