You might consider using a voltage sencing relay from you starting battery to your other batterys (one for each extra battery) and the out board will charge the other batterys when it is being charged. Also when you kill the engine it will disconnect the TM battery or other battery from the starting battery so that it will not discharge the starting battery(s).Checkout Voltage sencing relay it drops out at 12.8 volts and kicks in when starter motor is running..