wrong MX5 Battery from Battery World? (acid based?)

[93_Clubman]

A battery venting gas in itself isn't the problem - the problem is when those gases are not vented outside the boot, which is not the case with the MX5's OEM Panasonic battery. It's a so called sealed battery, but not fully sealed as it has a vent tube either side at the top to which a two into one pipe attaches, which then vents through the boot floor.

The best replacement battery is a fairly regular topic to which, like many questions, there is no definitive answer - it depends on the users requirements, constraints & their philosophical approach. If you've not already seen these not too old posts then they'd be worth a look as they should answer your questions:
viewtopic.php?f=18&t=25325&hilit=battery
viewtopic.php?f=18&t=39216&hilit=battery
viewtopic.php?f=55&t=28764&hilit=battery

[bigdog]

My Supercharge MF43 is now 4 years old and working just fine. Calcium batteries are a 'Low Gas' design, so they don't require vent tubes. $90 for more than 4 years service is a definite win in my book.

[93_Clubman]

BD, from what I've read the Supercharge battery would be my next choice in a battery purchase, which hopefully isn't any time soon. I've only had to buy one battery since early '91 when I bought my '89 NA6. Its original Panasonic battery lasted until '03 when I replaced it with the appropriate AC Delco (aka Delkor) battery which cost about $100. Sold the NA6 the following year when I bought the Clubman, which is on its 2nd Panasonic, the 1st being replaced by the original owner in '00. While the early Panasonics were known to last well over 10 years, I'm not sure the later Panasonics have had the same longevity to make them cost effective against the Supercharge, given the former's cost of $350 approx.