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Just what it says. (This will be a bit long.) Suddenly as of last week, the battery goes dead over the course of a day or overnight (fine when I go to work, dead when I get out). Happened for the first time last week; it didn't jump so I had it towed to my shop where they found the battery was dead, they charged it, and it tested fine once they did. Almost a week later, happened again; I left work and it was a tiny bit slow to start, went somewhere else for a couple hours right from work, dead. Had someone jump me, drove it half an hour before going home. It started in the morning (without much issue if any), after work it was dead, and after jumping, I took it straight to my shop and dropped it off. The next morning, it was dead. Again they charged it, again it checks out just fine. They did other testing and can't find anything parasitic drawing on it. They kept it for the weekend with one of their batteries in it to see what happens by Monday.

They tested the alternator the first time and it was fine. It doesn't go completely dead-- headlights, dome light, etc. still function-- but cranks a couple times super slowly and that's it. No dash lights on about it. I've not done anything to it lately, no aftermarket parts (alarm systems, sound systems, etc.). I've only had it four months; the previous owner had it about a year before he upgraded, so I'm not that well-versed on its history (no records from first owner, and I think previous owner might've gotten it from a dealership so didn't know much). Means, of course, that we also don't know how old the battery is, but it doesn't look manky, and I would think that if the battery was dying, it would not test well, yes?

I had to have a transmission solenoid put in a month after I got it (#&$%@ automatic transmissions), but that's all I've done to it. Previous owner was a former auto tech so he did his own work to it, but nothing he did to it was electronic or anything that I might blame (he did put new headlights in-- LEDs IIRC?-- but I can't remember if he actually had to do anything with wiring or just put in new bulbs). It does currently have a bad TPMS sensor-- I bought it that way-- that I haven't fixed yet. Also, a couple weeks ago the key fell apart as I turned the car on, but I was able to tape it back together (it's my only key)-- just saying this in case something about that could be it.

In the interest of complete disclosure, I do drive it nearly every day and it never sits more than a day or two; my commute is about 13-15 miles, about half of that on the freeway-- the first time it went out, I'd driven it the day before and of course the day of. The second time it went out, I'd driven it a bit the two days before (not much, only went somewhere and about 6-7 miles each way, all surface streets), and the day of, and then of course drove it the next day before it went out again. So this car never sits for more than a day or two, though if I'm not going to work, it doesn't go far (probably not more than five or six miles, on surface streets)-- still, this isn't an "old lady only drives it a mile to church on Sundays" deal.

I honestly thought they'd find something was draining the battery (the two most common things I've read of are the AC compressor relay, and the rear defrost relay, and let them know of that), but nope-- they said nothing was drawing on it at all.

Is it possible the battery is going bad and just doing so in an odd way? (Well, it seems odd to me, but I'm no mechanic and not that well-versed on batteries, so maybe I'm wrong when I think this seems odd for a going-kaput battery.) Or is it possible something is draining it, but intermittently so there might be times when they test and don't find a draw? Any other possibilities, in case this turns into one of those sagas that is hard to diagnose? I read back through the forum but nothing exactly like what I'm seeing.

A relative looked it up and found reference to an electrical issue Civics were having, with some electronic component that would "wake up" on its own while parked and drain the battery. This was not something I found when I researched but searching specifically now, perhaps MICU?); I'm not even certain if it affected 2010 Civics as they said apparently it was only a recall issue for 2016 and later Civics.

 
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It turned out to indeed be the battery-- it charged well and tested okay but they suspected a bad cell.

 
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Test should have caught the bad cell, but maybe it wasn't totally dead yet

 
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You'd think test would've caught it, but the guys at my shop only realized it when they switched in their battery and noticed mine was still hot 5 hours after they finished charging it... (which was something I'd noticed myself one of the times it went dead, but thought I was just imagining things; I should've realized.)