I just installed a sealed WestCo Battery (model 12V31M) in my car http://www.westcobattery.com/miata.html# . My OEM battery died a few weeks ago and was replaced with a Douglas 51-675 battery. Both the OEM battery and the Douglas replacement vented gas and leaked fluid onto the terminals, battery tray and heat shield, causing corrosion. I wanted to upgrade to a sealed battery to eliminate this problem and found the WestCo battery at a Miata specialty store. It doesn't have as much capacity as the OEM battery, but it's smaller and lighter. Here's a table that I compiled to help compare them in case anyone else is thinking of upgrading. I'm also including a sealed battery, model SVR 31-12 http://www.svrbatteries.com/battery_page.php?bid=22 that is a better choice for our car since it has a higher capacity and still fits. I don't know why it's listed as being lighter than model 12V31M - doesn't make sense to me.


Cold Cranking Amps (CCA) are listed for 0 and 32 deg. F

...............OEM........12V31M......SVR 31-12
CCA 0......430.........365............410
CCA 32....520.........475...........500
Length "...9.4..........7.6...........7.75
Width ".....5.1..........5.2...........5.0
Height "....8.8..........6.75.........7.25
Weight #..31...........25............24


I paid $75 for the WestCo model 12V31M battery, and the Douglas OEM replacement cost $50 at a local Batteries Plus store.