To assemble my auxiliary battery pack, I purchased four Headway H-38120P LiFePO4 cells on the recommendation of Dimitri at CleanPowerAuto LLC, maker of the MiniBMS. These cells are 3.2 VDC each, which result in 12.8 nominal volts when four are ganged together in series. My meter showed an actual 13.4 volts of factory charge.
The auxiliary pack is electrically tied in parallel with the NG1 DC/DC converter/charger, which makes sure the auxiliary battery pack is topped-off every time the main pack is charging. Conversely, when the main pack is under momentary high current load (such as acceleration), the auxiliary pack chips in to prevent a brownout of the 12 volt accessories. Headway makes it easy to modularize and scale your install with their spacer blocks and bus bars. In the shot above, the cells are ganged together using four interlocking spacers, and linked in series with three bus bars.
The perfect spot for the auxiliary pack is on the backside of the DC/DC converter bracket, held in place with Velcro Extreme Titanium strips. The placement puts the pack in proximity to the DC/DC output, making it easy to tie into the 12 volt system. These lithium cells are rated at 8 amp-hours, and will output a continuous 30 amps for about 10 minutes, or a momentary peak pulse of 100 amps. The auxiliary pack also performs the critical function of supplying emergency accessory power in the event of catastrophic failure. That boils down to functioning headlights and an operating BMS if the DC/DC shuts down, and hazard lights if everything comes to a complete halt. Hopefully, these auxiliary cells are an insurance policy for an emergency that never happens.