Auxiliary Power - 12V power and starter motor considerations
Replacing the 36lb lead acid battery with a custom 16 Ah Headway LiFePO4 pack and integrated BMS is an attractive option, especially at 4 lbs. However the discharging / charging requirements for the Headway batteries are more stringent. For most applications, going with a smaller 45 Ah 19 lb lead acid battery connected to a DC/DC converter from the traction pack is a more manageable solution, as is the current solution for the Mustang.
one step at a time
A system for delivering auxiliary power allows for the traditional vehicle systems to function in the event that the traction battery is disabled. For example, items such as lights, horn, radio and especially the engine control features (coil/distributor) should be powered by an auxiliary power system.
Hybrid vehicles may be designed with start / stop functionality. This design allows the electric traction motor the capability of starting the gas engine. Otherwise, the auxiliary battery must also be capable of starting the gas motor.
Three options for an auxiliary power system are presented. The first is the usage of the typical vehicle battery, a larger lead acid battery.
The second being a smaller lead acid battery of approximately half the weight and reserve power.
The third option is a LiFePO4 custom Headway battery pack with an integrated Manzanita Micro MK3x8 Digital Lithium Regulator and relay system. Battery specifications are listed below.