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Join Date: Oct 2017
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Venting batteries on older 911

'69 E, new to me for 5 months. I noted for the first time that the batteries are vented to the outside across the RF fender well. The old batteries that I'm replacing have a single vent per battery and that battery has cells to fill with distilled water, the new ones are sealed and have two vents per battery. Is this necessary?? Looks like a PITA. thanx. s
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Old 08-17-2019, 02:02 PM
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911s seem to have an issue with failing voltage regulators -- a failing voltage regulator can let 17+ volt current spikes into the electrical system, which can cause the batteries to overcharge and boil acid -- better to vent to the ground than let the acid eat away at your chassis.

Modern AGM=type batteries reduce this risk as they encapsulate the acid in a glass mat instead of as a loose liquid.
Old 08-17-2019, 02:06 PM
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Outgassing battery electrolyte is the scourge of 911 sheet metal. Aim for an AGM battery. On vehicles that provide more space, consider using a plastic marine battery box.

Sherwood
Old 08-17-2019, 02:29 PM
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I have been using Miata sized AGM batteries for years in my 1973. They are sealed and they can't leak.

I just paid a little over $100 each for a pair of them. Bought on Amazon Prime free shipping from NJ to CA and very well packaged in tons of well done padding. They can be shipped Priority mail because they are sealed. They even put a sticker on the outside of the box stating they were sealed batteries.

Hunt around you can find them in lots of places.

My battery boxes are perfect......NO CORROSION.

DO NOT PAY HIGH BUCKS FOR A LIQUID ACID BATTERY IN THIS APPLICATION.
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Old 08-17-2019, 04:34 PM
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Jumping on the bandwagon.

I spent a great deal of time cleaning up my battery boxes after acid damage. Nothing but AGMs for me.
Old 08-17-2019, 07:45 PM
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Thanx again. i bought the Miata style AGM batteries but if they are sealed (and they are), why are there two vents per battery? Yeah, i spent an hour or so, just getting started cleaning up the mess from the old lead acid batteries. will spend another couple of hours on this tomorrow. argghhh. thanx again. s
got mine from batterymart.com
fast shipping, more than fairly priced, and, as noted, shipped like a piece of fine china. s
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Old 08-17-2019, 08:30 PM
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Quote:
Originally Posted by steve meltzer View Post
Thanx again. i bought the Miata style AGM batteries but if they are sealed (and they are), why are there two vents per battery?
Because at least on the early Miata's, of which one I own, there are a pair of vent tubes that connect to a T and then a single vent line out of the chassis for the battery that is located in the front right corner of the trunk.
Old 08-17-2019, 09:46 PM
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Thanx so much. I guess the primordial vent openings on an AGM are really vestigial. Right" thanx again. s
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Old 08-18-2019, 06:19 AM
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I'm not sure about that. The AGMs are not immune to venting if the voltage regulator fails. I've seen some photos of some seriously swelled AGMs.

If I remember, Optima batteries don't have vents, but they do have pressure relief ports on the top. You can even get a special shroud to go over those ports to vent outside.

Mark
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Old 08-18-2019, 06:46 AM
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Both vent, but what they vent differs.

A wet battery will vent sulfuric acid

A dry, not so much.
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Old 08-18-2019, 10:37 AM
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Quote:
Originally Posted by island911 View Post
Both vent, but what they vent differs.

A wet battery will vent sulfuric acid

A dry, not so much.
I think the vents may not be active. I am sure my battery was upside down and on it side in shipping.

One more note on Miata batteries, 911 battery boxes are quite deep and the Miata battery is narrow........if you push the battery all the way in you can have a situation where the positive terminal can touch the battery box metal.

One solution is to put a spacer behind the battery so it doe not go in all the way. Like a 2X4 block of wood or I have used an aluminum rectangular extrusion scrap. This will keep the positive terminal outside the battery box.

Other way to to put a piece of insulation plastic on the "roof" of the box in the area of the + terminal. Ihave done this and glued the plastic in with silicon seal.
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Last edited by dicklague; 08-19-2019 at 08:52 PM..
Old 08-19-2019, 12:38 PM
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