The most common batteries used today in small to household – sized setups fall into two categories – Lead acid and Lithium.

Lead

Lead batteries fall into several different categories;

Flooded

Flooded lead batteries are commonly used for cranking (the battery under your bonnet is almost certainly a flooded lead – acid). They contain a liquid Sulfuric Acid electrolyte. Every time the battery is charged or discharged, some of this electrolyte degrades to form hydrogen sulfide gas, which is flammable. Flooded lead batteries can produce enough hydrogen sulfide to build up and explode, so large installations require a specialised ventillation system. Flooded lead batteries can also be spilled; The electrolyte can produce dangerous / flammable fumes depending on what it comes into contact with.

AGM / Gel

AGM and Gel batteries are different but in terms of safety they fall into the same category – Non spillable, valve regulated lead acid.AGM’s immobilise the sulfuric acid electrolyte in a special absorbent glass mat, which reduces the hydrogen sulfide produced. Gel batteries, similarly, contain a sulfuric acid electrolyte that is mixed with a gelling agent. Both of these batteries are recommended to ‘not be charged in a gas tight container’ but are safe to have in an enclosed space such as a vehicle or a building. Lead – Carbon batteries generally are also either AGM or Gel batteries in terms of electrolyte, but because of their better charge / discharge efficiency and their slightly lower operating Voltage, Hydrogen Sulfide production is even lower.

Lithiuim

There are two broad categories of lithium batteries. Lithium – Ion is the general term used for lithium – Cobalt, lithium – manganese and a few other varieties. LFP, LiFePO4, lithium – Iron (not ion), or LiFePO is the term used for Lithium Ferrous Phosphate batteries.

Lithium ion batteries have advantages in weight and charge / discharge ratings but if mismanaged present a serious fire hazard as they contain a flammable electrolyte that bacomes explosive on contact with water.

LFP batteries are comparitively very safe. They handle the heat better and last longer, but are slightly heavier. Whilst the electrolyte is technically still flammable under extreme conditions, fires starting from LFP batteries are exceedingly rare and typically only occur when a high voltage bank gets short circuited, or when battery cells are mechanically damaged and then get wet. Prismatic LFP cells are some of the safest batteries available.

Fusing

Regardless of chemistry, batteries store a lot of energy and an electrical fault caused by a short circuit or an undersized conductor can start a fire simply because of the heat produced by the current. It is essential to ensure that all cables and links are correctly sized. Anything directly connected to a battery should be fused. Single – pole fuses and isolators, by convention, go on the positive terminal of your battery, and with good reason – In a vehicle, the starter battery is earthed to the chassis, i.e. the entire body of the car is negative. This means that if your positive cable comes into contact with the body of the vehicle at any point, it can create a short circuit (in layperson’s terms, this will cause electrical current to be drawn from the battery extremely fast, producing heat, which could case a fire). Fusing any cable going to the positive terminal of a battery, as close to that terminal as possible, is therefore an essential safety measure.

In higher Voltage / stationary systems, a ‘double – pole’ fuse or magnetic circuit breaker is used and generally double – sheath cable or conduit is used between the battery and the breaker. There are strict standards on the protection equipment that can be used and installation layout for larger systems.