So, I can't use my car this morning. Looks like the remote battery has died, although the LED indicates some current running through the circuit. Using the physical key to lock/unlock the door doesn't stop the alarm.

Do 7/11 have these batteries? If not, where can I find them? No car dealers in the vicinity here... Not sure what the battery type is called, but it pretty much looks like a shrunk copy of an AAA.

Amorn, but sometimes 7/11 & gas stations have them. Or try a car stereo shop that installs alarms.

I find the car key batteries at the major markets sometimes but always from one of the stores in Pantip. And you can get them from the car dealer.

My experience is that the cheap batteries sold at Thai markets, 7-11, and even Amorn only last about 3 months in an alarm remote. The ones sold at specialty shops like Amnuay, Daeng Kong Pratu, etc. last a year or more. I went through 3 of the cheapies then bought one at Amnuay for 200 THB which seemed like a lot but it lasted for more than 14 months. Bought the last one at Daeng Kong Pratu for 150 THB and it is working fine 7 months later. The ones at the other places are only 12 THB or something but they don't last.

Bought mine at Tesco and still going after more than two years.

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Thanks to everyone who took the time. I went to Runway near Nong Hoi market and got one for 80 baht.