Guest Commentary: Technology Choices for Telematics Back-up Batteries

Emergency back-up batteries for vehicle telematics systems must be capable of providing instantaneous high power, have an operating temperature range of –40°C to +85°C, be safe for use within the passenger compartment of a vehicle, and have a shelf life of up to 10 years. These parameters narrow the choice of battery chemistry to a non-rechargeable lithium solution.

There are a few different lithium primary (non-rechargeable) systems to choose from, and these can be divided into two types: those with a liquid cathode and those with a solid cathode. Liquid cathode systems, such as lithium-sulphur dioxide (Li-S02) and lithium-thionyl chloride (Li-SOCl2), must incorporate a chemical passivation layer on the surface of the lithium anode to prevent the cell from instantly discharging. The lithium becomes heavily passivated after storage and this is exacerbated at the high temperatures often found inside vehicles. As a result they may not be able to operate for several minutes, if at all, at low temperature. This is not the case with solid cathode systems like lithium-manganese dioxide (Li-MnO2) or lithium-carbon monofluoride (Li-CFx) where the anode and cathode are physically separated avoiding the need for a chemical passivation layer.

Liquid cathode electrolytes are based on sulphur compounds that are both toxic and corrosive. If one of these cells leaks or vents, releasing the electrolyte into the passenger compartment then breathing will rapidly become a challenge. The electrolyte in solid cathode systems is neither toxic nor corrosive. It is however flammable, but providing the cell does not vent on short circuit no electrolyte will be expelled from inside the cell. This is achieved through the use of a ‘shut-down’ separator between the cathode and anode that closes the cell down in the event of a short circuit. This system cannot be used in liquid cathode systems.

Sealed-lead acid (SLA) rechargeable batteries have been considered for telematics back-up, however, these present disadvantages including added weight and volume, and the complexity of a charging circuit with temperature compensation. A typical 12-volt SLA battery weighing 1.1 kilograms with a volume of 432 CC’s is nearly five times heavier and three times larger than a 12-volt lithium battery weighing 232 grams with a volume of 132 CC’s.

In the event of a vehicle fire it is essential that the back-up battery pose no additional safety hazard to fire fighters. That is to say that they should not explode if exposed to excessive heat or fire. Because liquid cathode systems are pressurised, any safety vent that they have must operate at relatively high pressures (600 to 1000 psi), whereas the solid cathode systems are not pressurised and therefore have a low-pressure vent that operates between 200 and 250 psi. Clearly this low-pressure vent presents a lower level of hazard.

The power requirements of the telematics transceiver are considerable and the back-up battery must be able to meet these across the entire temperature range. There are also inevitable space restrictions meaning that the back-up battery must be both small and powerful. Li-CFx batteries are not renowned for their ability to provide high currents, especially at low temperature and as a result a significantly larger battery is needed than with manganese dioxide.

In order to ensure that the telematics unit, in the event of an accident, can notify the emergency services, it is essential that a back-up battery be fitted. It is also vital that that battery should operate under all conditions and that it should pose no risk to the vehicle occupants. The choice of battery technology is critical, and all the indications are that the only choice is a primary lithium-manganese dioxide battery.

About the author. . .

David Richards, Telematics Business Manager, is responsible for the sales and marketing of Ultralife’s products in Europe. Ultralife is the world’s leading developer, manufacturer and marketer of the widest range of customised and standard lithium primary and polymer rechargeable batteries for diverse applications. David Richards can be reached in the UK at: +44 1235 542600.

Publishers Note: It has always concerned me, that as a reliance on the emergency capabilities of Telematics is sold to the public, how come there is no back-up battery power for this lifesaving technology? It can't be cost can it?

Bob Gordon, Co-Publisher

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