Repair ...Fixin' what's broke
Maintenance ....Fixin' what aint broke...yet.

When you discharge a Lead-Acid battery the lead oxide on the positive plate and the lead on the negative plate combine with the sulfuric acid in the electrolyte to produce lead oxide on both plates, water and free electrons (a lot of them). When a battery is fully discharged the positive and negative plates are chemically identical (lead sulfate) and the battery can then, theoretically, be charged in with the opposite polarity. 

Lead, lead sulfate and lead oxide all have different densities so that during the charge and discharge cycles the plates are expanding and contracting slightly. One of the limiting factors in battery life is mechanical fatigue of the plates. Over time they disintegrate and , in wet cells, fall to the bottom and short the cell.

Batteries designed for starting engines have a low cell resistance and a high peak current capacity. Batteries designed to provide standby power are optimized for maximum power density and are referred to as "Deep Cycle" type, as they are designed to be discharged until their cell voltage falls to a fraction of the initial voltage.

A typical (in this case) battery is composed of cells. Each cell produces a nominal 2.0 volts so several cells are connected in series to produce the needed voltage. Because they are manufactured at the same time, these cells are nearly identical in performance, which is a good thing because if one cell fails the battery fails. A battery only has the capacity of the weakest cell. Because both charging and discharging produce heat and the cells are stacked side by side, the middle cells run a little hotter than the cells on the end and therefore end up having slightly different performance. This difference is small and insignificant MOST of the time. The time it becomes significant is when the battery is completely discharged.  During a complete discharge, one cell reaches exhaustion first but it is still part of the circuit so the other cells are still trying to push electrons through it. What happens when you push electrons through a battery rather than it doing the pushing? It charges, but it charges in the reverse polarity (remember the plates of a discharged battery are chemically identical). This "cell reversal" is very damaging because a large portion of the remaining charge in the stronger cells is dumped in to to weakest cell. The mechanical stresses on a "reversed" cell are very high due to the density changes mentioned above. To make matters worse the internal cell resistance increases  as the battery discharges so the batteries ability to accept a charge current also decreases. This is why the charging current on a completely dead battery starts out low  then climbs rapidly and then falls slowly as the battery charges.

Battery life is frequently specified in charge discharge cycles and a cycle is usually specified as a percentage of capacity (e.g. 80%). Using only a portion of the capacity will extend the life and using ALL the capacity will shorten the life. Running the battery all the way out to cell reversal will GREATLY shorten the life. How much is that? I can't say. But it would be a good idea to replace the badly abused battery at the next annual or major service.

The energy capacity of a battery decays with time and use. If you wait until you notice that your battery performance is marginal it will likely be the time you need it most. It is better to replace on "time" as a maintenance item than on "performance" as a repair item. Depending on service, temperature swings, and flight hours, replacing your battery every two to three years would be a good plan.

Regards
Brent Regan