I find it really odd - it appears modern bikes discharge their batteries its something you hear a lot of all the time - maybe its alarms and all the gizmos people run, maybe they have poor batteries, Other than a Moto guzzi I owned most of my bike including Laverdas, BM airheads, Cagivas and KTM to name a few have been used for everything from daily rides, for commuting and as everyday transport as well as long haul travel bikes never really suffered discharged batteries in storage and very rare has it been when I have not been able to start a bike off the button or the kicker. I have never used these trickle charge gizmos, never ever had an alarm on a bike - that's i think is just asking for trouble and have always done good maintenance on the electrical system ie kept fitting and plugs well cleaned and lubricated with electrical grease, earths kept clean , battery terminals cleaned and greased, once a year seems to keep on top of it all - that includes winter riding - so I think the bike you have must have a leakage to earth through a bad connection, broken wire or something is staying on and agree a good auto electrician should be able to sort the problem. But if it were me I would get rid of the alarm, clean and lubricate every electrical connector, check where you can for possible broken wires or shorts, take your battery off give it a good charge and get a drop test carried out at a local car shop, if that seems ok you may have solved the fault if not the auto electrician is the man to turn to next. I would not be inclined to trust BMW main dealers they are not auto electricians there 'mechanics' are mostly fitters of sorts, cost too much and often have neither the skill or the experience and knowledge of a good auto electrician. Good luck with it though.

Short answer - NO!

It's all to do with the achitecture and chemistry of the each capacitor and some confusion marketing too. The two types of battery you quote are different devices doing the same job so you have to rely on the makers' explanation and specs. to enable an "apples with apples" comparison when making your choice;

Lead-acid makers have used AHr(capacity) ratings as shorthand to indicate cranking ability, rather than a real usable capacity. The lead-acid capacity rating itself is based on a complete discharge at a low discharge rate. Under actual cranking conditions they will deliver considerably less than spec capacity. And because lead-acid batteries begin sulfating when only a small percentage of the capacity has been used, and their internal resistance rises as they are discharged, the actual capacity which can be USED may be as little as 20% of the mfg. rating. Discharge in excess will not only damage the lead-acid battery, it may not allow proper starting as voltage sags.
Shorai LFX are based on a completely different chemistry. Not only do they have less than 1/3 the internal resistance per capacity than do lead-acid, they are also the ultimate "deep-cycle" battery. The internal "completely discharged" capacity of a Shorai LFX is 1/3 the rated "PBeq" capacity. For example, the LFX18 12V series have 6Ah cells internally. But the cells are capable of 90% discharge without damage and while retaining more cranking ability. As such, the USABLE capacity(or "reserve capacity") of an LFX18 12V battery is on or very near par with 18AHr-rated lead acid batteries, while providing superior cranking performance and a vast reduction in weight. The Shorai PBeq AHr (lead-acid equivalent) rating system therefore allows users to compare a very different technology from lead-acid, but on a close apples-to-apples basis when making a choice.

Shorai's explanation. Colebatch is correct, Shorai sell an equivalent 18 Ah equivalent battery which is 6Ah but will give the same or better real world performance as an 18Ah lead/acid battery FOR STARTING. A motorcycle battery is a STARTER battery and purposed to supply the required volts to start to engine and not to do any other task. Also remember, contrary to popular belief starting uses only a tiny amount of the battery's stored energy and is replenished within minutes. It's all the extras attached to the bikes electrical system combined with the inadequate OEM design of the loom R/R and alternator that stresses the battery.

Adventure 950 - Laverda's; now they were real bikes! lol. BMW's "technicians" are only allowed to plug into the CAN bus now. Your bike might be a smouldering puddle of solder but if the computer says "no fault found" then there's nothing wrong with your bike. It's the box ticking "computer says no" syndrome plaguing modern British life :(

Who would be interested in a two battery solution (one exclusively starter, one deep storage) with properly designed alternator, three stage charging and wiring loom?

Um.

So what I understand from this is that the Shorai will be good for starting.

I'm still not sure whether they will be better or worse than Lead-Acid if you left them slowly discharging - whether that is because or a fault/current leak or because you left some device on while parked, e.g. an alarm.

My guess is that my problem was because I left my alarm on despite the manual's insistence that it shouldn't matter.

I've now ordered a Shorai LiFeP04 LFX14A2-BS12 - seduced by the weight saving and lack of maintenance obligations - I guess I will see if it was the right decision...

Thanks to all for their time and insights!