how to charge one bike battery pack ?

This case would be optimal. Which batteries do you have for it?
Do protected batteries fit in the case?
Has your lamp a low voltage protection?

Don’t buy the soshine h2, take a look at the review here

Banggood.com has a lot of nice and cheap brand Batteries, don’t buy any fire batteries as they are almost always bad.

The BLF coupon code gives additional 8% on banggood

I4 pro for 21$

some example unprotected cells…
Good Samsung cell for less than 5$ Not so much capacity but lots of cycles

Panasonic ncr18650pf 2900mAh 7.20$

If protected cells fit, you should consider them for extra safety
Sanyo 2400mAh with lots of cycles for 12$ a pair

Panasonics ncrbs are the cells with the highest capacity, these are cheaper at wallbuys.com
Ncrbs for 15$ a pair