Nickel metal hydride batteries are based on a mature technology that has been used commercially for over 25 years for a variety of applications including consumer products, electric vehicles, hybrid electric vehicles, and stationary power applications. The introduction of batteries into cars are now driving the safety regulations both for vehicles and other battery applications, such as energy storage.
A significant part of the Battery Management System (BMS) functionality is to keep the battery from stages, which may affect reliability and safety. This mainly concerns the prevention of overcharge, over-temperature and short circuits.
But even in the case of BMS failing or malfunction, Nilar batteries show a high degree of passive safety under abusive treatment3:
The Nilar Hydride® design includes a safety vent to prevent an internal pressure buildup that may occur during abusive operation. Nilar systems can handle overcharging for 30 minutes at a 1C rate and up to 5 hours at a 0.2C rate without needing to vent. However, higher rates and longer periods may cause the pressure to exceed the maximum pressure limit of 5.5 bar. Consequently, the safety vents could open, releasing a minimal amount of gas to relieve the pressure.
Over-discharge and reversal
Over-discharge of a Nilar battery does not cause a pressure or temperature increase so it is not considered safety critical. However, this kind of abuse does lead to long term damage of the battery and should be avoided. In addition, over-discharge can lead to reversal in the cells where their polarity shifts to an opposite state. If this is continuous, this can eventually lead to an internal pressure build-up, triggering the safety valves to open.
A short circuit can occur when current is able to proceed through a circuit it
shouldn’t, such as when the positive and negative terminal of the same battery are
incidentally connected. To prevent a short circuit, Nilar has installed fuses on both the
positive and negative side of each string.
External heat / fire
If an external fire were to occur near a Nilar battery system, the fire should be extinguished like an electronics fire, utilizing water or CO2. If the fire is not extinguished, the heat will eventually cause a pressure buildup leading the
Nilar batteries to ventilate. The water-based electrolyte that will leak in the case of
ventilation is not flammable.
1 Electrode materials in NiMH batteries are chemically stable when in contact with the electrolyte. There are no heat generating reactions taking place between the electrode materials and the electrolyte and no solid electrolyte interface is needed to protect the electrolyte from electrode materials. This can be compared to chemistries containing highly flammable organic electrolyte. If the organic electrolyte is catching fire, explosive and poisonous gases are released.
2 Propagation is a dangerous phenomenon that can occur in batteries based on other chemistries, where one cell that has run into thermal runaway can spread the heat to other cells, in that way initiating thermal runaway in other cells and causing a cascade effect.
3 Several battery chemistries require a very strict safety region when it comes to upper voltage limits, temperature limits and current limits. If you pass the set limits you enter the safety critical region where thermal runaway can be triggered by internal short circuits and/ or external heat. For example, during deep discharge or overcharge.
When compared to other chemistries, one advantage of the Nilar battery pack is that United Nations (UN) approved packaging and marking is not required for transport by sea, road, rail and air.
There is no dangerous goods documentation required when transporting Nilar battery packs by air, road or rail.
For transportation by sea, a dangerous goods declaration is required for quantities exceeding 100 kg in one transport unit. Nilar battery packs are then defined as dangerous goods, class 9. The associated UN number is UN 3496 and the Proper Shipping Name is Batteries, Nickel-Metal Hydride.
In addition, the International Maritime Dangerous Goods Code (IMDG) provisions do not apply to NiMH batteries contained in or packed with equipment, according to Special Provision 963.
For transportation by air, an Air Waybill or similar is required. Nilar battery packs are not classified as dangerous goods by the International Air Transport Association (IATA) and belong to the entry “Batteries, dry” in their list of dangerous goods. According to the IATADangerous Good Regulations, an Air Waybill must include the words “Not Restricted” and the substance description must include Special Provision Number A123.
For several other battery chemistries, heavy regulations apply for all modes of transport, especially regarding transport by air. For chemistries classified as fully regulated dangerous goods, strict regulations and even training courses may be required for the personnel involved in the transportation.
• Transport UN38.3 Test T1 Altitude & Test T3 Vibration
• Transport ADR-S SP238 Test A