[time]

I have seen the 'second' battery referenced many times.
I believe there is one 'normal' rechargeable battery, and the second is the actual power converter that transfers light into energy. So technically its 1 battery.
Once the eco drive module fails, the battery will not charge, and will die.
It is possible that this is what the person is referring to.

 

[time]

I think the primary is the actual eco drive system which acts like a battery/charger and stores energy while transferring it to the battery.