In this third insight, we invite you to perceive that autonomy of a wireless vehicle detection systemis the result of a multivariate equation. We will also share with you how HIKOB acts on all these levers to offer you the more long-lasting system possible.
The battery life of a communicating wireless hardware system is related to the nominal capacity of the battery but also to many other external parameters that impact the energy consumption (quality of the wireless radio network, consumption mode of the embedded firmware, level of stress, …) including the deployment environment (temperature variations, electromagnetic context …).
What is the battery life?
- Disposable batteries called primary batteries are electrochemical systems that store energy in chemical form and return it in electrical form. The chemical reaction decreases until the battery no longer provides power.
- Battery life is the amount of time for which a device can operate without having to replace its battery.
- Battery life depends on many parameters inherent to its design, environment, and intensity of consumption.
What is battery life management?
The device energy control unit is a crucial part of the system. It distributes optimally the energy available to other modules (microcontroller, radio chip…). During the design phase, it is about setting up the best configuration according to the usage and taking into account each one of these factors in order to determine the best balance between the potential capacity of the battery and the expected performances. It’s about finding the best activation and processing sequence to get the best measurement possible while keeping consumption to a minimum.
The battery life of a wireless vehicle detection system is the result of a multi-variable equation. It is determined, among other things, by the management of the energy resource of each of the devices composing the system, the relevance of the communication protocol, the deployment environment, the deployment topology, the congruence of the configurations, the meteorological reality, but also of the frequency with which the system is solicited.
Battery life management seen by HIKOB
Estimation of lithium-ion batteries self-discharge phenomenon
The available energy capacity of a battery gradually decreases during its use. The more intensive the use, the faster the capacity decreases. This available energy capacity is also impacted by the self-discharge phenomenon.
Self-discharge is an electrochemical reaction that implies that when an accumulator is not used, it will still gradually discharge, leading to a decrease in the available capacity at the time of use.
According to their manufacturers, the lithium-ion batteries used in HIKOB WISECOW integrated sensors usually lose between 1% and 3% of their available capacity each year at a stabilized ambient temperature due to the self-discharge phenomenon. The reality of the weather being highly changeable and dependent on a given place, the actual phenomenon of self-discharge is therefore a complex phenomenon to model.
Self-discharge has a very low impact on available capacity. But the effective consumption of the integrated HIKOB WISECOW sensor is itself extremely low and comparable to that of self-discharge. The more we advance in time, the more the share of the decline in capacity linked to self-discharge, becomes important.
Under these conditions, designing devices capable of wirelessly transmitting complex data in real time with low power consumption is a real challenge.
Figure 1 : Share of self-discharge in energy consumption
Low power electronic architecture
Throughout the product development process, HIKOB has to master the logical and technological architectural optimizations in order to satisfy the constraints of processing speed, consumption and equipment costs of HIKOB INSTANT systems.
The various electronic cards used in the devices are provided for the best management of the “energy” resource possible, in particular by relying on low-power electronic components. HIKOB also uses microcontrollers with low computing power (32-bit microcontroller) and therefore less energy consumers while deploying advanced detection algorithms to retrieve accurate information.
The low power consumption of the devices makes it possible to reduce their need for energy and thus to supply them with space saving sources. The result is long-lasting and very unobtrusive systems.
For the targeted applications, HIKOB INSTANT systems demonstrate both high reactivity and high energy efficiency. We consider that saving energy and providing a level of service with controlled latency are the primary features of HIKOB INSTANT systems. They rely on a dynamic communication protocol that uses the energy resource just needed to transport the detection data while guaranteeing the required level of service (availability of the communication link, speed of transmission).
During operation, HIKOB devices alternate between periods of measurement, signal processing, communication and standby. The “duty cycle” of a HIKOB WISECOW sensor refers to its activity rate, excluding the standby period. The challenge on consumption is therefore to reduce the duty cycle to increase the sleep time and save a maximum of energy. The complete control of the acquisition chain and the radio communication stack optimizes the activity time and maximizes the “sleep” time.
In addition, the process of self-organization of communications towards an optimal path, reinforced by the possibility of intervening by submitting a preferential connection, helps to build stable radio links, a prerequisite for a controlled energy consumption.
Challenges raised by HIKOB POLAR STAR
Dynamic management of listening time (duty cycle)
HIKOB has invested an extra effort to reduce the energy consumption of HIKOB WISECOW integrated sensors. With HIKOB POLAR STAR, the communication protocol adapts its “duty cycle” according to the actual connection.
To provide the expected level of service in the wireless network, the equipment is constantly seeking to connect with each other to create a communication link. The establishment of the communication link follows a synchronization process which includes a learning phase of the time drift between the clocks of the different devices. Subsequently, each communication contributes to better apprehend this drift and thus to improve the synchronization.
The listening time of the device can consequently be reduced while guaranteeing the good reception of communication.
Figure 2_ Dynamic management of listening time (duty cycle)
Evolution of the battery life prediction model
All the stake of a good battery life prediction model lies in the fine qualification of the parameters, related to the meteorological conditions of the sites where the systems are deployed.
Over the years, HIKOB has been collecting realistic temperature profiles from different field deployments. Climatic chamber tests have also been used to measure the consumption of HIKOB INSTANT devices at different temperatures. This work makes it possible to build differentiated types of consumption patterns.
Jointly, the accumulation of experience and field feedback has given HIKOB a more in-depth knowledge of battery self-discharge. It is this empirical process that makes it possible to further refine the accuracy of the battery life estimates announced for each application. Adjusted over time, the HIKOB Battery Life Estimation Model incorporates a pejorative factor that has increased of 25% in 6 years.
Figure 3_ Oceanic climate – Percentage of time spent, by temperature, during a year
Figure 4_ Energy intensity requirement of the HIKOB WISECOW electronic card according to the temperatures