Smart Key not working

I'm having an issue with my smart key unlocking the doors. Forgive me if this has been addressed elsewhere, but I've searched this site (and the web) and found no matches.

I have a 2010 Prius, ~75k miles. The issue is 2 or 3 times out of 10, when locked, the car doesn't seem to sense my key. I have to push the unlock button on the key to open the doors. The other 7 or 8 times out of 10 it works fine, which is to say I approach the driver/passenger/hatch door with key in pocket and the car unlocks upon touch.

There has never been an issue with starting the car. The key is always sensed then. Also, replacing the battery in the key solved nothing, and this happens with both keys.

(One other thing I just noticed, which may be related, is I left the headlights on recently and they did not shut off automatically. I returned to the car ~15 minutes later and the lights were still on. Doors were locked, and I had to push key button to unlock. So the car didn't sense the key, and maybe it also didn't know to sense I was gone and it should turn off lights [?]. May not be related...)

Does anyone have any thoughts as to the cause of this? Or any suggestions for a solution? Any help is greatly appreciated. I saw a couple posts related to Gen 2 issues similar (but not the same) to this, but they didn't seem to apply.

David

 

Battery is fine, plus it's an issue with both keys—i.e. it seems to be an issue with the car, not the keys.

 

I actually replaced the 12V battery about a year ago. A yellow Optima, bought new from NAPA. But since everyone is suggesting this same thing, I went and checked via this method:

Check Prius Battery | PriusChat

and I measured 8.4 V. Car has been sitting overnight. Is that right?! Isn't that so low that it wouldn't work at all? I thought it should be 12.x V.

 

Thanks bisco.

After some driving I checked voltage again, and got readings 12.6-12.8 V. I'll continue to take readings. The question is: if the battery continues to drain during non-use, what could be doing that? As best I can tell, no lights (inside or out) are ever left on.

 

Can this be done with a voltmeter? And are you saying while the engine is running a healthy battery should show 12.x V? Thanks

 

Thanks, I'll take you up on that offer! I registered 12.8 V with all off. Then with just headlights on, 12.3 V.

 

Thanks ...does that leave me at 12V battery is fine? I.e. back to square one with key sensing issue?