Welcome to the forum! Unlkess you have manufacturer information to the contrary, the safe practice would be to connect both vents. You can use a plastic tee fitting with hose barbs from any auto parts store and vinyl tubing to tie the two together to a single vent tube outside of the car.

Battery vents & acid level
 

Bobson2 .............. venting Jaguar battery ...

my 2007 XK has a Jaguar label battery that was replaced 3 years ago

This battery has two vents ( one on each end) ........ vent was connected to
left side (driver side) and a plug on the other vent. Based on the condition
of my 2007 XK I believe the battery was replaced by a Jaguar dealer.

Although these batteries are maintence free ........... you still need to check acid level. My battery recently failed ... 4+ volts .... refilled with distiled water ... recharge .... battery appears ok .... takes charge and hold charge.

2007 XK ........... size 49 battery

Duracell does not show a replacement battery for 2007-2009 XK .........
900 CCA ...... same size ....... dimensions are the same ...
think the Jaguar label battery was rated at 850 CCA

Holy Krap .. 4,000 amp AGM Group 49!
 

Guess my OEM 2003 XK battery is due for replacement .. so followed this thread .. and stumbled across this offering from .. NewEgg .. ru kidding!!!

$283.10 with free shipping .. such a deal .. lol. Just thought I'd through this into the mix .. charging specs are on the listing. Amazing the stuff that NewEgg is selling these days.

Newegg.com - Group 49 Braille Endurance AGM Battery B10049

Checking the battery's cells regularly and adding water as needed greatly extends the battery life. Our 2005 S-Type and 2006 XK8 were both built in May 2005 and both still have their factory batteries running strong. I check the cells at least every couple of months, and more often than that during our miserable summers....

+1

I replaced the original May 2005 battery on my XK8 last week following random ASC warnings at start up over the last three weeks and having eliminated every other possible cause.

You could be getting to the end-of-life on your 2005 batteries. be interesting to hear how long you do eventually get out of them using a similar maintenance routine to me.

Graham

Graham,

When our May 2005 factory batteries finally give up the ghost and have to be replaced, I'll post that information. It will be in the S-Type section for that vehicle, and in this section for my wife's XK8. I will not be surprised if both of these batteries make it into 2013. Beyond that, who knows....

On my Duralast H8-DLG battery, only one vent hole is used. The mfr's instructions say:

"2. If the vehicle battery has a venting system, you will have to use the plug (attached to the positive terminal cover) to complete the safe installation of the battery...
3. Connect vent tube (if provided) into closest vent hole on battery end.
4. Remove vent plug from terminal cover.
5. Install [in] vent port opposite vent tube."

Both vent holes lead to a common chamber on my battery (if I blow compressed air into one vent hole, it comes out the other).

I used one hose and one plug.
https://cimg5.ibsrv.net/gimg/www.jag...3c199a02e2.jpg
Just wish I had had a right angle connector

https://cimg3.ibsrv.net/gimg/www.jag...00622d107c.jpg
On my Duralast H8-DLG battery, the mfr gives you a red plug to block off the vent you are not using. In my car, it is the one closer to the negative terminal.

I suppose if you use both vent holes and tee them off into the floor grommet, it would accomplish the same venting result, but that takes more parts and plumbing, so I used just one.

Although there are two vent holes on your battery, the second hole is only there for your convenience.

The battery mfr doesn't know where the vent tube grommet is on your car, so they gave you a vent hole on each end to keep that vent tube short as as possible.

The main takeaway is, you must still address both holes. You can't put a vent tube in one and forget the other. You can't have hydrogen venting into the trunk, since it is highly explosive in the presence of a spark...remember, you do have electric motors in your trunk that run the trunk lock and power antenna.

Hope this helps. I'm sure there is more than one way to skin this cat