I intended to run this article next year but several recent examples of motorhomes and caravans being built with unventilated battery enclosures has caused me to publish sooner.

Once a lead acid battery is charged beyond 70% (about 14.3 volts for a conventional 12-volt battery) it begins to give off hydrogen and oxygen. This gas is colourless and odourless and can be ignited at concentrations of about 4% (when mixed with air). It then burns with a sort of determined fizzle. But if ignited when the concentration exceeds 10% the explosion may blow a structure apart. The early airship industry experienced this in a tragic way.

Despite this known risk, some builders of caravans and motorhomes install conventional lead acid batteries in unventilated or all-but unventilated compartments. One person told me last month that he had built several in this way, before he realised the danger. Another allegedly told a Member that 'I've made several like that and they are all okay so far - there's no risk because I only use a low output charger'.

How Big a Risk?

Battery makers argue that a normally charging battery presents little risk. And that is so - providing adequate ventilation is provided.

There is also only a very minor risk where batteries are charged by a standard vehicle alternator/regulator because serious gassing only commences when batteries exceed 70 or so per cent charge and a standard alternator rarely exceeds that level. Nevertheless, excess build-up can occur if the charging system goes haywire, or the battery develops an internal fault.

The risk increases when solar modules, battery chargers, or three-step alternator regulators charge batteries because all can charge batteries close to 100%. This is electrically desirable but can cause high concentrations of gas in unventilated enclosures.

The Worst Case

The worst case is where an excessive charging rate (not uncommon with cheap battery chargers), or a shorted cell, causes the electrolyte to boil. Then, 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%, and thus explosive, concentration will build up within 60 seconds. (The data is at the end of this article).

Even this does not require a huge amount of ventilation. Hence the worst-case situation is best considered when designing the enclosure.

No Industry Standards

An RV battery enclosure may be relatively simple. It mainly requires that adequate fresh air enter the bottom, and the lighter and warmer gas allowed to escape to outside atmosphere via unrestricted outlets at the very top. But the RV industry has no standards regarding size, shape, or volume of battery enclosures, nor a guide to vent size.

The general practice is to provide a few 25 or so mm holes at the top and also close to the bottom of the enclosure, such that fresh air is drawn in and gases vented to the outside. But following legal advise, I cannot comment on the adequacy or effectiveness of this.

Several other matters need to be taken into account. Wind can generate areas of high pressure around the exit vents and can cause the rising gas to be 'pushed back' into the enclosure: no problem 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 low down.

Causes of Ignition

Hydrogen will only explode when it is ignited, but it needs only a small spark to set it off. The most common causes are poorly secured terminal clamps and cables and (particularly) rigid battery connectors that work-harden and crack. Almost any electrical, or moving device, may generate sparks: so may worn bearings. It's not good practice to install battery chargers, and particularly solenoids etc, in poorly ventilated battery enclosures. I would not even install them in a well-ventilated enclosure.

Adequate ventilation may also extend battery life. Battery life increases but available capacity reduces as temperature falls.

Gel Cell Batteries

Gel cell batteries are sealed to atmosphere and contain the electrolyte in gelled form. Gassing forms a maze of tunnels within the gel and these tunnels allow the gas to travel to the opposite electrode where they are ‘consumed.'

This does not mean that gel cells require no ventilation. Internal absorption works up to a certain limit but if that is exceeded, automatic vents release the excess pressure - and a lot of gas is instantly released. Maker's advice that gel cells may be installed unventilated has long since been withdrawn.

Much of the above comment applies also to AGM (Absorbed Glass Mat) batteries.

Low Gas Lead/Calcium and Gas Recombination Batteries

These batteries produce only small amounts of hydrogen if charged following the maker's instructions but one cannot rely on chargers not going wrong. These batteries too should be well ventilated.

The Enclosure

A battery normally begins to gas quite gently. In a ventilated enclosure, the small amount of gas is diluted by the air in the enclosure and (being warmer and lighter) is vented to atmosphere. Even in the worst case, gassing usually commences slowly.

Whilst simple naturally vented enclosures can be criticised, decades of experience indicate they are adequate for motorhomes and caravans. The venting in most is adequate, providing the enclosure has holes at the bottom as well as the top - and there is no deep top lip that can trap quantities of the lighter-than-air gas.

I cannot, for legal reasons, suggest that such and such a size vent is adequate or otherwise, however 'Advanced Renewable Energy Systems' suggest a single 50-mm vent above and below a battery bank is adequate for small domestic battery banks (up to about 500 A/hr). A prudent approach might be to exceed that by a factor of two or three.

Forced Ventilation

If installing a fan, that unit absolutely must be of spark-free operation and made specifically for use with explosive gases. The fan needs to supply at least 25 times the volume of hydrogen/oxygen emitted under the worst conditions. This is not as arduous as it might appear and is readily moved by a fan of a mere 20 - 25 mm diameter. The fan should suck or blow cold air into the bottom of the enclosure - not force vent the top.

Whilst easy enough to do, I prefer not to rely on forced ventilation as a dangerous build-up may occur if the fan were to fail

The Basic Arithmetic

In the worst case (ie. a seriously overcharging battery beyond 70% of full charge) produces about 0.2 litre of hydrogen and 0.1 litre of oxygen for every watt/hour of charge. Keeping to a safe 2% concentration requires about 10 litres of air per watt/hour - or 2.5 litres per watt/hour to stay below a borderline 4%.

A 12 volt 100 amp/hr battery charging at a runaway 35 amps accepts about 14.4 volts X 35 amps/hour - approximately 500 watt/hr. It will produce about 100 litres of hydrogen/hour - requiring 5,000 litres of air/hour to dilute to 2%- or 1250 litres of air/hour to dilute to 4%.

Summary

Because there are no existing (RV) standards or reliable research in this area, this article can only really emphasise the importance of battery ventilation - and explain why it is necessary. In other words the risk is known but what represents an adequate solution has yet to be defined, in this field at least.

The article's main purpose is to attempt to persuade professional and home caravan and motorhome builders that adequate ventilation is necessary - even for sealed batteries. Secondly, that it is not good practice to have anything within a battery enclosure that can generate a spark.

The content of this article should not worry people whose batteries are housed in enclosures that are well vented at top and bottom to atmosphere. It is a different matter however if they are vented within the vehicle. Here you are pushing your luck.

Remember that hydrogen is totally without colour or smell. There is no way a human can detect it without instrumentation.

UPDATE - 1/9/03

The following method is suggested by the SEAI (Sustainable Energy Industry Association) for determining MINIMUM vent size. The size given is for each vent (i.e. one at the top and one at the bottom).

Area in sq cm = 0.006 X 'n' X I.

Where 'n' - the total number of CELLS in the battery/s

'I' = maximum charging rate in amps.

For example a motorhome with two by 12 volt batteries (12 cells) and a possible maximum charging rate from the alternator and solar input combined might be 100 amps. Then A = 0.006 X 12 X 100 = 7.2 sq cms.

As the above is a minimum requirement a prudent amount of ventilation could thus be slots top and bottom - each 10-14 centimetres long by one centimetre deep.

More detailed information on batteries and battery charging etc is available from www.caravanandmotorhomebooks.com. It is also covered in depth in Motorhome Electrics, available direct from the CMCA (use the secure site to order).

Ventilation Is Vital & No spark producing devices inside the box!