How well your battery is recharged by the vehicle is entirely depependent on the vehicle's voltage regulator.
As for how much capacity is actually required to start a modern fuel injected vehicle, it is very little.
I have an AGM battery which when being charged will basically eventually take less than 0.05 amps at any voltage when fully charged.
If I start my engine on this absolutley fully charged AGMbattery, when the amperage tapers back to 0.05 amps or less, the battery can be again considered fully charged.
How long does this take? For amps to taper back down to 0.05a, at 14.7v, after starting my engine?
Under 45 seconds.
back when engines required much more cranking this was different, but the fact is that not much of the batteries capacity is required to start the engine. The parasitic draw of the door locks, engine computer, ect in a 48 hour period cumulatively draws more from the battery than truning the key to start for 2.5 seconds.
problem is, when the battery is discharged to 95%, it takes ~1.5 hours to get it to 100% charged, at ideal voltages, which wuor alternator will not be allowed to seek.
So 95% to 100% takes 1.5 hours of driving, or 1.5 hours on your plug in charger.
Which is more efficient?
Your battery wants to be at 100% at all times to be happiest.
How much effort are you willing to go through to have the happiest battery possible?
Most people could care less, and simply replace the rented battery when the rental contract expires. Those wanting to extend the length of that contract to the maximum, plug into the grid to top up the battery monthly, or after any significant level of depletion has occurred, intentionally or otherwise..
Then you have the something for nothing crowd, who seek to kill a battery within the warranty period and get something for nothing.
I'd hate to be a battery retailer. What other product can be intentionally ruined, and have replaced for free?
