Not fully charging batteries

[hhow55]

No matter what type of batteries your truck has, to be cost effective long
term, you need to always ensure that the batteries are not only of the same type,
and capacity, but also close to the same age. Keeping your batteries as a 'Matched
Set' as much as possible will increase their lifespan greatly. As a comparison to
this analogy, think about your truck engine for a moment. What do you think
would happen if each cylinder of your truck engine was a different size and/or
type. Your engine would not only run very rough, but the cylinders doing the most
work will always cause you the biggest problems. Truck batteries need to be the
same capacity, type and roughly the same age to ensure they perform together as a
group. Buying used batteries, and/or mis-matching them will only result in more
battery problems long term, costing you unnecessarily.

Btw, this is from Rawze book.