NomadBob said:
And how much Hydrogen in the semi-sealed battery? Water levels in a 6 cell battery would allow about golf ball sized amount for all 6 cells per 12v battery. Very little 'head room' in those batteries for *that exact reason* So now the *excess* is dispersed into the atmosphere. Albeit an engine compartment, a battery bank house or a cab. Whats the saturation over x hours? What is x hours?
Seems silly to even guess at something that is so easy to follow manufacturers' recommendations and provide ventilation for a battery box. Copy/Paste:
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.
[img=346x0]http://caravanandmotorhomebooks.com/wp-content/uploads/2014/09/battery-exploded-1024x674.jpg[/img]
An exploded battery is not a pretty site.
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.
