[Al Skierkiewicz]

When you put batteries in parallel they will self discharge to each other even if nothing else is connected. You can use diodes of the appropriate current rating but each diode has a voltage drop. Schottky diodes have 0.2-0.3 volts and silicon has 0.6 volt. So we are talking real world, generally batteries have more than 0.5 volt cell voltage. Flashlight cells and other "dry" cells have 1.5 volts per cell. A 9 volt radio battery has 6 1.5 volt cells inside in seeries. In our robot battery there is 6 cells in series of about 2 volts per cell for a an overall rating of 12 volts. Each cell can produce 18 amp hr at specified discharge currents.
When you put batteries in parallel one of them will always have a slightly higher voltage. Even if that voltage is only 0.002 volts higher than the other cell, current will flow into the lower battery. As they approach the same voltage the lower cell will now have moved slightly higher and it will start to feed current to the other battery. This see saw effect will continue until both batteries are depleted (zero volts)