Here is a hypothesis of what may be happening with regard to the
auxiliary-battery problem with a plug-In Prius:



Hypothesis:

In EV mode the hybrid system, after using the auxiliary battery to start
the system, does not charge the auxiliary battery from the system
battery in order to reserve the system battery energy for motion.
Instead, it uses the DC-to-DC converter to provide 14 volts for the
dash, etc. Out of EV mode the DC-to-DC converter does charge the
auxiliary battery. Thus, a plug-in Prius does not charge the auxiliary
battery as much as a Prius without the possibility of EV mode. So, a
weak battery in cold weather might not get charged enough to keep it
above the voltage level needed to start the plug-in system, or even
enough to start or keep running the hybrid system.



If this hypothesis is correct, a plug-in system probably could solve the
problem by using a AC-to-DC converter off of the plug to charge the
auxiliary battery while the plug-in battery is being charged. I have not
had the problem, even in cold weather, since I installed a separate plug
out the trunk lid connected to a trickle charger that is connected to
the auxiliary battery.



I got this idea because the auxiliary battery ran down on my electric
pickup, which I detected because I had a device connected directly to
it. However, the pickup still ran because the battery is vestigial and
no longer is needed to start and run the electric motive system.



Dave Roper



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