I'm not a boffin by any means and ,at the risk of being proven wrong ,would venture to say that ;
the battery has to supply some voltage to energise the alternator in order to produce a charge .
If battery voltage is too low , this will not happen ,the alternator will not provide sufficient charge and the battery will discharge itself quickly .Any momentary boost from another vehicle will only have a short lasting effect .On a modern bike and especially one with FI , you need a good battery -period - full stop .
If there were a permanent magnet alternator on the bike [which is self energising] you would PROBABLY be OK as the alternator would provide 14 v regardless of the battery condition and keep the bike running once you have found a way to get it started in the first place .
A battery with a bad cell will never charge successfully as it will always draw the good cells down to it's own voltage .

Theoretically ,I have never tried this , you could use some flashlight batteries to energise your alternator and then tow or bump start the bike .
After a long enough tow ,with the engine spinning over ,the system might reach the critical voltage for the FI to kick in and give it life .

Or

You could try this .


YouTube - Emergency Starting Technique

Now I really am confused! Not sure what you mean by 'energize the alternator' - as I understand it, this is a magnet rotating around coils of copper wire which induces an electrical current in the coils due to the changing magnetic flux - it is 'energized' by physical motion of the magnetic field around the coils of wire and does not need a primer charge. The resulting AC is then converted to DC by the rectifier and the voltage regulated to 14 V or so by the regulator which is upstream of the battery and FI/ECU. How does the ECU distinguish between 12V that has come from rectified and regulated AC off the alternator as opposed 12 V from some other source applied across the battery terminals, be that other batteries or solar charger? Dunno and in fact I'm not convinced this is the case....

When I checked the power rating of fuel pumps, they came out at between about 2 and 10 watts depending on the size and flow rate; also need enough juice to activate the injectors and run the ECU as well as generate a spark in the plugs, perhaps 20 watts or so. Should be possible to generate this by a number of methods....

There are two kinds of alternators generally fitted to bikes .The one you have just described is a permanent magnet rotor alternator [ with actual magnets ].
Like the old Lucas alternators fitted to Brit bikes .

The other kind uses electromagnets [coils] to produce the north and south poles of the rotor ,these act like physical magnets when spun around a stator .This kind has to have a reasonably healthy battery voltage in order to work .

A good explanation Dodger - it sounds logical to me.

Sadly the emergency starting technique in the video will only get you so far down the road if you find yourself in my situation though. My battery died on the Khabarovsk - Chita corrugated gravel road. I had stopped 10 minutes earlier, started the bike without problem but stopped again to collect fresh water from a stream. As soon as I turned on the ignition I didn't even have enough power to turn the engine over (pressed the starter button and the LCD instruments faded to black). As it was late afternoon I pushed the bike off road, set up camp and waited till next morning to get back on the road. With a jump start the bike started instantly and I thought all was fine again - and it was, until approx. 100km down the road the motor just died and the instruments faded etc.. After flagging down the friendly local the bike would restart but die instantly the jumpers were removed. As soon as the battery was replaced the bike electrics ran perfectly again.

I can only assume the battery failure was due to the corrugations killing a cell as prior to that the battery had worked flawlessly for around 5 years in all road and traffic conditions. If it had simply been that the battery was getting old I would have expected some sort of early warning rather than an instant failure - as an example the bike had been shipped from Australia to Korea and had not been started for around 2 months - yet it had started instantly when I had uncrated it in Busan 2 weeks earlier.

A little strange explanation but as you said, if the voltage is lower than 11V, the bike won't start and you'll never get 14V from the generator.

Otherwise the voltage on the battery will always be the same as on the generator or there is to high resistance in the cabling.

Just the thought of battery acid on my bell-end is making me wince:eek3:

Ok, so acid burnt bell-ends aside, the question still remains..

You're out in the middle of nowhere, VRR Uckfayed, Battery Boiled...

Could you - theoretically - locate a 12v battery from somewhere, and hook it up to the bike, disconnect the alternator (on account that you don't want it to fry the replacement battery either) and run on battery power alone until you get somewhere you can source replacements?

G

Yes, but it won't last long if the lights are on and some bikes have lights on all the time (N America).

Yaaa agreed, I forgot to add I - hypothetically - would have disconnected all unecessary ancillaries too ...

G