The owner's manual states the following:
Battery TypesThere are two types of batteries used in this vehicle; a standard 12-volt battery that powers the airbags, the interior and exterior lights, and other standard 12-volt systems; and a High Voltage battery that is used to power the propulsion motors and recharge the 12-volt battery.
From the initial press media release:
NSX Direct Drive Motor
To help eliminate the response delay typically associated with turbocharged engines, the Direct Drive Motor acts directly on the engine’s crankshaft which, together with the front-mounted TMU, helps the NSX realize immediate, high-output and high-torque acceleration. The effectiveness of this design is particularly noticeable when accelerating from a standstill at low engine speeds.
The NSX utilizes its Direct Drive Motor to start the engine instead of using a conventional 12-volt engine starter motor. Eliminating the 12-volt starter ring gear reduced weight and layout complexity. To help improve fuel efficiency, the NSX comes equipped with idle stop capability. Using the powerful Direct Drive Motor as an engine starter ensures that engine restarts are smooth and quick. Engine idle-stop is also fully integrated into the operation of the Automatic Brake Hold system. Additionally, the Direct Drive Motor acts as a generator, helping to maintain the hybrid batteries at full charge to consistently support driver demands under almost any condition—even while lapping a track at full bore.
The Direct Drive Motor has a liquid cooling passage that provides consistent cooling, even when the system is being pushed to the limit, such as on a track excursion.
I am really confident that the 12 volt battery is used only to power all the ECUs and the various accessories and NOT the Direct Drive Motor attached to the engine. As for why a 600 CCA 12 Volt Battery: You need high output so that in emergencies things like emergency flashers can work for hours, etcetera. A lithium battery has power but only for short bursts.
A lithium battery cannot supply a steady current as does a lead acid battery like our 12 volt battery. So keep in mind if you replace the 12 volt battery with a lithium battery you will need to keep a special tender on the car. Not a big deal. I have also enclosed the 12 volt battery system that shows how it works in the car.
I did some research and came across a few Lithium batteries that were lightweight, having different price points and warranties. There has been some debate that some AntiGravity Lithium batteries that have the restart technology interfere with the electrical system of the NSX by turning itself off. I am not sure if this was happening because there was not a battery tender conencted or something else, however I am sure there is some more research to be done. I hope this is a start and answers some questions for us who are doing some research. None of the batteries mentioned below are direct plug and play (due to different LxWxH dimensions), and would require some modification (custom bracket to make it fit properly) unless you buy a H5/Group 24 size battery.
OEM Battery AGM 12V H5/Group 24 (600CCA)
LxWxH: 9.5x7x7.5 inches
Weight: Approx. 40lb
Model: LFX36A3-BS12 (36AH)
Warranty: 3 years
Model: 12v 23Ah Battery
Amp hours: 23Ah
Weight: 6.4 lbs
Warranty: 11 years
Amp hours: 24Ah (48 PbEq)
Weight: 7.8 lbs
Warranty: 5 years (3 direct replacement & final 2 pro-rated)
Model: 12V 33Ah
Amp hours: 33Ah
Weight: 8.6 lbs
Warranty: 5 years
Amp hours: 30Ah
Warranty: 1 year from date of manufacturing
Model: 50Ah Lithium LiFePO4 Battery
Warranty: 5 years
Model: 12V 35AH Ion Battery
Amp hours: 30Ah
Warranty: 5 years
Amp hours: 32Ah
How does "PbEq AH" capacity rating compare to lead-acid Ah ratings?
Lithium batteries compare ONLY in cranking power (starting Amps), but NOT in capacity (Amp-hour). To bring the cost closer to lead-acid, lithium battery manufacturers take advantage of the 3-4 times higher cranking power (starting Amps) and reduce the capacity (in Amp-hour) i.e. capacity is typically 3-4 times LESS than a lead-acid battery it is meant to replace. Some lithium battery manufacturers use the term PbEq (lead-acid equivalent) to make it easier for a consumer to choose an equivalent lithium replacement, but it is equivalent only in cranking power (Cranking Amps), not overall capacity (in Amp-hour). For that reason it can be 5-8 times lighter (pnds / kg). Lithium Iron Phosphate (LiFePO4) batteries have a power density 3-4 times higher than lead-acid, in other words, a lithium battery of the same Amp-hour capacity as a sealed AGM (Absorbed Glass Mat) lead-acid can deliver 3-4 times higher cranking power. A lithium battery can deliver its rated cranking Amps down to 10% charge remaining where-as a lead-acid battery delivers it’s rated cranking Amps at 50% or more charge remaining, below that cranking Amps reduces rapidly. A lithium battery will maintain a higher voltage for longer when delivering current.