For a hobby project I would like to switch between two power supplys. The two power supplys are two 6s LiPo batteries. If one battery is empty i want to immediatelly switch to the other battery. (the load is a motor) For switching a 90A, 2.2mOhm n-channel MOSFET with a highside driver will be used. I already did some research and made a first schematic drawing: 
The maximum current is about 50A and the battery voltage varies between 18V (empty) and 25V (full).
So if e.g. BATT. 2A is empty (18V) and BATT. 2B is full (25V) and Q1 is therefore switched off and Q3 on (to only use the full battery), there is about 25V potential at the source of Q3 (and hence 25V at source of Q1), but only 18V at drain of Q1. So there is a difference of -7V between source and drain of Q1 and the parasitic diode of Q1 between source and drain will probably conduct which leads to an short circuit (at over 100A battery current for a short time, so the MOFET will be destroyed).
How can I avoid that transverse current?
Thank you for any help in advance!
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