Wow! And I thought having a flat battery was a big problem. It pales into insignificance compared to blowing up a car

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"Investing my Super in a Cooper" &
"Re-living my Surfing Past in a Woody"

Charging a lead acid battery gives off hydrogen gas.
Ever heard of the Hindenburg.
https://en.m.wikipedia.org/wiki/Hindenburg_disaster

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Find a job you love and you will never have to go to work !

Hydrogen release is only as a battery reaches full charge (just before, >95% full charge ) and increases dramatically with the input current rate as the battery tries to dissipate its overcharge. The release of hydrogen is proportional to the charge the battery is being forced to dissipate. The point of a trickle charger is to maintain the battery at this peak "sweet spot". It does so by putting a small amount of current in to make up for losses, and only as much as to get the battery to the peak voltage only.

Cheap chargers have a voltage output higher than the battery, and will overcharge the battery much more than a trickle charge. That's why smart trickle chargers cost more, because they're paid to manage the battery intelligently and avoid degrading the electrolyte by creating byproducts such as hydrogen and heat. The management of peak charge voltage means that the release of hydrogen will be extremely small if the battery is in good shape, and only in small periods as the battery occasionally tops up a little.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

So do I open my boot or not open my boot?? CTEK charger on maintenance mode on a brand new fully charged Optima Yellow top

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Lillee - 1969 Morris Mini K

If you can I would leave it open, not sure how you would get the leads through otherwise to touch the terminals?

All depends on the state of the battery and the amperage it is drawing.

A charged battery on a very small trickle should not give off too many fumes. If your battery is heating up and giving off fumes then yes. These fumes can also condense into droplets and corrosion can start.

I open my boot and remove the boot board when the battery is low. If the battery is charged then I just hook up to the battery lead on the starter solenoid.

I was going to ask whether I could just hook up the "comfort" lead to the solenoid.

Not sure if opening boot will make a difference if I have a car cover over the car as well? This would surely make things worst in terms of ventilation. Sometimes I don't drive her for up to 6 mths...

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Lillee - 1969 Morris Mini K

On most Minis the boot seal doesn't, really.
I have a 12A Nilsen charger which I connect at the solenoid, it drops to zero when charged. It is often left on for days, I have had no problems.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

The hydrogen release is directly proportional to the current. If the battery is already charged then the hydrogen release will be tiny, especially on a long term/not turned over scenario. A battery conditioner is designed to keep it topped up without over charging. Over charging is when the battery has to burn off the extra energy chemically, through heat and hydrogen loss.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

OPEN it or leave a good gap....why take unnecessary chances

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"Show me the Mini!"

Just leave a window open.. the boot is not properly sealed from the cabin.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Hydrogen is lighter than air, which means if it's in there in any quantity it'll be either at the top of the boot, or have got through the parcel shelf and be somewhere up near your hood lining, therefore cracking a window is better if it makes it into the cabin.

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Roads need more corners
A Deluxe(CG13DE), 2 Clubbies(998 and 1275) and 2 Morris 1100Ss

Why not approach the problem from the other end and remove the potential for spark
If you have a ctek you can get leads that attach to your battery by bolting to the battery terminal fittings which eliminates spark in the boot (this is in lieu of the typical alligator type connection). They have the normal double push in fittings to connect to the charger and you can feed this lead permanently out of the boot through a rubber plug and have it say, wrapped around the rear subframe, where it can be connected to the charger. it can also be feed out through the boot opening and is thin enough not to be cut by the closure of the boot (you can cover the area through the the boot seal to protect it if concerned)
I had this on my MG for 3/4 years and had the charger in the pool pump enclosure in the car port and simply drove the car in, fished out the connection from the gap between the headlight and the guard (its a pommy car after all) and plugged it in all the time. The connection is fused as i found out when my sister in law took the car for a drive without disconnecting the charger, lots of shredded wires but no sparks or shorts
If there is no spark potential in the car then there would be no more danger than any car that just sits closed up in the garage but would suggest leaving windows open if possible

Alright, I have made a makeshift bungee chord to prop the boot open slightly. But I use a car cover over the whole car for long term storage so I might have to pump some holes in the car cover too!

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Lillee - 1969 Morris Mini K

What material is the car cover, if it's the usual very light stuff it would be porous anyway.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.