As of late August 2019, the official JPL Open Source Rover specifications call for this battery pack. Based on specifications listed on that page, it appears to be built from 18650 Lithium Ion battery cells in a 4S2P configuration. (4S2P means four cells in series, two sets of them in parallel, for a total of eight 18650 battery cells.) The key feature that made this pack desirable for JPL is the extra safety it offers: this battery pack features an integrated battery protection circuit board backed up by a polyswitch. This is great protection against battery abuse such as over-charging and over-discharge including short circuits. Like many facilities working with leading edge engineering, JPL had its own experiences with runaway batteries so it's no surprise they would recommended the safest thing available.

The safety, however, comes at significant cost as the pack costs over double that of a commodity battery pack popular with remote control vehicles. (Multi-rotor aircraft, monster trucks, etc.) And that's before factoring availability and its impact on shipping costs. The rover specifications already include a 10A fuse on board, plus a power monitoring module that can be programmed to sound an alert when the battery has been discharged too low. This provides a baseline level of protection so rover builders like myself can choose to forgo the belts-and-suspenders safety of a premium battery.

But in order to use commodity battery packs, we'll need a different battery tray, and that's where this project came in. It also makes the battery more easily accessible via a rear door for charging, replacement, or in the worst case scenario, yank it out of the rover quickly in an emergency.

This battery tray was designed for a 4S LiPo battery pack (*) with a hard outer shell for physical impact protection, and the tray bolts on to the bottom plate of rover body. CAD file is an online Onshape public document for anyone to modify to suit different battery packs. For those who don't need to make modifications, ready-to-print STL (and DXF for updated rear panel) have been posted on Thingiverse, and a video walkthrough has been posted to YouTube:

https://youtu.be/ArVRFoZtYyA

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.