Backup battery

[Gringo]

Sorry if this question has already been asked, but I've searched and can't find the answer.

I now seam to be running my Agattu on longer & longer rides, getting closer to max battery range. I'm now concerned about running out of juice with miles of hilly peddling to get home.
Here's the question.
So before spending my hard earned on a new lipo (about £45) what are the pitfalls to putting a 7s 5000mah 25c lipo in place of the std battery. like having a reserve

Note, my std battery has 5 connection points 1/+, 2/C (0v), 3/22.9v, 4/28.4v, 5/28.4
The battery connects to 3 pins on my bike that match these on the battery 1/+, 3/22.9v & 4/28.4v
Thanks in advance for any advice

 

[Gringo]

A bit more reading and I think I've found what I was after !
The 2 outer pins on the drive unit = + & - the middle pin is for the Battery Meter.
My plan would be simply to disconnect the main battery ( if & when its to low to get me home ) and connect a 7S 5 amp lipo (£43) in its place ( I'd use a low voltage alarm (£2) plugged into the balance lead )
sound like a plan