Problem unlocking and starting car. thought it was battery but.....

This is the first bit of trouble I've had out of my 2006 prius. I do have the transponder option so I don't have to us the fob in the dash.
Yesterday, when I tried to open the door it tried to unlock but couldn't, I heard the horn beeping very low (the alarm going off i guess) like an almost dead battery. I manually open the door and I can see that the lights are very dim. I hold down the brake and press the power button. The lights are still dim and the dash lights upfront are also much dimmer than normal. So I think it must be and issue with the 12v accessory battery. I push the power button again, wait a few seconds and turn it back on. This time it comes up with full brightness on the dome lights and the dash and the car runs and drives normal. I have driven it 4 more times and everything seems fine now. So, the question is, if it was the battery, why did it come back on like it was fully charged by just turning it off and back on again? What else can be going wrong to cause this and do you still think it is the battery? It seems weird to me that with it acting so dead and then coming back on at full power that it may be something else.

Thanks all,
Swami

 

I found the post on how to go into maintenance mode. My battery is at 11.9v instead of 12.4-12.8 like it should be. Under load it dropped to 11.5-11.8. When put in ready mode it went to 14.2 so it looks like the battery is running low.

I guess it was right on the edge of being able to get it to ready mode so the other battery could take over and let me start the car so turning it off and back on it made it to enough voltage that time to turn it on. I think my problem is I have only been driving the car for .25 miles a trip for the last year or so since I lost my job, and not every day.

Do you think I should turn off the mode that allows me to unlock the doors by just grabbing the handles until I start driving it farther to keep from discharging the battery as quickly? Do you think if I go out of my way to make sure that I charge the battery more often (drive the car more and longer distances) that the battery will recover and last a while longer since I think it just slowly discharged due to lack of driving distance and not driving for multiple days? If I do replace the battery do you recommend getting the optima upgrade for the $161 or should I just stick with the oem battery?

Thank you,
Swami