Exploding lead acid batteries

1. Introduction

Thirty seven incidents of exploding lead acid batteries at coal mines, metalliferous mines, and quarries have been reported to the Mines Inspectorate over the last 11 years - an incidence rate of 3.4 per year for mining and quarrying operations. These batteries, used in stationary and mobile plant and vehicles, have exploded, with casings shattering and the hazardous internal electrolyte, a blend of water and sulphuric acid at low pH, being expelled.

Injuries have resulted, mostly from the impact of plastic shards from the exploding casing and chemical burns from the electrolyte

2. Types of batteries

The two types of batteries commonly used for the above applications are:

2.1 Valve regulated lead acid (VRLA) batteries (also called 'sealed' or 'maintenance free' batteries)

There are two primary types; gel cells (with silica dust) and absorbed glass matt (AGM), with the glass or gel used to immobilise the electrolyte.

In VRLA batteries the casing is a sealed, more modern design. Most of the oxygen (O₂) and hydrogen (H₂) produced during charging is converted back to water when the battery supplies current.

Excessive internal gas pressure, produced for example when the battery is charged too long or too fast, is released through a one way pressure relieve valve. Normally only a small quantity of gas is released.

2.2. Vented batteries

These are an older design, with non-sealed casings which freely vent H₂ and O₂ to the atmosphere. If the area around the battery isn't sufficiently ventilated, an explosive atmosphere can be created.

This type of battery requires regular topping up with distilled water. As the sulphuric acid has a low vapour pressure, it seldom needs topping up.

3. Incidence rates

Battery explosion incident reports show that in mobile plant and vehicle applications, VRLA batteries explode significantly less than vented batteries. For stationary plant, incidents are reported for both types of batteries.

For stationary plant on standby (e.g. fire pumps) it is perceived that the incidence rate increases significantly if the charging characteristics are inappropriate for the application.

4. Recommendations for mitigation

This bulletin cannot account for all possible battery applications, but if a battery application is about to be introduced or changed, a proper management process, taking into account the particular circumstances and the latest relevant technical information, needs to be undertaken.

The risk of injury due to a battery explosion can be reduced by:

• risk assessing all activities involving batteries, including:
  • introducing or changing a battery use application (mobile and stationary plant)
  • selecting or changing the type of battery
  • the characteristics of the charging circuit
  • the extent and frequency of maintenance, inspection and audits
  • the process for jump-starting vehicles.
• following original equipment manufacturers' (OEM) recommendations for the battery and/or equipment
• consulting with the OEM, and if applicable, use VRLA batteries instead of vented batteries
• using correct personal protective equipment when working with batteries, which may include acid resistant gloves, chemical safety goggles, foot protection and long sleeved acid resistant clothing
• using the correct type and rating of battery for the application e.g. for some applications, a 'deep cycling' battery may be more appropriate
• ensuring regular, appropriate battery inspection and maintenance
• following correct charging characteristics. Do not overcharge batteries. Standby equipment e.g. fire pumps and generators, may use continuous float (trickle) charging instead of charging at a higher
• current. Consult OEM for guidance. Note that Reference 3 provides guidance on starter batteries and back-up diesel engine systems.
• when jump-starting, ensuring correct polarity of connections. Some operators use Anderson plugs, which help to ensure that polarities are connected correctly.
• not jump-starting from a battery of a different nominal voltage e.g. 12V vs 24V. (Anderson plugs can also be used to help ensure that similar voltages are connected)
• before removing terminal clamps, ensure current isn't being drawn from the battery e.g. open the battery isolator switch. Sparking at disconnection can be an ignition source.
• noting that metal objects such as tools may cause sparks (and ignition) if they simultaneously touch a positive battery lead and the vehicle body.
• safely discarding batteries with distorted plates which may cause internal shorts and ignition.
• in multiple battery sets, ensuring all units in the set are of similar rating, type and condition. Do not mix old and new. When needed, exchange all batteries in the set at the same time.
• determining if battery-charging stations need explosion protected electrical equipment
• establishing and maintaining a register of batteries used on site
• establishing a site standard on battery types and their applications
• regular auditing to ensure conformance to the site standard.

The following applies especially to earlier design vented batteries:

• Keep ignition sources away.
• Charge in a well-ventilated area.
• Maintain battery fluid levels. The volume available for hydrogen to accumulate increases as the levels drop, which also increases the risk of internal shorting.
• When jump-starting a vehicle, see References including:
  • Connect the cable to the live terminals first (i.e. positive for most vehicles).
  • Consulting with the OEM, connect the cable to the earth polarity (i.e. negative for most vehicles) at a position away from the battery, such as on the chassis, body, engine or special cable receptacle such as Anderson plugs. Then any sparking during cable clamping is removed from areas of potentially elevated hydrogen levels.

5. Background information – Why does a battery explode?

5.1. Mechanism

H₂ and O₂ gases are produced inside the battery casing during charging, and when almost fully charged, the H₂ and O₂ production rate increases. It also increases during overcharging or too rapid charging.

The 2:1 volume ratio for the H₂:O₂ produced, excluding the effect of other gases that may be present, creates a very explosive mixture.

The 4% lower and 75% upper explosion limits for hydrogen in air means it can explode over a wide range of concentrations, so partial dilution/ventilation is probably insufficient. The H₂ concentration needs to be below 4%.

5.2. External ignition

Any ignition source in the zone where hydrogen is within its explosive range increases the likelihood of an external explosion. If a path to inside the battery exists - e.g. for a vented battery, the flame front may continue into the casing, igniting any gases there, and increasing pressure inside the casing. The casing, not designed to withstand this pressure, will probably explode, potentially injuring bystanders.

For a VRLA battery, an external flame can't enter the sealed battery casing, the vent valve releases one way only, and only small quantities of gas will be released. In the unlikely event gas from the vent valve is ignited, it will not be contained and no solids or chemicals will be propelled.

5.3. Internal ignition

Although less likely, it is possible to have an ignition inside the battery if there is electrical shorting between battery plates. This can occur when there is:

• an old, neglected or continuously overcharged battery. Damaged batteries can cause plates to touch creating an internal ignition source.
• impact damage to the battery.
• excessive vibration over time e.g. the battery of a tracked dozer used for ripping.

5.4. Chemistry

The explosion reaction is 2H₂ + O₂ => 2H₂O + Heat. Fewer gas molecules are present after the reaction than before, which in principle should decrease the pressure, but the rapid rise in temperature causes a net expansion effect, resulting in an explosion.

Authorised by Russell Albury - Chief Inspector of Coal Mines

Contact: Theo Kahl, Inspector of Mines , +61 7 4936 0127

Issued by Queensland Department of Natural Resources and Mines