trying to build is a Lithium Ion Battery

Hello and thank you for reading our post, what we are trying to achieve is a Lithium Ion Battery that will operate at least 8 to 16 hours of use, with a preferred weight is half a lb or Less per battery. We are trying to create a reliable LI battery that is easily charged by both solar power and or wall charging either indoors or outdoors for backpack purposes so as not to be limited by weight or capacity. Any guidance on configuration, pitfalls and potential hazards is welcome. Here is what we have learned so far that in order to build the power pack from the inside out to the completed, usable product we will require: IE from the battery cells to the PCB to the power cable to the barrel plug, please keep in mind we are not experts this is a home created item to fit our needs. We are looking for advice on creation or possible schematic design. Question, if you know of a source of supply that will meet our needs would you recommend we purchase the battery pack or rather build the battery pack? What are the advantages to either scenario?

What we are trying to build is a Lithium Ion Battery for a device that’s design parameters are rated for the following sources:

PSA2 (Wall Unit Charger w/ Interchangeable International Plugs)
BAT4 (8 Hour Battery Pack) 4SP1
BAT8 (16 Hour Battery Pack) 4SP2

The existing Lithium ion Battery Unit in use is a:
BAT8
14.4V, 5200mAH, 75Wh
Configuration 4SP2
With what we think is a EIAJ-05 or a J5BP5 barrel plug that goes from the battery to the device specification can be provided as soon as we can acquire a caliper
The Wall Plug in Current Use is a :
PSA2
Input: 100-240V
50-60Hz, 1.0A
Output: +18V—, 1.67A

The components we must specify, if we are to fabricate, are:

Male Barrel Plug (From Device to Battery Pack)
Wire that goes from plug to Core question what kind of wire would you recommend? Or how do I gauge the existing wire?
PCB & Charge Indicator Light On the Housing
LI core that is configured in a 4SP2 w/ UL parameters of 14.4V, at least 5200 mAH (What might happen if we up the mAH?)
Female Barrel Plug in the Housing(to accommodate wall or Solar Panel charger)
Housing to Accommodate Everything
We are considering a EarthTech R14 (14W) as a solar source in the field. Our existing wall plugs should be fine for locations where electricity is available.