Typically, environmental monitoring systems are battery-operated. This is because they are usually in remote locations or need to record data when other forms of power are unavailable. The focus of this article is on battery systems, specifically alkaline batteries and other cell-type batteries.
Required Battery Capacity
The first step in determining an appropriate battery technology is calculating the required battery capacity:
Required battery capacity = (monitoring system’s current drain) x (reserve time) / (0.8)
The system’s current drain is the calculated average drain in Amps; the reserve time is the amount of hours that the system should be able to operate without charging; and 0.8 is used as a factor of safety to take into account that a battery should only discharge 80% of its capacity. For example, a ‘standalone’ data logger with a wind sensor that has a current drain of 10 mA and needs to be able to last 30 days without charging should use a 9 AHr battery.
0.010 A x ( 30 days * 24 hours/day ) / 0.8 = 9 AHr
Note: 1 mA = 0.001 A
Increasing battery capacity
The capacity of a power source can be increased by connecting two power sources in parallel. A battery is connected in parallel with another battery when the positive leads of each battery are connected together, and the negative leads of each battery are connected together.
Batteries connected in parallel should be identical in type, age and capacity. For example (2) fresh 12 V 6 AHr batteries of the same model and manufacturer could be connected in parallel to make a single 12 V 12 AHr power source.
A blocking diode is used to prevent current from flowing in reverse. This should prevent batteries in parallel from charging each other, which can overcharge and damage both batteries. As shown in diagram 4.2, the diode is wired between the positive terminals of the batteries. (Cathode – towards the circuit | Anode – towards the farthest away battery)
Required Battery Voltage
The required battery voltage of a monitoring system should be based on the middle value between the minimum and maximum allowable voltages of all data loggers and sensors powered directly from the power source. For example, a data logger with a battery voltage requirement of 10.7-16 V could use a single 12 V lead acid battery. It could also use (9) AA alkaline batteries (which typically start at 1.4-1.5 V and drop to 1.2 V).
Increasing battery voltage
Battery voltage can be increased by connecting two power sources in series. A battery is in series with another battery when the positive lead of one battery is connected to the negative lead of the other. The un-connected negative lead is the negative lead of the power source and the un-connected positive lead is the positive lead of the power source.
Batteries should be identical in type, age and capacity. For example (2) fresh 12 V 6 AHr batteries of the same model and manufacturer could be put together in series to make a single 24 V 6 AHr power source. This method of connecting batteries in series is used for power sources that utilize multiple small batteries due to their relative inexpensiveness and size, such as alkaline batteries.
Primary cells, commonly referred to as non-rechargeable batteries, are an inexpensive means of powering environmental monitoring systems. They are best suited for applications which can last several months without changing the batteries. The most common primary cell batteries are described in the following sections:
Carbon-zinc batteries were the most common form of primary cell batteries before alkaline batteries were sold at the same price point. Batteries that are not specifically listed as alkaline or have names such as “Heavy Duty” are usually carbon-zinc. Compared to alkaline batteries, carbon-zinc batteries have smaller capacity and shorter shelf life. Carbon-zinc batteries are no longer recommended as their alkaline counter parts are now just as cost effective.
Alkaline batteries are the most common primary cell batteries. Alkaline batteries come in various shapes and sizes. The most commonly found sizes are listed in table 4.3.2.
The Normal Voltage is the voltage level a full capacity battery will output. This voltage level can be lower depending on how long the battery was stored and in what conditions (figure 4.5). Since alkaline batteries have a near linear discharge rate until it reaches its voltage cut off, the lower the voltage level, the lower the remaining battery capacity. For example, an alkaline, AA battery, at 1.1-1.2 V has about half of its remaining capacity.
When an alkaline battery reaches the voltage cut off, it will quickly drop to 0 V. If an alkaline battery is left in a powered device after it has reached its voltage cut off, the battery will begin to reverse polarize. When this happens, the battery will switch to a negative voltage. This will cause damage to any other batteries in the device, as well as potentially cause batteries to leak, which will damage device electronics. It is recommended to only use batteries that are at the same discharge level in a device that requires more than one battery to avoid this occurrence. When one battery reaches its voltage cut off, all of the batteries in the device should be replaced.
The typical capacity of an alkaline battery for low power consumption devices is listed in the table. However, alkaline battery capacity is heavily dependent on the amount of current used by the device(s) it is powering. For example, a AA battery powering a system with an average current draw of 100 mA or less will have the typical capacity of 2,850 mAHr, as listed in the table above. However, the same battery, when powering a system with an average current draw of 2000 mA, will only have a capacity of 285 mAHr. For high drain applications, Duracell Ultra and Energizer Advanced Formula batteries do last approximately 30% longer than standard alkaline batteries.
Another effect on alkaline battery capacity is storage temperature. Alkaline batteries self-degrade over time, about 3%/year when stored at room temperature (figure 4.5). This number can be improved by storing the batteries in a colder environment. If alkaline batteries are to be stored for several years they should be stored in a freezer to preserve their initial typical capacity.
Primary-cell Lithium batteries are typically used to power real time clock circuits on sensors or data loggers. In a coin shaped package, these batteries usually supply 1.5 or 3 V with battery capacities exceeding 1000+ mAHr. This battery keeps the real time clock circuit on a sensor or data logger powered while the sensor or data logger is powered off. This allows the circuit to continuously keep time even while the device is not powered. It is easy to tell when this battery needs replaced, as the time on the sensor or data logger will reset to a time in the past, such as 1/1/2000 or 12/31/1969. Ideally the battery is designed with enough capacity to power the real time clock circuit for the life of the instrument. However, sometimes these batteries only last 3-5 years and will need to be replaced when they are drained. Lithium batteries have a flat discharge curve, which means they will keep a near steady voltage throughout their life. When these batteries reach the end of their life, their voltage output will quickly drop to near zero.
The size of a lithium battery coin typically depends on its capacity and voltage level. These coin shapes also come in two varieties: soldered-lead or battery-holder. Soldered-lead batteries have the positive and negative connection of the battery soldered directly to the circuit board it is powering. Replacing a soldered-lead battery involves de-soldering the leads and soldering a new soldered-lead battery in its place. These batteries usually are not meant to be user replaceable. On the other hand, battery-holder batteries are simply battery coins that slide into a battery holder soldered onto a circuit board. This makes them easier to replace when needed.
Lithium batteries are also available in standard sizes such as A and AA, but are very expensive (sometimes nearly 10x the cost), and carry less capacity than alkaline batteries. Therefore they are not recommended as an alkaline battery replacement.
Water activated batteries are one-time use batteries. They do not contain electrolyte to produce any electricity before activation. To produce a voltage they must be soaked in water for several minutes. As such, they can be stored inactivated for years or even decades without any degradation. However, after activation, they have a high self-discharge rate. Because of this they should only be activated prior to use.
These types of batteries are designed to be environmentally friendly as they lack the heavy metals commonly found in other battery technologies. The most common application for these batteries is powering high altitude weather sensors, called radiosondes. Radiosondes cannot contain heavy-metals as they fall to the ground or ocean bottom surface. More information about radiosondes can be found on the NOAA (National Oceanic and Atmospheric Administration) website: http://www.ua.nws.noaa.gov/factsheet.htm
Secondary cells, commonly referred to as rechargeable batteries, are a more expensive means of powering environmental monitoring systems. They are best suited for continuous charging applications. The most common secondary cell batteries are described in the following sections:
Lithium-ion batteries are typically used in portable devices because of their high energy-to-weight ratios, ability to not lose charging capability (lack of memory effect), and slow discharge when not in use.
Li-ion batteries have several advantages compared to other secondary cell battery technologies, including:
– High energy density. They have higher battery capacities and voltages compared to other batteries of the same size and weight.
– Operation at higher voltages per cell than other rechargeable battery technologies; 3.7 V as opposed to 1.2 V NiMH (Nickel Metal Hydride) or NiCd (Nickel Cadmium).
– Low self-discharge rate. Once they are charged, they will retain their charge for a long time. For example, NiMH and NiCd battery technologies can lose 1-5% charge per day, while Li-ion batteries will retain their charge for months.
– Special circuitry to protect the battery from damage due to over or under charging.
However, Li-ion batteries:
– Are more expensive than other technologies as they are more complex to manufacture and made in smaller numbers.
– Are not available in standard alkaline cell sizes (AAA, AA, C, D) like NiMH and NiCd batteries.
– Require intelligent chargers to monitor the battery charge. Additionally, each battery model must have its own charger as there are not standard Li-ion battery sizes. This makes Li-ion battery chargers more expensive and harder to find.
– Are not as rugged or durable as other battery technologies and are prone to explosion under high pressure, extreme temperatures, and improper charging.
A unique drawback of the Li-ion battery is that battery life is entirely dependent upon its age from time of manufacture regardless of the number of discharge cycles, usage, and other factors that affect other secondary cell battery technologies battery life.
However, temperature does still affect the self-discharge rate of a Li-ion battery. At room temperature (25°C), a Li-ion battery typically loses 20% capacity per year. At freezing (0°C), a Li-ion battery typically loses 6% capacity per year.
Since Li-ion batteries are constantly evolving with new ion variations and the expensive nature of the battery and associated charger, Li-ion batteries are best left for laboratory equipment and other specialized equipment that will not be used in field applications.
NiMH and NiCd batteries
NiMH (Nickel Metal Hydride) and NiCd (Nickel Cadmium) batteries are available in the same package sizes as alkaline batteries. They are usually a drop-in replacement for alkaline battery applications. Additionally, while older NiMH/NiCd batteries typically had much lower capacities compared to Alkaline batteries (such as only 800-900 mAh in AA sizes), they are now available in higher capacities (such as 2500-3000 mAh in AA sizes), which match or exceed Alkaline batteries. And unlike alkaline batteries, NiCd/NiMH batteries do not lose capacity under high current draws.
One important difference between NiMH/NiCd batteries and alkaline batteries is that NiMH/NiCd batteries have a flat discharge rate. They operate at a steady 1.2 V throughout their entire discharge cycle. Alkaline batteries, however, have a linear discharge rate. They begin at 1.5 V and slowly drop down to 0.8 V throughout their entire discharge cycle. Because alkaline-powered devices are designed to handle voltages as low as 0.8 V, a steady 1.2 V will power these devices.
Additional advantages of NiMH/NiCd batteries include:
– Not being damaged by complete discharges (like lead acid batteries)
– Availability in alkaline battery sizes (AAA, AA, etc.)
– Use inexpensive chargers compared to lithium-ion or lead acid batteries
However, NiMH/NiCd batteries:
– Are more expensive compared to alkaline batteries
– Have a lower capacity than alkaline batteries (The total lifetime of NiCd/NiMH batteries is longer as they can be recharged)
– Have a self discharge rate of 1-5% per day at room temperature (They will retain over 90% of their charge for a full month if kept at 0°C)
– If charged after only partially discharged, will develop a lower voltage and capacity. This effect is called “voltage depression” or “the memory effect.” It is reversible by fully discharging and then charging the battery several times.
– Are toxic to the environment (only Nickel Cadmium)
NiMH batteries are similar to NiCd, but are less toxic and offer higher capacities. However, they are also more expensive than NiCd batteries.
Charging NiMH/NiCd batteries
NiCd or NiMH batteries can be damaged by overcharging and over-drawing current. NiCd and NiMH batteries sometimes come with timer-based or simple trickle chargers. If an incorrect charger is being used or if the batteries are left on the charger too long, overcharging will occur that will permanently reduce the battery capacity. Be sure to only charge batteries that a charger was designed for. Additionally, if NiCd or NiMH batteries are kept in a device that is draining current from the batteries after the battery has run out of capacity, the discharged battery will reverse the polarity, rendering it unusable. It is recommended that batteries be removed from any system that is not in use.
Even with the known disadvantages and higher cost, NiCd or NiMH batteries are typically a better option for systems where continuous access to the system is available for battery charging. In systems where long-term deployments without access are required and battery charging is not feasible, alkaline batteries are better suited due to the higher battery capacities available.
Due to the type of metals contained in batteries, they should never be disposed of improperly. Batteries such as Nickel Cadmium, if disposed of in landfills, can seep toxic cadmium into the water supply with serious side effects to human health.
See http://www.epa.gov/safewater/contaminants/dw_contamfs/cadmium.html for more information.
In 1994, the Rechargeable Battery Recycling Corporation (RBRC) was founded to promote recycling of rechargeable batteries in North America. RBRC is a non-profit organization that collects batteries from consumers and businesses and sends them to recycling organizations.
See http://www.rbrc.org/ for more information, including drop-off sites.
Companies such as Inmetco and Toxco are two of the largest battery recycling companies in North America. These companies are good for recycling batteries in large quantities.
AC to DC power supplies
AC to DC power supplies supply constant current and voltage to an environmental monitoring system as long as AC power is available. They typically come in two varieties; switching and transforming. Switching power supplies are small, light weight, and inexpensive as they use integrated circuits to convert AC to DC power. Transforming power supplies are typically bulkier, heavier, and more expensive than switching power supplies as they use a large coil of wire, called a transformer, to convert AC to DC power. However, transforming power supplies are usually more rugged and offer protection to the monitoring system. If the AC power spikes, it could cause damage to equipment connected to it, but a transforming power supply will short and only damage itself. A switching power supply on the other hand, unless listed as a specification, will send damaging voltages onto the system it is powering.
As power outages or outlet failures can cause an AC to DC power supply to stop powering an environmental monitoring system for the duration of a power outage, this type of power system is not recommended for applications where critical data is obtained.
Battery chargers are suited for applications where an AC power outlet is available. They are inexpensive compared to solar charging systems and are therefore recommended when it is available. The type and size of battery charger should be selected based on the type of battery that is being charged.
Simple or Trickle Chargers
Simple or trickle chargers supply a constant charging current to the battery and can be used with all rechargeable battery types. Typically, a simple charger has a low current output and takes longer to charge a battery to avoid battery over-charging. This simplicity in design makes a simple or trickle charger inexpensive. However, a battery will overheat and become permanently damaged if it is kept on a simple or trickle charger for too long. This type of charger is never recommended unless used under monitored conditions.
Timer-based chargers operate in the same manner as simple or trickle chargers, except they stop charging after a set timer has expired. They can be used with any battery type but are best suited for NiCd or NiMH batteries. Typically these chargers are bundled as a package with a set of NiCd or NiMH rechargeable batteries that are well suited for the timer settings. Timer-based chargers should only be used with the batteries they are designed for or came with. They will over-charge batteries with a lower AHr capacity and under-charge batteries with a higher capacity than the charger was designed for.
Intelligent chargers monitor a battery’s voltage, temperature and/or charge time to determine the best charge current to send to the battery at the current moment. An intelligent charger will stop charging a battery when it determines that the battery is fully charged. This type of charger is ideal for newer battery technologies such as Lithium Ion, which is easily susceptible to over-charging. Intelligent chargers are higher priced than simple or timer-based chargers.
Fast chargers use special control circuitry located inside NiMH batteries to rapidly charge the battery without permanently damaging it. They were first developed by VARTA for VARTA brand NiMH batteries and were designed to overcome the several hours’ charge time required for NiMH batteries. Most fast chargers have a cooling fan to keep the temperature of the battery at a stable and acceptable level. They are also capable of performing as a standard NiMH charger for NiMH batteries that do not have special control circuitry.
USB (Universal Serial Bus) chargers use an available USB port on a computer to charge an attached battery. Many consumer electronic devices can already be charged directly from the USB port. NiCd and NiMH batteries chargers can also be purchased with a USB-based charger. These chargers simply use the 5V, 100mA power supply from the USB port. Due to the low voltage and current of the USB port these chargers are best suited for small batteries in very low powered devices.