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MG MGA - Battery Venting

Just noticed that my old battery has a plastic plug at both ends of the cells. I presume these should be removed when the battery is in use? If left in what are the consequences?

For what its worth they have been in for the last 6 years on the car!


Steve Gyles

If you actually have a serviceable battery, the plugs are for adding distilled water, and should be loosened slightly to allow it to vent while on the charger. But unless you have a vintage battery or replica thereof, chances are you do not have a serviceable battery. In that case, the plugs were there for the manufacturer to add the electrolyte, and should not be opened.
Del Rawlins


The battery was new 6 years ago. It has been okay for 5 years but recently started to drop off in performance. I have just had another look at the plugs. The one on the other side is in fact a vent. It has a tube in it with a small foam filter. All six cell caps unscrew, presumably for filling. I guess the plug is used for setting the fluid level as with our back axle?


Steve Gyles

I believe it is impossible to water a battery through a single fill hole, as the cells are separate inside, so each cell must be filled individually. Pressure relief valves in a sealed battery may be connected to a single exhaust port, but those act like one-way check valves, no good for filling.
Barney Gaylord

That's correct, each cell is serviced through its own hole. I can't remember the last time I saw a serviceable battery in active use on a car or truck though. It's highly unlikely that the plugs in your battery were meant to ever be removed once they were closed at the factory.

We still use serviceable batteries in aviation... Here's an example showing the service ports:

Those plugs are removed for adding distilled water, and are designed to serve as vents when loosened. You would check the condition of this type of battery by measuring the specific gravity of each cell, using a hygrometer. Modern sealed batteries are good enough that I don't know why anybody would ever bother with any other type unless you are trying to score points at concours. The aircraft battery I linked is actually a pretty poor excuse for a battery (despite its high price), which I only post as an example.
Del Rawlins

It was the colour of the plug that alerted me. A red colour plastic push plug such as this would suggest its removal prior to use after installation, in much the same way that both terminals come protected by red plastic covers. How I missed that red plug six years ago and asking the question I am asking now beats me.

Steve Gyles

Just had another look. All six screw stoppers are vented into a common gallery (see photo with metal rod passing straight through from end to the other). I can understand the black vent plug with the foam filter/absorber at one end, but why the red stopper at the other? Why not two vented black plugs?


Steve Gyles

I have bought a fair number of batteries (on-line) in the past months. Normally there is a label on the battery saying to remove the plug before fitting. I assume they are to limit spillage during transport.
Neil McG

It appears that people are talking about different holes in their postings.
Those mentioning adding water are talking about the holes on top for each cell and with an "X" groove for removal.
Steve is talking about the horizontal holes at each end of the battery case, one black and one red.
When batteries are charged hydrogen gas can be formed. It appears that it is meant to vent through the black hole with the filter.
If that filter blocks the red plug at the other end will blow out.
Just my 2 cents worth.

M F Anderson


I favour your analysis. However, my whole question really boils down to the colour of the plug - red. In my career in the flying game, all the bits and pieces that had to be removed before flight were either painted red or had red flags attached. This made them conspicuous and less likely to be missed. Emergency switches and handles that we may have needed to use in flight were normally black and yellow striped - like my ejection seat firing handle sitting on my shelf in my study!

With this battery, applying the same logic, the plug should be pulled once the battery is in situ. If it is designed to stay in place and act as an emergency pop-off it should be coloured black or striped yellow and black. Interestingly there is nothing on the battery labelling.

Steve Gyles

The batteries are desirned so they can be in the boot (trunk) or under the rear set in some cases. A tube is attached to the black vent end and out through the floor so the car does not fill with explosive gas !!
The red plug is just that - a plug - coloured red so even the most visually challenged person can see it is in place. The vent / plug can swop ends to suit various aplications / cars.
A J Dee


Happy to go along with the reasoning, makes sense. I agree that the plug/vent plugs can swap ends to suit installation requirements. However, again, I repeat, regardless of visual impairment, that having different colours for two plugs that should both remain in place is not a wise idea. For example, if they had both been black I would not have started this thread. Because they are of different colours there is the potential for confusion.

Steve Gyles

So Steve, is your handbrake lever taped black and yellow?
Art Pearse

No, that's for standard use, not emergency!

Steve Gyles

Have we thought about contacting the maker of the battery? Maybe they have an explanation!
Barry Gannon

This thread was discussed between 29/10/2013 and 31/10/2013

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