Updated in March 2016

All lead acid batteries, AGMs and gel cells, generate explosive gas. Even though most are sealed, makers stress that battery ventilation is vital still. Confusion exists over this is due to some makers of such batteries stating (around 2000) that ventilation was no longer required. Or it was advisable but not essential. That advice was withdrawn shortly after. Many batteries have long since had a warning notice like that below. 

Battery ventilation notice

Despite warning notices like this, many builders of camper trailers, caravans and motor homes install batteries in unventilated compartments. In 2012, one RV maker threatened criminal defamation when I published to the effect that battery ventilation is vital. They withdrew when sent photographic evidence that the very batteries they were fitting in their own current products had such warning notices. 

Explosion risk

Hydrogen has no colour or smell. Without instrumentation, a human cannot detect it. Unsealed batteries smell whilst charging, but that odour is not hydrogen.

Excess charging voltage causes the electrolyte to ‘boil’. A charge rate of 20 amps at 15 volts produces about a litre of hydrogen per minute. In a typical battery enclosure with about 10 litres of free air space, a 10% (explosive) concentration builds up within 60 seconds. When mixed with air, ignited hydrogen starts to fizzle at a concentration of about 4%  It can explode when concentration exceeds 10%. 

Sealed batteries cope with low levels of overcharge, but to prevent a bomb-like explosion these vents open when pressures reach dangerous levels. As long as the batteries are ventilated (and there is no nearby source of ignition) this gas is likely to dissipate harmlessly to atmosphere.

 Lead acid battery explosion is rare but can blow an RV apart. One well known fifth wheel manufacturer, that had not provided adequate ventilation, blew the bottom out of one of his products through just that.

As assuring basic ventilation is so easy it seems ridiculous not to provide it – let alone (as do some) argue against doing so.  


battery exploded

An exploded battery is not a pretty site. Pic: http://www.rayvaughan.com

Hydrogen only explodes when ignited but an almost invisible tiny spark can do that. Common sources include insecure terminal clamps and cables, and battery connectors that work-harden and crack. Sparks can also be caused by any electrical or moving device, e.g. from worn bearings. Battery chargers, isolating relays etc, should never be installed in battery enclosures. Such battery ventilation is vital.

Battery ventilation is vital – venting details

The battery enclosure must enable fresh air to enter at its base. The lighter hydrogen must be able to escape to atmosphere via unrestricted outlets at the enclosure’s very top. The RV industry has no standards regarding this but general practice is to provide a few 25 or so mm holes at the top and also close to the bottom of the enclosure.

In 2003, the (then) Sustainable Energy Industry Association suggested the following minimum. The size given is for each vent (top and bottom).

Area in sq cm = 0.006 X ‘n’ X I.

Where ‘n’ – the total number of cells in the battery/s (for this purpose each cell is nominally 2.0 volts)

‘I’ = maximum charging rate in amps.

For example a caravan with two 12 volt batteries (each of 6 cells) and maximum charging rate from the alternator and solar input combined might be 50 amps. Then A = 0.006 x 12 x 50 = 3.6 sq cm. As the above is a minimum requirement such ventilation could thus be one or slots top and bottom – each 5.0 cm long by 1.0 one centimetre deep.

Whilst naturally vented enclosures have been criticised, decades of experience indicate they are adequate for caravans and motor homes. Wind however can generate areas of high pressure around the exit vents and prevent the gas escaping. This is less of an issue if adequate lower vents are provided but many an enclosure is vented only at the top. If yours is like that, cut a few holes at the very bottom.


Lithium-ion batteries

These batteries use a technology that is different from lead acid. It is unlikely that any lithium battery would explode but were it to happen they typically emit dense white smoke. This strongly irritates and may harm the respiratory tract, mucous membranes, eyes, and skin. The electrolyte reacts with moisture to form hydrogen chloride and sulphur dioxide. Some also release bromine and chlorine. There are also anecdotal and other reports of fire risk if they are overcharged. Here too battery ventilation is vital.

There is little advice re this currently available from vendors. It seems prudent however to locate them in a ventilated enclosure in the same manner as lead acid batteries, ideally using fire-proof materials. (See also Article Lithium-ion batteries in caravans.)



Because there are no legally enforceable (RV) standards in this area, this article can only emphasise that battery makers stress that battery ventilation is vital. Some explain why.

There is no risk if batteries are housed in well vented enclosures. With sealed batteries the risk is not high. It exists however if they are within (say) a closed bed base that traps any the gas. As eliminating this risk may consist of little more than cutting two slots, or drilling a few holes – why take the risk?


Further information

For information on batteries and battery charging see also Articles: AGM batteries for caravans, Lithium-ion batteries in caravans, and Speeding battery charging from generators.

In-depth coverage of batteries and battery charging is included in my books Caravan & Motorhome Electrics, Solar that Really Works!  (RV-related), and Solar Success (home and property systems). My other books are the all-new Caravan & Motorhome Book, and the Camper Trailer Book.  For information about the author please Click on Bio.

This topic often arises on RV forums. If you feel this article might assist others, please consider posting this Link to it on the relevant forum thread.