High current buffer battery power management
You can't get around having some kind of power management board for a 12S lithium battery. Fortunately most of them come with an integrated battery management system (BMS) that takes care of shutting off before overdischarge/overcharge, but it may not be quite what you want since it will do that by disconnecting from the load.  Note that you cannot just charge the battery at 48V forever without damage -- you will need a proper CC/CV charger.  Additionally, 3.6Ah sounds like you're only putting one of these in parallel, for a 12S1P setup.  I'm not sure exactly what cells you're planning, but 140A is a very large amount of current to pull from a single series of cells -- you may find that your batteries will last much longer if you parallel more than one.  I think if you went this route you'd probably find that building a custom power management board would be the way to go.

That said, for this particular application, have you considered using lead acid?  High current for a short time followed by a slow trickle charge is exactly what car batteries are optimized for, and will probably perform better than Li-ion in this case -- lead acid excels at high current bursts.  Weight and size would go up, but cost would go down, and no BMS or custom board would be required.