Took a 38 month old dead battery into Walmart last night. Since it had a 3-year free replacement/ 5-year total warranty and was only 2 months over the three years I figured I'd get near full credit towards a new one. NOT TRUE! I only got 37% toward a the new battery. Here's the shocking truth about how that works. A 5-year battery loses 1/60 of it's replacement value each month. So at 36 months it's actually down to 24/60 or 40% of it's value but since the 3-year free replacement still applies, it's credited at 100%. That all goes away on the 37th month leaving 23/60 or 38%. Moral: get your battery replaced IN the 100% period or suffer what appears an unfair decreased credit.
Got me thinking about my RAV4 battery, so did some fun with numbers. I did get a warranty paper for my Les Schwab (west coast chain) Johnson Controls battery. The free replacement period was only 24 months, but the prorated warranty is 84 months. So I risked shorter term failure, but I fare better over the long haul.
I lose 1/84 replacement value each month. At the half-way point of 42 months, I would get 50% toward a new battery. My OEM battery lasted 43 months, so it might be a good bet. I'm at 37 months now on my replacement, will see if this one beats that.
So my moral: consider longer term on the proration side of things for slightly better payback. (Note: I did pay upfront cost of $105--probably higher than Walmart--thus my 'depreciation' is $1.25/month over the 84 months. If turns out to be a higher quality battery and lasts longer, so much the better.)