Morris Minor Owners Club

Do 'modern' car batteries have vents? Presumably they still produce gas when being charged?

Or are they 100% sealed?
Or do they have 'plugs' in a vent that should or should not be removed?
Or do they have some sort of valve?

They are described as 100% maintenance free.

This came up in conversation I have no idea what the answer is and would like to know...

Sealed lead-acid batteries are also called "valve regulated batteries".

In normal use there is no venting of gasses. The gases that are given off (Hydrogen and Oxygen) are recombined in the battery into water so there is no loss of electrolyte. In abnormal conditions, a pressure regulating valve opens to prevent the battery from exploding. An abnormal situation might be a battery being overcharged by a faulty charging system in the car or an old-fashioned battery charger that does not automatically shut off the power when the battery is fully charged.

Some info here: https://www.batterycharged.co.uk/shop/b ... cEnTM-fU0C

Stephen

I recently fitted several new maintenance free batteries, different makes and they all came with vent plugs fitted for transport that have to be removed and left open to atmosphere once installed

Bill_qaz wrote: ↑Thu Jun 19, 2025 10:35 am I recently fitted several new maintenance free batteries, different makes and they all came with vent plugs fitted for transport that have to be removed and left open to atmosphere once installed

Yes I had one delivered like that too. When I removed the vent plugs there was a slight hiss of escaping gases.

Stephen

So...in theory...they are not sealed in use, they have a valve. (I can be pedantic!)

If they are supplied by a garage or factors then these transit bungs ought to have been removed by them. If they are sent by courier etc the bungs may still be there?

Of course, some of will be using 'sealed' batteries whose parentage and age is unknown. If they've lasted 15 years so far I hope we can assume that they are not about to explode!

michael4 wrote: ↑Thu Jun 19, 2025 1:28 pm So...in theory...they are not sealed in use, they have a valve. (I can be pedantic!)

If they are supplied by a garage or factors then these transit bungs ought to have been removed by them. If they are sent by courier etc the bungs may still be there?

Of course, some of will be using 'sealed' batteries whose parentage and age is unknown. If they've lasted 15 years so far I hope we can assume that they are not about to explode!

Yes, whether you get a battery delivered, or buy it from a shop, it will undoubtedly have transit bungs fitted to comply with transport regulations for batteries.
Every new battery I've brought in the last couple of decades has carried a warning sticker on the top to remind me to remove them from each end.
Cars that have vent pipes attached, those with batteries in stupid, hard to reach, places like many top brands, will connect the vent pipe(s) to where the bungs were removed from.

Sealed, maintenance free, just means no removable plugs for top up not air tight. I don't believe its even a valve, it has to vent due to heat expansion and draw air when cooling. It been that way on bikes for years but usually have a vent pipe to below the bike away from paint.