Power Distribution

Porsche 928 Electrics

Electrical information & supplies for late model 928s

Power Distribution System Upgrade with Auxiliary Battery

Power Distribution Systems Design Issues:

I have complained for a long time about Porsche's design of the basic power distribution for the 928. As it evolved over time it became more and more of a compromise of practicality Vs. apparent design goal.

I believe Porsche original design goal we to locate all the relays and fuses on the central electric panel for clarity and standardization. However given the varied locations of all the equipment this creates extra wiring distance and the cabin location of the Central Electric panel and rear battery mounting battery create additional supply challenges.

The extensive use of electronics at the cutting edge caused Porsche to do some odd things to reduce noise for the engine control and safety equipment electronics that would generally be considered very bad basic electrical practice.

Examples are the separate small feeders direct from the battery to the 2 main ECU's, to the ABS & PSD systems and to the radiator cooling fan driver stages.

Some of these feeders are remotely fused while some or not fused at all (ECU's).

Meanwhile the supply to the Central Electric panel is fed via 2 rather undersized conductors and is not directly fused at all.

Backup/Auxiliary Battery:

In my 8 years of ownership my car has left me stranded with a no start condition only 3 times - every time has been due to a battery fault. Anticipating this continuing I decided I wanted to add an auxiliary battery being charged while running and capable of starting &running the car alone and yet significantly smaller in size & weight to the stock battery.

Combining power supply routing & functional improvements with the auxiliary battery allows solutions to some of Porsche's concerns on electrical noise as well as addressing some of my own concerns on electrical safety & reliability.

However its a lot of work and certainly not for the faint of heart or for anyone without solid high current fabrication skills and a willingness to spend on top quality components. If you implement this poorly you will likely make your car much less reliable...!

Steps to the solution - Concepts:

I like the Central Electric Panel Concept but it may not be possible to have all the fuses in one location

However a small number of fuse locations must be maintained ideally only 2-3:

Existing Central Electric Panel (and in my case an expansion panel due to running out of space)

New Battery Adjacent Fuse Panel

New Engine Area Fuse Panel

Fuse/Relay locations should be either inherently dry or specially protected

Fuse/Relay locations should be relatively easy to access (similar to CE panel)

All Power feeders should be fused as near to the source as possible (batteries/alternator)

Only the Battery/Starter/Alternator direct connections should be un-fused.

Key Equipment:

I chose an Odyssey gel type battery for my auxiliary - these batteries are small and reasonably lightweight but are quite easily capable of starting the 928.

I also specified a large battery configuration switch to connect in the auxiliary battery or to switch over fully to the auxiliary battery (I already had a usable main battery disconnect switch).

I use a high current relay to link both batteries together for charging once the engine has warmed up but also have a a diode bypass to ensure fail over of the primary over to the backup battery since I use it in an interesting configuration to power some hardware directly (ECU's, clock, alarm etc)