Well unless the array power was less than the minimum 'do nothing loads', particuarly with the sholder heater, in which case the battery is not going to charge anyway. The issue is: do you want to conserve power in the battery or do you use every drop trying to stay alive..

...the Great Ghoul of Mars, that has claimed so many other probes, finally turns its gaze on little Oppy and stops her in her tracks...

When the tau levels rise to 1, it limits the activities the rovers can perform and values of 2 or greater can be fatal. During most of Spirit's mission on Mars, tau values have ranged anywhere between 0 and 1. Not surprisingly then, there was real concern over the holiday weekend about whether or not Spirit was going to make it to its anniversary when the tau suddenly rose to .9 and then tipped to 1 at the end of December, and the rover's power levels began dropping....On Sol 1066 (New Year's Day 2007), the tau rose to 1.136 causing the rover's solar array energy to drop to a low 276 watt-hours, a level it had experienced briefly before in the depths of winter at Low Ridge....Once the engineers at JPL reanalyzed Spirit's power situation, they realized the rover could survive on less power now that spring was moving in. "For this time of year -- when temperatures are getting warmer and it's not getting super cold at night -- the amount of power that Spirit needs for survival is considerably less than it was in the wintertime. What the rover needs to survive in winter is something between 200 and 250 watt-hours, whereas now we can survive on something less than 200 watt-hours, so even with relatively low power numbers we're able to safely operate the vehicle and get lots done," Squyres explained.

There are so many questions I could ask here - eg, if the batteries are receiving little or no charge from the arrays, but are fully charged to begin with, how long will they retain their charge?

I don't know the specifics of the PCB, but is it intelligent enough to be preprogrammed for an expected wake up array voltage of X volts (when X is a lower number right now ) - which could be estimated to be 11am - then the master sequence for the day is sleep all day, wake up for the earliest Odyssey pass at which point you both uplink for the next day and downlink for that day, and then enter deep sleep till 19 hours later.

Edited to add: Just noticed Doug's comment re the heater. Doh!, I'd forgotten that and it is quite likely that the temperature is high enough around the middle of the day to keep it off so that's a relief.
Also with reference to James Canvins comment regarding the wakeup current - I thought that wake up current was 2.0 amps which would keep it asleep until quite late in the day. I don't know what the panel output voltage range is but assuming it is only 10v then it would only just rise above 2A at the moment.

Ah, I'm glad sombody knows this. I've only ever been able to find reference that all *loads* are removed from the battery. As clearly something of the solar array power gets to the BCB* (as it is this power that tells the BCB to reconnect the battery in the morning) I was never sure if the battery could be charged while in deep sleep.

To add a little bit more to our stress : is there one of the two rovers in restricted sols at this time ?

James

I'm doing some research to write about the storm and just thought I'd share some now-comical quotes from one of A. J. S. Rayl's rover updates from January 31 this year, regarding tau values measured by Spirit:
...
How naive we were...I'm thinking of you, Opportunity, hunkered down under storm-darkened skies...

Spirit is on restricted sols, but given the current situation it's not a big deal: plan for solX = plan for solX+1 = plan for solX+2 = tau measurement.