High Drain Best Lithium 3.7V 5000mAh 26650 Panasonic Li Ion Battery

Visually inspecting your Panasonic 26650 batteries
The appearance of your battery

Should not have

Scratches
Rust
Discoloration
Dirt
Deformation
Leakage

You should also take a look at the pictures below and see that they match your own batteries.

Profile Shots

Notes on building lithium battery packs
If you are building rechargeable battery packs, Panasonic cautions the following on design:

Designing a lithium battery back

Can not connect to unauthorized chargers, equipment, or other devices
Terminal shape should be designed to prevent short-circuiting
Over current protection function to prevent external short circuits
Terminal shape designed so it can’t connect backwards and short-circuit
Design should prevent static, electrolyte, or water ingress* issues
Protection circuit should be accessible to be inspected
If electrolyte leaks out of the battery, it should not be able to reach the PCB
Cells should be fixed in place so they can’t jostle around
Use glue, and be careful if ultrasonic welding
Should be sealed shut so end users can’t touch cells
The protection circuit for the battery pack requires:

Overcharge protection
Over discharge protection
Over current protection
Avoid connecting cells together by soldering. Many DIYers weld directly on the top-cap of the batteries which can (pretty easily) lead to short-circuiting. As the negative and positive terminal meet right near the top of the battery, a bit of solder which slides to the edge of the battery is all it takes for a thermal runaway event.

The proper way to connect batteries is with lead plates and spot welding.

Water ingress is the leaking of water into an enclosure.