The component between batts is a diode.
Hi guys this is my first post, hope to help u. I've reading about the cranking shutdown problem. The solution suggested by Draco is right, but the component between the two batteries is just a diode with amp rating above your PSU amp requirement (suppose 20A will be good enough for most systems @ less 5US$).
This is how its go: connect your two batteries in parallel. Then between the the two positive poles put your diode forward to the PSU battery, this is anode to car battery, cathode to your PSU battery (the cathode is the line marked terminal). Done. When you crank, if the voltage on the car battery goes low enough, the diode will be reversed so no current will draw from the PSU battery, wich will be supplying your PC system, while you crank. When you finish cranking the voltage rises and your generator will charge your PSU battery without need of manual intervention.
The diode will not waste power when you stop your engine. It will just get a little hot when recharging the PSU Battery (maybe some heat sink needed). It wont lower the full charge voltage of the PSU battery.
I suppose that any small UPS battery like 12V@5Ah at ~30US$ will work for your PSU Battery.
This is the correct setup to correct this problem, cause is very healthy to the PSU, and should be the solution used when the PSU dont hold while cranking.
Remember, no manual switches, no relays, just a diode.
Thankx. Please send your comments and questions.
Sys Batt just work while cranking.
Here's the diagram.
Really i haven't tried, BUT THIS WORK, maybe if your system is heavy loaded its better 7ah, but dont think bigger than that is needed. For example my 450VA APC UPS uses one 12V@6Ah, and holds about 5 minutes at full charge, REALLY I DONT THINK YOU'LL CRANK FOR 5 MINS, really i dont think your system takes 450VA (thats a full pc and >15" monitor), and your using it directly not loosing energy on an inverter.
5Ah should work for most systems, 7Ah WILL WORK for "any" system.