If you are charging two batteries set up in parallel, will they always see the same charge current?
The voltage on a lead acid cell varies by about 10% or so with the state of charge. So (all other things being equal), the less-charged battery gets more charging current and things tend to even out. This effect distributes the charging evenly both across the surface of the plates, between the plates within a cell, and among the component batteries of a battery bank.
As others have pointed out, the big caveat is "all other things being equal". A lot of things besides state of charge affect the voltage of a battery: Age, temperature, geometry, chemistry (SLA vs. glassmat vs. gell, water level, etc.), amount of sulfation, degree of equalization between cells, whether there are defective cells, and so on.
If one or more of these factors are NOT equal the battery tending to lower voltage comes to full charge while its paralleled partner is at significantly less than full charge. Then on discharge the less-full one reaches minimum charge while the more-full one is still above it. The result is that the capacity of the paralleled bank is reduced by the percentage difference between the most-full and most-empty battery. If the lowest one is at 80% charge when the highest one is full, you lose 20% of the ENTIRE BANK's capacity.
So ideally all the batteries you intend to parallel should be purchased at the same time from the same manufacturing lot, wired and mounted with care to equalize their thermal (ambient temperature and cooling) and charge/discharge environment. Failing that, at least they should be the same model from the same manufacturer and with about the same history.
But if, say, you get a good deal on a bunch of used forklift batteries that have been rotated out on scheduled replacement rather than failed in service, go for it. Then charge 'em up, check 'em for approximate balance, hook up the ones that are close, and maybe try to rejuvinate the rest into a better match. Check 'em from time to time for balance and pull those that are starting to cause problems.
Having five sets of batteries rather than four will compensate for a 20% mismatch in charging states, and having a whole bunch of big, cheap, batteries will give you more power storage than a small number of pricey, minty-fresh, perfectly balanced, small ones.