The ideal discharge curve of a lead acid battery is on a flat discharge curve, the amount of current that the battery can deliver remain more or less constant for quite a while and then drop off rapidly when the limit of it capacity has been reach.
Sealed lead-acid batteries are generally rated with a 20-hour discharge rate. That is the current that the battery can provide in 20 hours discharged to a final voltage of 1.75 volts per second at a temperature of 25 degrees Celsius.
Lead-acid batteries can only be discharged up to 50% before irreversible damage occurs. In practice this means that you will only be able to use half of your battery capacity. Semi-traction batteries such as AGM en Gel lead-batteries are often used as Marine batteries for water sports and angling. These batteries can often be discharged up to 70%.
A deep-cycle lead acid battery should be able to maintain a cycle life of more than 1,000 even at DOD over 50%. Figure: Relationship between battery capacity, depth of discharge and cycle life for a shallow-cycle battery. In addition to the DOD, the charging regime also plays an important part in determining battery lifetime.
Table 4 shows typical end-of-discharge voltages of various battery chemistries. The lower end-of-discharge voltage on a high load compensates for the greater losses. Over-charging a lead acid battery can produce hydrogen sulfide, a colorless, poisonous and flammable gas that smells like rotten eggs.
Lead acid batteries typically have coloumbic efficiencies of 85% and energy efficiencies in the order of 70%. Depending on which one of the above problems is of most concern for a particular application, appropriate modifications to the basic battery configuration improve battery performance.