shop-talk supported in part by Fat Chance Garage
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>> I've been using a Miata replacement battery from Westco
>> for a few years in my 67.5 Datsun Roadster.. Last time
>> I bought one it was about $65, including shipping,
>> rather than the $110+ that Mazda gets.
>
> Miata batteries from Mazda have a tubing system to vent
> the gases out of the trunk. The replacement battery would
> be great for secondary uses, but I was cautioned to use
> the original equipment type as a replacement.
The OEM Miata battery is a sealed AGM battery. The vents are there for
"emergency" - any catastrophic problems that would cause the battery to
vent gases. Normally an AGM battery will never vents gas like a
traditional lead-acid battery.
The WestCo battery is also an AGM battery and is a direct replacement
for the Miata's OEM battery (after about 9 years, I finally had to
replace my OEM battery with a WestCo).
The AGM batteries behave differently than regular batteries -
specifically when discharged they must be charged at a slow rate (only
an amp or so). If people let the battery run dead (leave the lights on
for instance), a jump will get the car restarted, but driving around
will not re-charge the battery. This lead to lots of complaints and
Mazda having to replace batteries that really only requires 24-48 hours
on a slow charger.
The result was that Mazda's "official" dealer replacement battery is a
traditional lead-acid battery (the OEM AGM batteries are NOT available
from Mazda). Lead-acid batteries have to have the "emergency" vent
tubes connected since the lead-acid battery constantly gives off fumes.
Unfortunately, lead-acid batteries have other problems (corrosion for
instance), and the best solution (for a Miata anyway) is to use a
replacement WestCo battery (or other AGM battery).
Even if I have a normal battery in the engine compartment, I try to get
one with vent tube fittings. I will install a plastic hose to the
vents, and run the tube to the underside of the car. I've greatly
reduced battery corrosion problems with $0.50 of tubing...
Tim Mullen
Chantilly, VA
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