When you say "enough plate material left to provide enough amps for the starter motor" -- would a visual inspection clue you in on that? ie. would the affected plates be whitish?
Probably not. "Whitish" means sulfation, which is related to plate area and relates to poor state of charge history. The white sections are a block to chemical action and electrical flow.
Or could the battery look 100% fine from a visual inspection, read 13volts after charging, and still have "not enough plate material left" ?
Yes, that is so. When the battery discharges, ions flow from plate into electrolyte. Recharging puts the ions back on the plate. But it is only the separator technology and design that helps guide the ions back. Still, it is seldom deposited at the same place that it was removed from during discharge. The deposition of materials over many discharge /charge cycles can leave big holes in the plates reducing the area that the plates have, and the quantity of electrons it can supply in a short time period. In other words, the voltage potential can only be maintained at very low current flow numbers. If the current demand is too high for the capacity, the voltage drops to levels that aren't useful for the starter motor. Since power is Current x Amps, a low amount of either, diminishes power.
A battery has a rating. The 550 battery is rated at 12 Amps (Your CX500 is rated at 14 amps.) This rate is called "C". Normal charge should not be over 1/10 C which would be 1.2 amps. 2 amps is a little bit hot for the 550 battery. It can be used, but you have to monitor the battery's heating and also not allow it to charge the battery beyond 14.5V (for a good battery), which is 2.4V/per cell.
Batteries have different failure modes, one of which is a single dead cell. You can charge a battery with a dead cell to 13V, which overcharges the remaining 5 cells to 2.6V and they should be bubbling. Removing the charger and measuring immediately will show the battery at or near this surcharge. Let it rest and the good cells will stabilize at their full charge state, which is 2.1 V/cell. With 5 cells operating however, the voltage will fall to 10.6-ish, simply sitting without any connection to it. This is enough to condemn a battery.
But, you can actually test each cell for this. I use a nail as a sacrificial probe for the meter leads. And insert them into the electrolyte of each adjoining cell. The meter will read each cell voltage. One cell at the end will be read between fluid and battery post. A dead or bad cell will read much lower than the others.
Anyway, many assume a "trickle charger" can be applied indefinitely. This is not true unless the charger is finely matched to the battery's rating numbers. It is not the same as an automatic charger which monitor's the battery voltage for you.
For example, a car battery can have a 40 amp rate (NOT it's CCA rating). 1/10 would be 4 amps. And your 2 amp charger could be used on this battery almost indefinitely without worry (it would take a long time to charge this battery if depleted). But, the rate is too high for the small 550 battery unattended.
Finally, there are many types of battery chargers. Some are constant current, some are constant voltage. The latter changes it's current with the battery charge state and is "better" for the battery at attaining full electrolyte saturation. I can't tell from the Wabash label which type it is. It could be 2 amps at 12v and it taper's down to below 100 milli-amps at 14.5V. Many of the old, cheap chargers behaved this way. With a known good battery, you could characterize the charger's current and voltage to find out.
Cheers,
