If battery charge warning light was not on light the last time you drove your vehicle the alternator is working correctly.
The Battery Tender Plus is clever than most other chargers and is able to get past this problem by adjusting for temperatures changes when charging.
If the battery is sealed, the battery could have low electrolyte, especially in a hot climate.
Most newer vehicles have a voltage regulator which is part of the alternator, but older vehicles have the regulator as a separate part.
With batteries in series, if discharging unevenly or use of non-identical batteries can not be avoided, then use an isolated multi-bank charger, single bank charger with an external diode isolator adjusted for the voltage lossor combiner to recharge all of the batteries at the same time.
Capable to jump most vehicles.
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Just follow up with the wire brush, if necessary.
This is because more CCA requires greater plate surface area and in order to fit more surface area in the same space, this means thinner plates.