Cell checker

Having one of these guys in your tool box is a solid decision. Being able to easily check the voltage and capacity can be crucial in maintaining the longevity of your batteries.
 
This gizmo from Overlander Batteries allows you to check:
Total pack voltage
Total battery capacity (0-99%)
Individual cell voltage (Lithium)
Individual cell capacity (0-99%) (Lithium)
Voltage difference between Lithium cells
 
It also has a balance function for Lithium packs, bringing all of the cells within the pack to the same voltage, maximising the life of your battery. It also has a built in servo tester. Handy if you also dabble in RC modelling but totally useless for us nerfers.