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Optocouplers are useful when controlling mains voltages to avoid any risk of electrocution or mains voltages appearing on the low-voltage control circuitry. When driving low-voltage DC motors, they don't serve much purpose.However, when driving any inductive load, the output stage needs to have a diode that 'clamps' the output, preventing the back-emf of the inductor from rising above the positive rail and potentially damaging the output transistors. In the L293D (but not the L293), each output is protected by an internal clamping diode. The outputs are rated to 600mA continuous, 1200mA peak. If you use motors that keep within those limits, there's no real risk of damage. The TI datasheet for L293/L293D is available at:http://www.ti.com/lit/ds/symlink/l293.pdf
To be accurate, amp-hours are a unit of charge, not energy (charge x voltage = energy). But Oddstr13 is correct that you should be saying "Well, if your battery miss 75% of its consumable 15Ah capacity, then it needs 11.25Ah to be pumped into it. As a charging process has no more than 75% efficiency, you need to give ~15Ah to the battery to get it fully recharged. With 150mA it will take 100 hours, or more than 4 days to fully charge your depleted 50Ah nominal capacity battery."