Vexatious forums and the curious case of the Mazda MX5 Battery!

Okay, so it sounds like a potentially dull subject for non petrol heads, but I assure you what follows is only peripherally concerned with car batteries. Here goes,…

I find public opinion is only of use when taken as an average. If one man is asked to tell the weight of a bull, he will likely be wrong by a wide margin, but if you take ten thousand men and ask each what they think, then add up their thoughts and divide by the number of men, you’ll get an answer that comes very close to the true weight of the bull. It’s called the wisdom of crowds. There’s nothing spooky in this. It’s a question of probability.

But the wisdom of crowds is harder to detect in the online forums which these days form a global network of talking shops that mull over the multifarious issues that vex mankind. If you have a problem and you search online, I guarantee someone else not only has that exact same problem, they have already asked the question of an online forum, and thereby unleashed a tsunami of uniformed opinion in response.

It’s rather like asking a question in a large public house when everyone is volubly chatty from a few pints, and bursting with the urge to press their wisdom on anyone who will listen. The opinions fly, the volume is raised as the evening progresses. Then certain comments are misinterpreted by the more prickly pundits as insults to their intelligence and they fire back with less veiled insults, and much pettiness. Indeed, to read them it seems the wisdom of online forums, as opposed to crowds, approximates not to the truth of any matter at all, but to zero.

Speaking of one such vexatious issue, and returning now briefly to the titular matter, I noticed my car battery was rather an ill fitting one, suggesting it was not the original, but a later replacement, and not a very expensive one either – just your regular sealed lead acid job. Further, there were some pipes dangling in the vicinity which appeared related to the function of the battery, while not being actually connected to it if you know what I mean.

The battery of a Mk 2.5 MX5, like the one I drive and love, sits in the boot (trunk), tucked away in its own little well, hidden under the carpet. The problem with this is that when charging, batteries release oxygen and hydrogen gases, and these can accumulate in the boot (trunk), or worse, leak through into the cabin as you go along.

Hydrogen is not poisonous, but it is impressively explosive especially when mixed with just the right amount of oxygen. Normally, with a battery mounted up front under the bonnet (hood) of a car, these gases vent harmlessly away to the atmosphere. Mounted in the boot (trunk) however, this is not so, as boots (trunks) tend to be sealed tight to keep the weather out and your stuff dry. I presumed those little pipes of mine were supposed to be connected to the battery to allow these gases to vent outside of the bodywork. That they weren’t was an oversight, or just sloppy maintenance, and was soon remedied by the procurement of some small, plastic elbow joints from my local aquatic centre. These fit snugly in the little vent holes which most batteries have, and allowed the vent pipes to be neatly coupled up again. Whatever the risk from outgassing was, it was now taken care of.

For so simple a matter, the subject of the MX5 battery is one on which whole volumes have been written, on the various online forums, and scary reading they make too. This is all the more puzzling since the world as far as I know is not assailed by exploding MX5’s. Still, it makes one pause before accepting a ride, let alone owning one since, thanks to the incompetence and penny pinching of your average owner, according to said forums, one is likely to blow the back end off by merely starting her up. Nor would one ever store anything precious in the boot (trunk) as it would likely be dissolved by the “corrosive gases” emitted by that most fiendish of all gremlins, the incorrect, and worse, “cheap” battery, again according to the opinions expressed on said forums.

And opinions vary wildly, fingers jabbing in all directions, certain pundits opting only for ultra expensive space age batteries, as nothing else will do for their treasured vehicle. Other pundits are more reassuring, telling tales of running with a regular, inexpensive battery for decades, and no problems. One can almost hear the sucking of teeth, see the raising of eyes at the foolishness of others, the tutting, the shaking of sage heads,… in short the forums were no help whatsoever.

Instead, I talked to my friendly local (independent) mechanic.

Of course batteries do not emit corrosive vapours. Tales of MX5 boots (trunks) partially dissolved by the “incorrect” battery suggest to me more a problem with a battery actually leaking electrolyte (acid) because it’s been damaged. This is a risk with all lead acid batteries – even the supposedly sealed ones, since a crack is a crack and yes, even dilute sulphuric acid (nasty, nasty, stuff) will make light work of anything in its path, be it human, animal, mineral or vegetable. So, if you want to eliminate all possibility of such a disaster you should by all means plump for the more expensive batteries which use an electrolyte in gel form to stop the acid from dribbling all over the place in the event of an accident or a spillage. But a properly fitting, and securely clamped battery should not crack, so, if the budget will not allow a high falutin’ space age technology battery, don’t worry about it. Sure, your regular, inexpensive battery may be dead in a couple of years, but it won’t cause your car to burst into flames, so long as it is properly vented, and it won’t melt the floor of your boot (trunk) either, so long as it’s properly fastened down.

So, yes, it’s sensible to connect those vent pipes up to get rid of any hydrogen gasses accumulating in the boot(trunk) and leaking through into the cab, but again the risk involved in this is unknown, and you’ll find nothing to quantify it on the forums – only a lot of diametrically opposed opinion from the lackadaisical to the apocalyptic. I ran mine for about a year before picking up on it, and came to no harm. And in the end we have only our own experience to go on.

All told then, we should enjoy our MX5s, and avoid at all costs the risk of becoming self diagnosing hypochondriacs with the online forums as our only reference. When seriously in doubt go talk to your friendly local (independent) mechanic. He’s been to college to learn about these things in those long gone days when colleges still taught useful vocational stuff, and he’s seen a lot more cars than you ever will.

Of course if you have any opinions on the correct battery to use in a Mk2.5 Mazda MX 5, feel free to share them. I’m full of opinions myself of course, but I’d never vent them on a discussion forum.

I keep a blog for that.

That way I always get the last word.

So, mine came with an AC Delco from goodness knows where. It fitted the original battery tray perfectly but sat too high, so I couldn’t get the boot (trunk) carpet down properly. The battery clamp also sat too high, and it was taking up valuable luggage room.

I replaced it with a Yuasa HSB063 from Halfords. This sat low enough to get the carpet back down properly but wouldn’t fit the battery tray so I made a new tray from impermeable packing foam, I also had to modify the clamp a bit. With some tweaking in the vice and few odds and ends from the garage, the clamp worked perfectly. Connected up the vent pipes, and the job’s a good ‘un.