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Rechargeable battery applications Moving on to rechargeable applications: These are a nice choice for higher power or higher drain IoT applications where primary battery replacement frequency is not an option. A rechargeable battery application is a higher cost implementation because of the initial cost of the batteries and the charging circuitry, but in higher-drain applications, where the batteries are drained and charged frequently, the cost is justified and soon paid back. Depending on the chemistry used,
a rechargeable battery application can have a lower initial energy than a primary cell, but on a longer term it is the more efficient option and, overall, is less wasteful. Depending on the power needs, another option is capacitor or supercapacitor storage, but these are more for shorter-term backup storage. Battery charging involves several
different modes and specialist profiles depending on the chemistry used. For example, a lithium-ion battery charge profile is shown in Figure 2. Across the bottom is the battery voltage, and charge current is on the vertical axis. When the battery is severely
discharged, as on the left of Figure 2, the charger needs to be clever enough to put it in pre-charge mode to slowly increase the battery voltage to a safe level before entering constant-current mode. In constant-current mode, the
charger pushes the programmed current into the battery until the battery voltage rises to the programmed float voltage. Both the programmed current and
voltage are defined by the battery type used – the charge current is limited by the C-rate and the required charge time, and the float voltage is based on what is safe for the battery. System designers can reduce the float voltage a little to help with lifetime of the battery if required by the system – like everything with power, it’s all about tradeoffs.
When the float voltage is reached, it can be seen that the charge current drops to zero and this voltage is maintained for a time based on the termination algorithm. Figure 3 provides a different graph
for a three-cell application showing the behaviour over time. The battery voltage is shown in red and charge current in blue. It starts off in constant- current mode, topping out at 2A until the battery voltage reaches the 12.6V constant voltage threshold. The charger maintains this voltage for the length of time defined by the termination timer – in this case, 4-hour window. This time is programmable on many charger parts. For more information on battery
charging, as well as some interesting products, read the Analog Dialogue article “Simple Battery Charger ICs for Any Chemistry”. Figure 4 shows a nice example of
a versatile buck battery charger, the LTC4162, which can provide a charge current to 3.2A, and is suitable for a range of applications including portable instruments and applications requiring larger or multicell batteries. It can also be used to charge from solar sources.
Energy-harvesting applications When working with IoT applications and their power sources, another option to consider is energy harvesting. Of course, there are several considerations for the system designer, but the appeal of free energy cannot be understated, especially for applications where the power requirements aren’t too critical and where the installation needs to be hands off – that is, no service technician can get to it. There are many different energy
sources to choose from, and they don’t need to be an outdoor application to take advantage of them. Solar as well as piezoelectric or vibrational energy, thermoelectric energy and even RF energy can be harvested (although this has a very low power level). Figure 5 provides an approximate
energy level when using different harvesting methods.
08 July/August 2022
www.electronicsworld.co.uk
As for disadvantages, the initial cost is higher compared to the other power sources discussed here, since you need a harvesting element such as a solar panel, piezoelectric receiver, or a Peltier element, as well as the energy- conversion IC and associated enabling components. Another disadvantage is the overall solution size, particularly when compared to a power source like a coin-cell battery. It’s difficult to achieve a small solution size with an energy harvester and conversion IC. Efficiency wise, this can be a
tricky one to manage low energy levels, because many of the power sources are AC, so they need rectification – diodes are used to do this. The designer must deal with the energy loss resulting from their inherent properties. The impact of this is lessened with increase in input voltage, but that’s not always a possibility. The devices that pop up in most
energy-harvesting discussions are from the ADP509x family of products and the LTC3108, which can accommodate a wide range of energy- harvesting sources with multiple power paths and programmable charge management options that offer the highest design flexibility. A multitude of energy sources can be used to power the ADP509x but also to extract energy from that power source to charge a battery or power a system load. Anything from solar (both indoor and outdoor) to thermoelectric generators to extract thermal energy from body heat in wearable applications or engine heat can be used to power the IoT node. Another option is to harvest energy from a piezoelectric source, which adds another layer of flexibility – a nice option to extract power from an operational motor, for example. Another device capable of being
powered from a piezoelectric source is the ADP5304, which operates with a very low quiescent current (260nA typical with no load), making it ideal
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