Car starts fine, I drive it to the store, out of the store, battery dead. Drive home after jumpstarting it and it starts later on

[deblythe]

I have a 1999 300 RX. It has been a great car but I have some sort of problem where the battery and the alternator checking OK, I drive it where it started perfectly after stored overnight, get it to a store, go into the store for a half hour, come out and the battery is clicking. So, in that it started fine after overnight, I don't think mine is a parasitic drain any ideas? I'm desperate.🙏 This doesn't happen every time I just never know when it's going to happen.

[RX400h]

I would first check the battery cable connectors on the battery posts. Are they both tight? You should not be able to rotate either by hand. Did you have the battery load-tested? Simply measuring the battery voltage will not identify if the battery is load-capable.
 

[deblythe]

I am not sure if it was load tested, I will take it in on the 9th Of September.  Somebody else suggested this could be my ECU, and yes, the engine check light is on. It is coming on now and then and then goes off on its own. I took it in several times in the past when the light came on they said nothing was wrong. So I do tend to ignore the light. Maybe it was a gentle warning about my ECU going out.

[Bykfixer]

I had a Chevy truck that did that. It was the ground wire to the battery. There was a wee bit of naked wires at the battery terminal that got just enough corrosion on them to cause occasional issues. 

I snipped off the end of the ground wire, added an aftermarket terminal and wrapped any exposed wire with electrical tape. Later I removed the battery and hit the electrical tape with a heat gun like you would shrink wrap in order to make sure it was weather tight and never had that problem again. Never have something like a heat gun or fire near your battery to prevent "BOOM".

Hopefully yours is something simple like that and not the ECU. But over time some of the solders at connect points of the ECU have expanded and contract enough times where a thing called fatigue resistance causes the solder to break. Think about bending a metal coat hanger over and over until it breaks. That's fatigue resistance and solders do not "bend" anywhere near as many times as a coat hanger would before it breaks. 

Best regards.