Batteries designed to be used in off grid applications need the proper charge current and maximum discharge current to achieve the longest possible battery life. This charge rate is being referred to as C-rate and is a percentage of the AH rating. Most battery manufacturers recommend a minimum C-rate of C 5 to a maximum of C 20.
If you have a battery bank of 8V X 415 AH batteries in series for a nominal 48V battery bank your minimum C 5 would be 20.75 A to a maximum of C 20 or 83 A. The maximum recommended continuous discharge rate is in most instances the same as the C 20 rate.
For flooded lead acid batteries the consequence for under charging is sulphation. The sulphuric acid and the distilled water start to stratify and the sulphuric acid starts crystallising effectively reducing the surface area of the plates. With a lot of effort, time and baby-sitting sulphated lead acid batteries can sometime be brought back to life with a de-sulphation process. However de-sulphating batteries also takes lifespan away. The process to prevent sulphation is called equalisation. This is a process where the regular charge voltage is exceeded by a certain percentage to dissolve minimal amounts of crystallised sulphuric acid.
On the opposite side, charging batteries at a too high C rate will cause excessive heating of the batteries, excessive water loss and excessive gassing of hydrogen gas that is explosive.
So be careful with the addition of PV modules and upgrading charge controllers without consulting with a professional.
For AGM batteries under charging them, as long it is not deficit charging, is a much lesser problem as there is no liquids involved that can stratify. The electrolyte is absorbed into glass mats. However, over charging will dry out an AGM battery bank. Should that happen there is no way to re-instate the capacity of the batteries as they are completely sealed.
As some may see, there is a bit more than just going out to buy batteries and PV modules (Solar panels), hook them up and hope for the best.
As always, have a sunny day.