I'm looking for suggestions on what the best replacement battery would be for the 8. Best meaning lightest and most powerful!

 

Im not sure what the CCA requirement is for the 8, im sure it says in the user handbook. Maby the panasonic battery out of the miata could be adapted. Personaly I just want to ditch this battery and get a sealed battery, optima makes a good battery but its a little pricey, you could look into the lightning audio batterys, they are pretty good.

most batterys that come with cars are 'starter batterys' made for 1 thing, to start the car, get a good sealed battery with the right ammount of CCA, esp in canada where you get colder winters than say, FL

 

One with the most cranking numbers and biggest that will fit. I got one from Cosco, three
year full replacement and since at three years a battery is pretty much junk I get my money
back and get another one.

 

The Interstate MTP-35 (640 cca) is one Mazda-recommended replacement battery. There is a newer recommended battery (see the TSB's) but I don't know the brand-model for this one. You can try calling Tommy at Rosenthal or Marshall at Trussville to get the brand-model information so that you can purchase it on the general retail market for a lot less $$$ than Mazda wants.

I installed the Interstate several months ago and it seems to spin things up a bit faster than the puny OEM battery.

 

Once u go optima u never go back.

 

There's more to the equation than just cranking amps.

I switched to the Yellow-Top Optima 35-series. Check the DIY section.

This battery is designed to handle repeated deep-discharge situations with minimal loss of capacity, vs. ordinary car batteries which fade quickly after even one or two deep discharges. Furthermore, it maintains voltage much further along the discharge curve. This is important for 2 reasons

1 - should you need to use the de-flood procedure this battery will deliver more useful cranks because it will be able to provide not just the amps, but also maintain voltage. The last thing you want in a flood situation is a battery that cranks slower and slower. You need to keep those cranking rpm high. And, once you have a successful start, the yellow-top will still charge up to its normal capacity. An ordinary battery will be weaker if it was significantly discharged in the deflood attempts, which will only increase your odds of a future hard-start or flooding event.

2 - modern cars draw current even when sitting. There have been plenty of threads of RX-8 owners who encounter hard starts or no-start after the car sitting for only a couple weeks. Again, the ability to withstand discharge is important and the yellow-top will not only increase your chance of starting but will "survive" the discharge in much better shape.

The damage from these kind of discharge events is cumulative. Ordinary batteries won't take many of them. The Optima yellow-top is designed to withstand 500 deep-discharge cycles and still retain 80 percent of its capacity. Most batteries will croak after just a few deep discharges.

Plus, the Optima batteries are the AGM type, like the original Miata batteries. That means no leaks, no corrosion, no need to add water, and they have more power per pound than a "flooded" (liquid electrolyte) battery. The original AGM battery on my Miata, despite its diminutive size, lasted 7 years.