Hi Trevor
Without knowing the type of ignition and lighting draw, it is impossible to know what you are actually drawing from your Shorai batteries. According to the Shorai site, divide the number in the model size designation by 3 to get actual amphours. The Shorai LFX 18 is then a 6 amphour battery, which in lead acid is also a very tiny battery. It should be noted that this 6 amphour capacity is when fully charged to 14.6 volts. If only charged to 13, volts, the Shorai LFX has just 30%of it's full capacity available, so just two amphours.There is nothing magic about the Li batteries , 6 amphours is 6 amphours, two ah is two ah, same as with a lead acid battery. A two AH lead acid battery would be a very tiny battery, but not many would consider installing a battery with such a tiny storage capacity. It is happening all the time with li batteries tho, because people, self included, get fooled by the big number, such as the 18 in the LFX18.
If you have a magneto ignition and low draw led pilot light, then almost any battery will work deadloss fo quite awhile. I have done this on a couple of trips when my charging system failed, only this was on a bike with high draw dual point dual coil ignition. Even so, a big lead acid battery would run the bike all day deadloss without problem, generally 500- 600 kms.. I found it was an annoyance having to locate a power source and extension cord for the charger at the end of a long day of riding, and even more annoying to forgo running my heated vest in the interest of conserving battery voltage.
The cost of two Shorai batteries and the special Shorai charger will get you most of the way to the purchase of a new Alton. The new version of the Alton is very good. You dont mention how many years you have been using the Shorai batteries in this way. Until it failed permanently at 18 months, I thought my Li battery great and was looking forward to the manufacturer's claimed life, "up to three times that of lead acid" . I typically see 4 years from a cheap lead acid so this meant 12 years of life from the LI battery! When the battery failed at 18 months it no longer was a cost effective purchase.
Using the starter to propel the bike could prove to be rather costly, especially if you cook a couple of $200 batteries. A charging system and one lead acid battery will suddenly seem very economical. Have a look here:
http://www.youtube.com/watch?v=ySNbSZTgplA
It might be that running deadloss and using the Shorai charger is the best way to utilize this type of battery on our old bikes. At least this way you know you have full battery capacity after charging, unlike the situation with a charging system that operates in a lower voltage range than Li batteries do.
Itis interesting that the Shorai site now lists a disclaimer on their batteries. They state that the batteries are only compatible with vehicles having a charging system that produces a minimum of 13.1 volts at idle! This excludes amost all of our older motorcycle charging sysems and most of the new ones made for the Vincent as well. The Walkernator might be the only exception. These are just my thoughts after having a rather negative experience with what I thought was a fabulous new technology. After the problems, when I really dug down for hard numbers, it was easy to see why the battery failed. It is not so much a fabulous new technology as it is a fantastic new marketing strategy.
Glen.