Max profit mentality means they are built so that the higher percentage will likely last the warranty period.
Nowadays higher parasitic loads on vehicles can deep cycle a starter battery, which they do not enjoy as all lead acid batteries always want to be kept cool, and fully charged.
One cant charge an 80% charged battery to 100% in less than 3.5 hours and that is when it is held at at ideal voltages.
Your vehicle does not come close to holding it at ideal voltages, even if you were to drive it 3.5 hours.
You want a battery to last? Charge it regularly to full or nearly so by plugging it into a 'smart' charger, even though these too do not hold the ideal voltage for long enough.
The Worst constructed battery kept fully charged will last longer than the best battery chronically undercharged.
Many modern vehicles will attempt to keep the battery at 80% charged or less, so that when one lifts their foot off the gas to coast or brake, the depleted battery can duck up 20 to 50 amps when the voltage regulator controlling alternator output seeks mid 14's. It is said each 25 amps that the alternator produces requires 1 engine HP.
This is an attempt to get those epa MPG numbers up a smidge, and this is a battery killer.
A fully charged battery cannot accept much amperage, so this minor regenerative braking scheme in the quest for a smidge more MPG, requires the battery be dischrged to the point it can accept higher amperage when the system voltage is increased
Automakers do not care about battery life. But that MPG number going up a tiny fraction means they are rewarded. Even though the energy required to recycle lead acid batteries is way way greater than any fuel efficiency gained.
Put a digital voltmeter in a ciggy port. When you lift your foot off the gas or hit the brakes and voltage climbs up into the 14s from some point well below that, your vehicle is sacrificing battery life for a miniscule amount of extra MPG.
Thank you again C.A.F.E.
Not much one can do when one's vehicle is trying to trade battery life for MPG, although the battery will still appreciate being brought back to full charge via a grid powered charger, even if the next time the car is driven its voltage regulator will allow it to discharge to the point it can accept higher amperage, in that sub 80% charged range.
A marine battery will have slightly lower CCA figures, weigh a bit more, and be slightly more tolerant of deeper cycling, but warranties on marine batteries are shorter or Non existent, and of course not available in all BCI group sizes.
Marine batteries also seem to require longer periods at higher voltages to reach a true full charge when they are regularly cycled, and it is hard to get a 'smart' charger to seek abd hold these higher voltages for the duration required to max out the specific gravity.
A hydrometer will prove that most smart chargers, are stopping the absorption stage(constant voltage held in the mid 14's) prematurely, and dropping to float voltage.
This has been coined 'premature efloatulation' in the marine world, and its occurrence is a battery murderer.
And modern vehicles do it on purpose as it seems the only times they allow 14+v is when coasting or braking.
Since I deeply cycle my batteries intentionally, I modified my voltage rgulation and can choose voltage by a dial on my dashboard, next to digital ammeter and voltmeter, but even my 30 year old vehicle would immuninate the check engine light if I did not trick it with a 10 Ohm 50 watt resistor, into thinking it is still connected to the externally regulated alternator.
and 30+ seconds of 14.8v when the engine computer is seeking only 13.7v still lights up that CEL and it goes open loop and MPG and power suffer until I reset engine computer.
It is a shame that automakers have sacrificed battery life for extremely slight possible improvements in mpgs, buut battery makeers also have done the math and statistics and cheapened their products to an acceptable number of warranty claims.
