2013~2016 Dead battery after only 3 years

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CX5 GT
no no no, I wasn't trying to measure CCA! haha I was just trying to see if there was a phantom power draw when the car is off, on the order of milliamps. But obviously I did it wrong.
you may be able to measure voltage drop from each of the incar fuses if they have exposed metal pins on top of each. Have to keep the car locked though so it gets to full rest/sleep. All the electronics are live for a while even id the car is not running. But the best way and safest way is using a DC clamp on the negative cable.
The parasitic draw should be below 20 miliamps. 0.020 a. Mazda claims below 65 but I think thats still too high. Mine is around 9-10 miliamps when I last checked 30 mins after car is stopped and locked.
Some of the early Miatas had issues with parasitic draws but I think its fixed on later.
 
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2018 CX-5 Sport
I don't think you have a parasitic draw, because you would experience slow cranking after the car sat for a week even if it is a new battery. I think it's the heat that kills them. As far as battery testers, I think the best one is the load tester that Harbor Freight and other stores have. It simulates cranking the engine over and then shows how fast the battery recovers, and the battery voltage. I had a computer battery tester and it said my tractor battery was good, but the battery couldn't crank the motor over.

Load tester ($16 with coupon) + tax
 
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2014 Mazda3 S GT auto, 2008 MX5 6-speed
What would be the point of doing that if I already knew the battery needed replacement? I didn't want to take it to an auto parts store because I knew I would not be purchasing a battery from one of those stores, due to the warranty I already had.
What I meant is that you should bring it to the store from which it was purchased. They should charge it and do the load test.
 
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2014 CX-5 Touring
What I meant is that you should bring it to the store from which it was purchased. They should charge it and do the load test.

I wasn't sure if Costco did that or not. So I just took the battery up there anyway, and swapped it out.