V10 Battery System
I just changed out my front battery...... because it wouldn't start. Battery was dead.
Everything was off. black dash, nothing, zero, nada.
when I connected a charger to the front battery, all of the dash stuff powered up
rear battery tested perfect.
car started fine on the bypass system.
I will back Drag on this, doesn't make any sense to change over both batteries just because one dies.
the starter battery may last for years. Front AGM battery, not much change out of $500, so not in any rush to change both.
given that there are two batteries, and taking into account the difference in duty cycles, the chances of them both failing at the same time is slim.
my two cents worth on that subject.
will throw some oil on the fire.... file from VW self study program.>
http://www.volkspage.net/technik/ssp/ssp/SSP_298_d1.pdf
Page 21 clearly shows the starter battery at the rear and the consumer battery under the seat.
after a quick read through, I'm reasonably sure that if the front battery is flat, the car won't start because the startup consumers ( engine control unit) are supplied from the on board power supply battery.
if the front battery is flat, the consumer items are supplied from the starter battery via the bypass system.
in normal condition, the front battery initiates the start, but the actual starter current is supplied from the rear battery.
Seems that if the starter battery is flat, the power supply battery is switched in and does the start.
I'm guessing this may be why the power supply battery is bigger. in the last case, this power supply battery has to supply the startup load as well as all of the consumer load.
Ian