Second, and this is just my personal preference, I would only use one battery at a time. (I'd still use the isolation diodes, tho.) Using both batteries will not improve performance, only extend the range. If you are commuting, I would use one battery in direction, and the other to get home. As I understand it, battery life is extended by not discharging them to the lower limit. (Alan please help me here)
I would definitely recommend running both batteries together in preference to one at a time. Combining the two batteries should produce a slight increase in the total available capacity
(and therefore total range) compared to using both batteries, one battery at a time.
Each cell's available capacity will vary according to the rate at which it is discharged, so by running the two packs in parallel you are effectively halving the load being applied to each pack, discharging them more efficiently, which will also output a greater capacity:
As each pack only needs to supply half the current, the voltage sag
(drop in voltage under load) on both packs will also be reduced, therefore the motor should produce slightly more power while drawing 30 Amps from two batteries than it would if it were drawing 30 Amps from just one, due to the slight variation in the voltage being delivered.
The above graph shows the voltage level
per cell at different loads (0.5C, 1C and 2.25C) I would not be surprised to see a voltage drop across an eight cell pack of at least 1V.
Even if the difference in voltage sag was only 0.5 volt, it would still provide an extra 15 watts of power for the motor, giving an increase of approximately 2%.
- 2x batteries supplying 30A @ 25V = 750W
- 1x battery supplying 30A @ 24.5V = 735W
So from my point of view, combining two batteries in parallel will produce a lower discharge rate on both batteries, which will effectively produce more power
and allow a higher capacity to be extracted from the batteries, which allows a greater range too.
I personally would not bother with the added expense and hassle of installing a couple of 50A diodes, as I would not want to reduce the available battery voltage by an additional 0.6V because I always want as much power as I can get.
If the two batteries were connected in series, then I may well consider connecting a suitable diode across each of the battery's output terminals, just so the motor could still run at half voltage if one of the battery switches or
BMS was to fail.
Regarding the suggestion to use one battery on the outgoing trip and the other to return home, it is highly unlikely that both trips will include identical inclines and headwinds etc., so it is inevitable that one battery is going to be discharged more than the other, which could greatly reduce the life expectancy of the most used pack in comparison to the other.
If the morning route to work was mainly downhill, the afternoon return trip would obviously be mainly uphill, this
(combined with a tired set of legs at the end of a working day) would produce a huge difference in the discharge state of the two batteries by the end of the day.
Lithium cells will generally provide a longer service life if you only consume less than 80% of their total capacity, so I think you'll agree that it would be a lot better to discharge both batteries evenly by 70% instead of one being discharged by only 50% and the other by as much as 90%.
If two identical batteries are being used simultaneously, it should not be possible for any imbalance to occur and their life expectancy should also be fairly similar.
I hope the above information helps to throw a little bit more light on the subject.
Alan 
