Solar and super-capacitor power source for WiSense nodes
We are testing a battery less solar power module for WiSense nodes operating as reduced function devices (RFDs). An RFD spends most of it’s time in deep sleep (all components on the node are put into their lowest power modes). It wakes up now and then periodically or in response to an external stimulus. On waking up it senses and/or communicates if required. RFDs do not take part in routing. They communicate with the rest of the network through a parent node (an FFD) chosen at the time of joining. This mode of operation allows an RFD to keep it’s energy consumption to a minimum. RFDs can therefore be powered by a constrained power source such as a coin cell or a low capacity rechargeable NiMH cell assuming the application requirements can be satisfied. If an RFD is deployed outdoors, it can utilize solar power to power itself. An energy storage device is still required to get through periods of low or zero solar insolation. The energy storage device can be a rechargeable battery or even a super capacitor.
A super capacitor is essentially a capacitor with an extremely large capacitance (usually in Farads). We are using a 2.5 F super-cap in our tests. The energy stored by a capacitor is given by 0.5*C*V*V where C is the capacitance and V is the voltage applied across the terminals of the capacitor. WiSense nodes can tolerate a maximum voltage of 3.6 V. At 3.6 V, the energy stored in a 2.5 F super-capacitor is 0.5 * 2.5 * 3.6 * 3.6 -> 16.2 Joules. Of course, not all of this energy is usable. Let us assume 1.8 V as the lowest usable voltage. The amount of usable energy is then 16.2 – (0.5 * 2.5 * 1.8 * 1.8) -> 12.15 J. How much is 12.5 J ? Let us calculate how long 12.5 J of energy can keep a node powered assuming it’s radio remains active. Let us assume that the node’s average power consumption is 30 mA and the super capacitor’s voltage drops from 3.6 V to 1.8 V.
- Average_Voltage = (3.6 + 1.8) / 2 = 2.7 V
- Average_Power = Average_Current * Average_Voltage = .03 * 2.7 = 0.081 W
- Time = Energy / Average_Power = 12.15 / 0.081 = 150 seconds
So, in the absence of any energy from the solar cells, the super capacitor can keep the node powered on for only 150 seconds !!. The super-capacitor is therefore only useful for very low duty cycle applications where the RFD spends most of it’s time sleeping. As long as solar power is available, it will keep the super-cap charged. The RFD can afford to have a higher duty cycle (wake up more frequently) when solar energy is available. When there is low or no solar insolation, the RFD has to reduce it’s energy consumption. Power management decisions (at any time) will depend on the following variables –
- Super capacitor voltage
- Energy required to transmit a packet
- Energy required to perform a sensing cycle
- Time of the day
Super capacitors have some advantages over rechargeable batteries –
- Available energy estimation is accurate (E = 0.5 * C * V * V). NiMH rechargeable batteries (on the other hand) maintain a constant voltage for most of the discharge cycle so it is difficult to estimate the amount of energy left based on the battery voltage.
- Number of charge/discharge cycles is very high (> 500,000 for the particular super capacitor we are using). Batteries have between 500 and 2000 cycles and need to be replaced eventually.
- Some batteries cannot sustain the burst loads (when the node wakes up and switches on the radio). Super caps have no such limitation.
One disadvantage of super capacitors is the high cost. We are using the “EMHSR-0002C5-005R0” from “Nesscap”. This is a 2.5 F capacitor with max voltage of 5.0 V.
Coming to our test setup, we are using two solar cells (KXOB22-04X3L from IXYS) to power a WiSense node configured to operate as an RFD. The specs are:
- Size – 22 mm x 7 mm
- Voltage (open circuit) – 1.89 V
- Current (short circuit) – 15 mA
- Voltage (at max power output) – 1.5 V
- Current (at max power output) – 13.38 mA
We are using two of them connected in series. The maximum voltage across the super cap will be 2*1.89 V minus the voltage drop across a schottky diode. This comes to 3.78 V – .18 V -> 3.60 V. This is well under the max voltage (5 V) which the super cap can tolerate. This is also the maximum voltage a WiSense node can safely tolerate.
One of the requirements for this design was to avoid the use of a dc-dc converter.
We are using a load switch (from TI) which is gated by a voltage monitor (from ST). The latter monitors the voltage across the super cap. The monitor’s output controls the load switch. As long as the super cap voltage is below 2.5 V, the monitor’s output will remain low which keeps the load switch off. When the super cap voltage climbs above 2.625 V, the monitor’s output will go high which will turn on the load switch. When the load switch is off, the load (which is the WiSense RFD) will not be powered. When the load switch turns on, the load will get power from the super cap. This circuit allows the super cap to get charged by the solar cell up to 2.625 V while the load is disconnected. Note that the MSP430 can start running at 1.8 V. We could have chosen a monitor IC with a lower threshold voltage of say 2.0 V. The voltage monitor IC has a hysteresis of just 0.125 V. We chose 2.625 V to make sure that the MSP430 powers up and is able to run properly without any danger of discharging the super cap below the selected threshold (note the small hysteresis voltage). When the super cap voltage climbs above 2.625 V, the MSP430 powers up and starts running. The default clock on power up is 1.1 MHz (DCOCLK). The first thing the RFD firmware does is to check the super cap voltage using the on chip ADC10 channel dedicated to measuring the MSP430’s power supply voltage. If this voltage is below 2.8 V, the RFD goes to sleep, waking up every 5 seconds to monitor the supply voltage. When the supply voltage exceeds 2.8 V, the RFD starts normal operations which involves initializing the system and attempting to join the network. This was our first attempt at testing the circuit so we were very conservative wrt to the voltage levels. We might choose a lower threshold in further tests. Testing was successful. A completely discharged super cap was used and it charged up to 2.625 V before the MSP430 powered on. The RFD firmware then monitored the supply voltage till it climbed above 2.8 V at which point the RFD started the network association procedure. The RFD then associated with the network coordinator and started sending sensor data every 5 seconds. The super cap voltage increased up to 3.476 V and stayed there. The RFD sent out around 374 packets (in around half an hour) before I terminated the test.
WiSense is going to offer this solar power module as an add on to WiSense nodes. Stay tuned for more updates.
See the test setup in the pic below (I am not good at soldering).