Well Matt, I suppose this is your personal opinion, which I certainly respect. But if you take a minute and check any of thousands of battery charging guides for Lead-Acid batteries available online you will find that, yes, a battery can store a charge while being applied the voltage you mention (13.5V), BUT it will never reach 100% charge state this way. A Lead-Acid battery in fact can only be charged to 80% of its capacity (at best) when the terminal voltage reaches 14.4V (or 2.4V per cell), and only THEN if this voltage is held constant for some time it will charge up the remaining 20% of the battery capacity. So if your alternator output is below 14.4V, you will have a chronically discharged battery that can never reach full capacity. This condition will cause Lead-Sulphate (a normal byproduct of battery chemistry during a discharge cycle) to accumulate on the plates, a process better known as "Sulfation", which slowly but surely robs the battery cells of useful plate area exposed to the electrolyte, until the battery is literally choked and is no longer able to crank the starter.

To fully charge a Lead-Acid battery involves a two step process. The first step (called the "bulk charging") will charge the battery all the way to 14.4V, from there on it will go to an "absorption charge" mode. A battery can only be charged up to a point in bulk charge mode, and it is impossible to reach a 100% charge state without going through the absorption charge step. If your electrical system only reaches 13.5V (or 14.1V in my case), it becomes more then obvious the battery will never even get close to the absorption charge step.

That is the reason why most if not all of the so called "Intellingent" or "smart" battery chargers feature a two (or even three) step charge process. First they bulk charge the battery in constant current mode (senses voltage increase), then when it reaches about 2.39~2.4V/cell the charger will switch to constant voltage mode (senses charging current) until the battery reaches 100% capacity. The charger will know when this point has reached because charging current will taper off. The third step most of the better chargers also incorporate is a float charge that will keep the battery at a nominal 13.4V (@ 25 deg C) since the cell voltage varies .022V (or 22mV) per degree C. This happens because the cell's internal resistance decreases with temperature, and the charger should ideally adjust the float voltage according to ambient temperature. If you ignore this, you will apply too much voltage to hot batteries and fry them, or insufficient voltage to cold batteries and so not fully charge them. BTW, most BOSCH voltage regulators have this temperature compensation build in, but still have the lower then optimum voltage setting shortcoming.

Since the only charger available to a Lead-Acid car battery is the alternator, this is a far from a perfect situation. It works well if the battery is not deeply discharged. But the alternator being essentially a "dumb charger" tends to overcharge batteries that are very low and the overcharge can damage the battery. Hence this is another reason why it is important to keep your battery at an optimal charge level which can only be attained if your electrical system is capable of reaching about 14.4V.

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W163 ML230 EURO SPEC 1999
W124 300E 1986