Compared to battery chemistries such as lithium manganese dioxide (LiMnO 2 ) , lithium thionyl chloride (LiSOCI 2 ) batteries can achieve higher energy density and better cost-per-watt ratios and are therefore commonly used in smart flow meters . But LiSOCl 2 batteries have the disadvantage of being less responsive to peak loads, which can lead to lower battery usable capacity. So in this article, we'll explore an effective way to reduce peak battery loads (hundreds of milliamps) to help extend battery life.
Maximizing the available battery capacity is important because it allows the system design to:
- Increase meter readings and data transfer while using the same battery.
- Achieve longer life with the same battery.
- Reduce battery size without changing operating life.
By applying the same design to more types of flowmeters, the above benefits can further reduce battery costs, maintenance costs and development costs.
Design Challenge: Extending Battery Life
A successful instrument design requires long run times (greater than 15 years) and functions such as valve control, data logging, and data transfer. Extending battery life is an effective way to extend meter run time. However, if the battery is connected directly to the load without any power buffers, the complex load profile of the meter may shorten battery life.
According to different currrent levels, the load consumption curve of standard meters can be divided into standby mode, intermediate mode and working mode. Each mode affects battery life differently:
- Current consumption in standby mode is 5µA to 100µA . The main power consumption items are the quiescent current (I Q ) of the metering, microcontroller and protection circuits . Although its absolute value is very small, it is usually the main factor affecting the life of the instrument. When in standby mode, the I Q of any connected DC / DC converter should be in the nanoamp range, and the leakage of the power supply buffer should be low to improve efficiency.
- Current consumption in intermediate mode is 2mA to 10mA . Typically, this type of load comes from the analog front end in the RX stage. In this mode, the efficiency of the power buffers is important to minimize power loss.
- The current consumption is highest in active mode. In active mode, the load usually comes from the drive valve and analog front end of the TX stage, requiring 20mA to several hundred mA. Drawing current directly from a LiSOCl 2 battery severely derates the battery capacity.
Table 1 shows the capacity derating of the Saft LS33600 battery based on the 17Ah rated capacity under different load and temperature conditions . At +20 ° C operating temperature , a 200mA load current results in a 42% capacity derating . Therefore, never use the battery directly to power the load. Peak currents can only be limited to less than 10mA with low leakage power supply buffers .
|
Capacity (Ah) |
-40°C |
–20°C |
+20°C |
|
10mA |
– 41.2% |
– 17.6% |
No derating |
|
100mA |
– 82.35% |
– 58.8% |
– 23.5% |
|
200mA |
not applicable |
not applicable |
– 42.0% |
Table 1 : Capacity and Current Characteristics of Saft Batteries LS33600 Batteries
TI's TPS61094 60nA IQ buck / boost converter extends battery life while maintaining excellent efficiency in standby, intermediate and active modes. The TPS61094 has three main benefits:
- Ultra-high efficiency over a wide load range. With V OUT = 3.3V and V IN greater than 5V , the TPS61094 achieves an average efficiency greater than 90% at loads of 5µA to 250mA , enabling a high-efficiency power supply in most flowmeter use cases.
- Limit battery peak current. The TPS61094 can limit its peak input current when charging a supercapacitor in Buck_on mode , or when powering a heavy load on V from the battery in supplemental mode . Figure 1 shows the configuration of the TPS61094 , and Figure 2 shows the peak battery current with a 200mA and 2s load pulse on V OUT . Peak current is limited to 7mA under heavy load conditions in Stage 1 . The device charged the supercapacitor with a constant current of 10 mA after the load was released in stage 2 . When the voltage of the supercapacitor is charged back to 0V , the device stops charging, but remains in Buck_on mode.
Figure 1 : Configuration of the TPS61094
Figure 2 : Oscilloscope showing battery peak current results under heavy load
- The energy available from the supercapacitor remains the same over the entire temperature range. Typically, using a Hybrid Layer Capacitor (HLC) or Electric Double Layer Capacitor (EDLC) as a power supply buffer improves pulse load capability. However, the energy stored within these passive devices depends on the battery voltage. As the temperature decreases, so does the battery voltage, which weakens the pulse load capability of the HLC or EDLC and increases the battery's supply current. To solve this problem, the TPS61094 keeps the voltage of the supercapacitor stable and does not change the voltage regardless of temperature changes.
The energy available in the supercapacitor depends on the capacity of the supercapacitor, the set maximum voltage across the supercapacitor, and the undervoltage lockout feature of the TPS61094 . The more energy available to the supercapacitor, the longer the operating time under continuous heavy load conditions.
Figure 3 shows a power buffer solution using the TPS61094 or just a supercapacitor, respectively. In the TPS61094 solution, the supercapacitor voltage is set to 2V . When supplying a continuous load, the TPS61094 draws power from the supercapacitor until the supercapacitor voltage drops to 0.6V . Therefore, the available energy on the supercapacitor can be calculated with the help of Equation 1 :
(1)
where ŋ is the average efficiency of the converter.
In the worst-case temperature of –40 ° C , the TPS61094 can achieve an average efficiency of 92% at an input voltage of 2V to 0.6V and a current of 150mA . Equation 2 shows that the calculation result is:
(2)
Figure 3 : TPS61094 with HLC/EDLC configuration
In an HLC or EDLC solution, the available energy varies with the battery voltage. At –40 ° C and 10mA , the LS33600 voltage drops to 3V . The available energy is calculated using Equation 3 as:
(3)
Comparing the results of Equations 2 and 3 , it can be seen that the TPS61094 solution has twice the available energy than the HLC and EDLC solutions. This means that more energy is delivered to the load and, in extreme cases, the battery's peak current is reduced. For example, if a 200mA load is used to drive a valve at 3.3V, the HLC or EDLC solution can only support the load for 2.8s . The TPS61094 buck / boost converter with integrated supercapacitor can support the load for up to 7.8s (assuming all loads are powered by the supply buffer).
Epilogue
The flow meter has a complex load consumption curve and requires the use of a power buffer to help prolong the life of the LiSOCl 2 battery. The TPS61094 provides excellent efficiency over a wide operating range, making it ideal for addressing length-of-life challenges. By limiting the peak current of the battery, this buck / boost converter maximizes the capacity and energy available from the supercapacitor , allowing the system to operate at low temperatures for longer periods of time compared to HLC or EDLC solutions.