I am curious, Steve, how does a battery become overcharged?
Easily. It happens when you try to charge them beyond the point at which they can store any more charge.
An analogous question is "How does a water tank get too full?"
I understand that the voltage source pumps electric energy into the battery.
That's a reasonable analogy
Let's take an AA battery, as an example, with 1.5 volts made available when it's 100% charged.
Firstly, when you 1.5V battery, I hope you are talking about a rechargeable (say NiMH or similar) battery, NOT a standard non-rechargeable AA battery.
In fact, depending on the battery, it may have 1.6V to 1.7V across it when completely charged (for a non-rechargeable battery). Rechargeable batteries will have between 1.2 and 1.6V depending on the chemistry (NiCd are 1.2V)
After using the battery some, the voltage drops to 1.4v. Let's ignore the calculations used to predict how much energy is still available in the battery because that will depend too much on its chemistry--I'm to understand--and varies from one type of AA battery to another.
At this point, the only thing we're concerned with is that the battery is NOT full, and that its maximum voltage (open circuit) is precisely 1.4v.
OK, I'll assume that.
To (re)charge it to 1.5v, the battery charger needs to deliver a sufficient voltage potential. Meaning, if the battery charger provides only 1.0v, the AA battery will NOT be recharged (instead, the battery would try to "charge" the battery charger, if such thing were possible).
That's correct so far...
What will happen if the charger provides EXACTLY 1.5v, at let's assume more than adequate amperage? Will the AA battery EVER be overcharged in this situation?
Well, it depends on the chemistry, the temperature, etc. Some rechargeable batteries WILL be damaged is charged this way. Others may not get charged fully.
Or will there finally be reached a state of equilibrium at 1.5v where the battery is charged no further because its charges sufficiently "push back against" the charges of the battery charger?
In some battery type (lead acid is a good example) float charging works like this. Most battery technologies will withstand this for some time without a lot of damage, but it is not recommended for any of them (aside from Lead acid).
Even with Lead acid batteries, the charge voltage is lower for float charging than for a periodic charging.
Li-Ion batteries are closest to Lead Acid in that they can be charged from a current limited voltage source, however the recommendation is to terminate the charge when the current delivered at the voltage limit falls below a certain value.
I'm guessing battery chargers are designed to deliver more voltage than the battery is ready to accept, but only because as the state of equilibrium was approaching, the rate at which the battery charges would slow to a crawl.
In a manner of speaking this is correct. Whilst you are charging the battery, the internal resistance causes a voltage drop which helps limit the current. This means the terminal voltage during charge is actually higher than the voltage effectively being used to charge the cell (which must always be in equilibrium).
So you'd end up having batteries that are charged to 95% instead of the full +100% (of rated capacity). Unless you're willing to wait, literally, an infinite amount of time. Please correct me if I'm wrong on the infinity part.
In theory (but not in practice) there will be an infinite wait. In practice, internal heating of the cell can reduce its voltage, allowing the same current to charge the battery more. This can lead to the "correct" voltage resulting in massive overcharge.
This is especially important in cells where, following a full charge, additional energy is expended as heat, and where this causes the cell voltage to drop (NiMH are an example of this). Lead-acid overcharging results in electrolysis of water and that expends energy in another way that results in less heat (but still damage -- especially if the battery is sealed).
Also, a side question that might help me and those reading this to understand what's going on:
If I connect two AA batteries together, which are SIMILAR in voltage but NOT IDENTICAL (say 1.45v and 1.52v), positive-to-positive, negative-to-negative, and leave them in that configuration for a few weeks, I expect that the batteries will end up having identical voltages.
Well, the voltage at the terminals will become *exactly* the same as soon as you connect them,
Furthermore, I expect that this will NOT overheat the batteries
No, this configuration can result in high currents flowing between the batteries, lots of heat, damage to batteries, and even fires.
What actually occurs depends on many, many factors.
and that this configuration can be left intact infinitely, the batteries surviving into their shelf life expectancies.
As long as the batteries survive the equalization process, yes, a parallel connection can be left indefinitely.
But note that both (all) the batteries must be the same type, and be at the same temperature.
