Am doing my homework prior to purchase. We go away for 3-4 months every summer. The Prius will sit home. Does 3 months of inactivity do any damage to the main battery? I would put a battery tender on the 12 volt..so the car will still start when I get home?
Toyota's recommendation is to start the Prius every two months and leave READY for 30 min. to recharge the traction battery. Since you won't be able to observe that recommendation you might want to force-charge the traction battery to make sure that it is at 7 green bars (or even 8 if you don't mind living dangerously ), hence having plenty of charge to withstand the storage period. To force-charge, make the car READY, put in D, then press the brake pedal with your left foot to hold the car stationary. Floor the accelerator pedal with your right foot so that the gasoline engine runs. Watch the traction battery SOC gauge. As the battery becomes fully-charged, the engine RPM will slow down.
Sounds like a plan, should work Make sure it's a "real" battery tender and not a simple trickle charger. A real battery tender will automatically keep a float charge on the 12 vdc battery, the better ones also desulfate the battery too. This is the one I use on my equipment at the hobby farm  12 Volt-1 Amp Charger and Maintainer As Patrick mentioned, a force-charge of the NiMH traction battery before storage might not be a bad idea.
..and if you purchase the option that includes the keyless entry/keyless start make sure that you disable the keyless function so that it does not drain the 12 v battery. This function is controlled by a pushbutton switch located bellow the steering wheel ( there are two buttons there, the one to the right is the reset for the tire pressure sensing system and the one to the left is the On/Off for the keyless entry). After disabling this function you will still be able to lock/unlock the car using the FOB buttons and you will be able to start the car by inserting the FOB in the slot next to the steering wheel.
Patrick: I, too, will be leaving my III Prius in storage for 4 months and am concerned, not about the 12 volt battery, but about the traction battery. Will it harm it to simply let it slowly discharge over 4 months? Is there anything I can do, short of driving it every 30 days, to help keep the battery in good shape? The dealer gave me pretty much a non-answer.
I know other snowbirds who keep their Prius in storage 6 months each year. I'd suggest that you make sure the traction battery SOC gauge shows near full before you store the car. You can force charge as described in my post above. I don't think you have to worry too much about a 4-month downtime period. Regarding the 12V battery, I suggest that you ensure that it is fully charged, then disconnect it before storage. Or else hook up a battery minder.
