Hi J-P,
Sometimes we see what we want and do not see the rest.
That 'Hint' says a 'bec' !!
The guys are talking about a 2nd battery , all be it a BIG one or in fact 2 cells from a BIG battery.
I will leave you to work it out.

Edit; I believe that the Emcotec Dual batt unit is the ideal way to use Chris's and Jim's idea - which is just brilliant for anyone that may be tight on the weight limit - like me.

Brian