A common question when deploying environmental monitoring systems is what type and how much power do I need. If under-designed the power supply could fail to provide adequate power, resulting in data loss. If over-designed the power supply can add an enormous cost to the system and take a considerable amount of space. With a little work ahead of time, selecting the power supply components should be a simple and straightforward task.
This section will give the information needed to make an informative selection for an environmental monitoring system's power needs.
Typically environmental monitoring systems are battery operated as they are either in remote locations, or need to be able to record data when power is unavailable. For the purpose of this section we are going to focus on battery systems with continuous AC or solar charging.
Power Supply Components
There are three main components of an environmental monitoring system power supply:
1. The data loggers and sensors, which make measurements and log results while consuming power from the power source.
2. The power source, which supplies the required voltage and current for data loggers and sensors to operate.
3. The charging source, which supplies a continuous renewal of energy to the power source.
Data Loggers and Sensors
The data logger and sensor power requirements need to be calculated in order to determine the type and size of an appropriate power and charging source. Even if AC charging is utilized, this power requirement calculation is important to ensure the system will stay powered during AC or
battery charger failures.
System Power Requirements
The first step in determining system power requirements is to determine how much current the data logger and the sensors will draw. This information will be available in the specification sheets of the data logger and sensors. The two important specifications to look for are how much and for how long current is drawn.
For example, suppose an environmental monitoring system consists of a data logger and an analog wind sensor. The data logger specification sheet lists the typical current draw at 1mA sleep, 8mA processing, and 36mA analog measurement. The wind sensor specification sheet lists the typical current draw at 10mA. The data logger manual states the analog circuitry is turned on for 15 seconds for each analog measurement and completes all processing in that time. The data logger is then sleeping the rest of the time. Note: sometimes sleep current is referred to as quiescent current.
Using the desired sample (or scan) interval of the system, a power calculation chart can be created.
For example, for a sample interval of five minutes, the following table can be formed:
| | Duration (sec) | Current Drain (mA)< /td> |
| Analog Measurement | 15 | 36 |
| Processing | 1 | 8 |
| Quiescent | 285 | 1 |
| Wind Sensor | 300 | 10 |
The average current drain of the system can then be calculated as:
Current drain (mA) =(Current Drain 1*Duration 1)+(Current Drain 2*Duration 2)
Log Interval
So the average current draw is 12.8mA:
12.8 mA =(15sec*36mA) + (1sec*8mA)+(285sec*1mA)+(300sec*10mA)
300sec
With the average current draw of the system, the power requirements the system needs to last a predetermine amount of days can be calculated. For example, if the system is required to operate for 30 days without charging it would require at least 9216 mAHr of battery capacity:
Required battery capacity = days without charging * 24 hours * Avg system current draw
For the above system: 30 days * 24 hours * 12.8 mA = 9216 mAHr
Note: 1mA = 0.001A, so 9200 mAHr is the same as 9.2 AHr
Conserving power
There are a few easy methods to reduce current draw if the required battery capacity is deemed excessive. The first easy method to reduce the required battery capacity is to lengthen the sample interval of the system. The example system with a 5 minute interval draws 12.8mA. However, with a 10 minute interval the system current draw is reduced to 11.8 mA.
11.8 mA =(15sec*36mA) + (1sec*8mA)+(585sec*1mA)+(600sec*10mA)
600sec
A 10 minute interval reduces the 30 day required battery capacity to 8.5 AHr. Increasing the sample interval to 15 minutes lowers the 30 day required battery capacity to 8.1 AHr.
Another easy method to reduce the required battery capacity is to only power sensors as long as they need to be powered to output a reliable measurement. For example, if the analog wind sensor only needed to be powered for 10 seconds to output a stable reading, the average current draw of the system can be significantly reduced. For this method to be feasible, the data logger must have the functionality to switch power on and off to the wind sensor based on the sample interval. At the five minute interval originally used, the average current draw can be decreased from 12.8mA to 3.11mA by implementing switch power to the sensor.
3.11 mA =(15sec*36mA) + (1sec*8mA)+(285sec*1mA)+(10sec*10mA)
300sec
Now the 30 day system would only need 2.24 AHr of battery capacity! If the sample interval is increased to 10 minutes the system draws 2.055mA, only requiring 1.48 AHr of battery capacity for one month of operation! At 1.48AHr battery capacity per month, the system could be powered by (8) C cell batteries (8 batteries in series to get to the required system voltage) for over 5 and 1/2 months!
Data logger telemetry
The above calculations have all been performed assuming that the data logger is not powering any telemetry, such as phone, radio, cellular, or satellite modems. The telemetry power must also be added to the equation.
If the data logger is powering a weather sensor with switched power for 10 seconds, and sampling data every 10 minutes, the average system current draw is 2.055mA, allowing one month of operation with 1.48AHr of battery capacity.
If the same data logger is also powering a phone modem, which transmits data to a computer once a day for 5 minutes, the average current draw
of the telemetry would be:
| | Duration (sec) | Current Drain (mA) |
| Quiescent | 86,100 | 1 |
| Transmit/Receive | 300 | 139 |
**Phone modems are typically only powered during communication using a method called power on ring**
1.479 mA =(300sec*139mA) + (86,100sec*1mA)
86,400sec
For telemetry calculations, the quiescent current is the power consumed by the data logger telemetry when the data logger is not sending or receiving data. The transmit/receive current is the power consumed by the data logger telemetry when the data logger is sending or receiving data.
The average system current draw can then be calculated by adding the data logger average current draw with the telemetry average current draw. In the example above this equals 3.534mA (2.055mA + 1.479mA). The required battery capacity for one month of operation is now 2.5AHr. One consideration is that if the data logger is interrogated twice a day, three times a day, etc then the average system current drain is also increased.
Since a phone modem is only powered on during communication it has a relatively light addition to the average system current drain. However, a radio or cellular modem must be constantly powered for communication to be available.
A cellular modem, which transmits data to a computer once an hour for 5 minutes would have an average telemetry current draw of:
| | Duration (sec) | Current Drain (mA) |
| Quiescent | 3300 | 40 |
| Transmit/Receive | 300 | 200 |
53.333 mA =(300sec*200mA) + (3,300sec*40mA)
3,600sec
The average system current draw can then be calculated by adding the data logger average current draw with the telemetry average current draw. In the example above this equals 55.388mA (2.055mA + 53.333mA). The required battery capacity for one month of operation is now 39.9AHr!
Conserving Telemetry Power
Just as the current draw of the wind speed sensor can be reduced by controlling power to the sensor, an easy method to conserve telemetry power is to only power the data logger telemetry when it will need to be accessed. If the above cellular system was only powered for 10 minutes every hour the average telemetry current draw would be reduced to:
23.333 mA =(300sec*200mA) + (600sec*40mA)
3,600sec
This gives a system average current draw of 25.388mA, and a required battery capacity of 18.3AHr for one month of operation.
The concern to this approach is that during the 50 minutes that the cellular modem is powered off a computer will be unable to communicate to the data logger using the cellular telemetry. Additionally the data logger time will need to sync with the computer time regularly so that during the 10 minute window the data logger is powering the cellular modem, the computer will be in the 10 minute window that it will retrieve data from the data logger. The clock on an embedded device such as a data logger can drift as much as 2 minutes per month.
An addition consideration in switching power to data logger telemetry is that radio, cellular, and satellite systems need time to latch onto a wireless signal before it can begin transmitting or receiving. Radio modems are typically the fastest, and can latch onto a signal in 30 to 60 seconds. Cellular modems can latch onto a signal as quickly as 30 seconds or take as long as 5 minutes. Satellite modems can take even longer. This is important to realize, because if a cellular modem is powered off and then powered back on, it may take up to 5 minutes before it can begin transmitting/receiving. This means a computer that wants to communicate to the cellular modem will need to wait 5 minutes before it should begin requesting data.
Therefore, typically the easiest power conservation method for telemetry systems is to power the telemetry off during the night, or to retrieve data less often.
Power Sources
After calculating the average system power an appropriate power source can be selected.< /p>
Required Battery Capacity
The first step in determining an appropriate battery 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 monitoring system's current drain in Amps from the previous section; 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 safety factor to take into account that a battery should only discharge 80% of its capacity. For example, a stand alone data logger with a wind sensor that has a current drain of 10mA, and needs to be able to last 30 days without charging should use a 9AHr battery.
0.010 A x ( 30 days * 24 hours/day ) / 0.8 = 9AHr
Note: 1mA = 0.001A
Required Battery Voltage
The required battery voltage of a monitoring system should be based on the minimum and maximum allowable voltage of all data loggers and sensors powered directly from the power source. For example, a data logger with a battery voltage requirement of 10.7-16V could use a single 12V lead acid
battery.
Rechargeable Batteries
Rechargeable batteries are the most common power source for environmental monitoring systems. When coupled with a charging source, these batteries provide continuous power for years without maintenance. These batteries are best suited for high power applications or where periodically changing battery is not feasible or cost effective.
Lead Acid Batteries
Lead acid batteries come in various forms, sizes, and weights based on their voltage output and capacity. They can supply large current draws of typical data logging telemetry, and are better suited for environmental conditions than other battery technologies. They are also inexpensive for the amount of capacity they offer, making them an ideal battery technology for systems where solar power is available, or where the system current drain is too high to be handled by other battery technologies, such as alkaline.
Lead acid batteries are generally characterized as either deep cycle or starter batteries. Deep cycle batteries are designed to withstand battery voltage drops down to cut off voltage level and back up again to fully charged battery voltage level. Starter batteries are better designed for high initial current draws and do not last many deep discharge cycles.
Deep cycle batteries include:
- Flooded: Wet cell batteries
- VRLA: Valve Regulated Lead Acid batteries
- AGM: Absorbed Glass Mat batteries
- Gel cell batteries
- Marine batteries
Starter batteries include:
- Starter, lighter, ignition automobile batteries
For batteries connected to photovoltaic solar charging systems, batteries specified for deep cycling are recommended. This kind of battery specification means the battery is less susceptible to degradation from regular discharge (down to 80% charge is considered a regular discharge) and can withstand deep discharges which would otherwise cause permanent damage. Both flooded and VRLA batteries are considered deep-cycle battery technologies.
With all lead acid batteries, make sure the manufacturer's specifications are checked thoroughly. Most manufacturers produce both high and low end batteries with important differences such as the number of charge cycles they will last, battery capacity, and amount of supply current required for charging. Therefore, buying just on the manufacturer's brand name or on price does not always ensure the best battery is purchased.
Battery life is typically specified by the number of discharge cycles a battery can withstand. A charge cycle is a drop in battery voltage and then a rise back up to pre-drained conditions from a charging source. The depth of discharge is the percentage of the fully charged voltage level the battery drops. For example, for a deep cycle battery, a discharge cycle is considered an 80% discharge. For a standard 12V lead acid battery, this would be a drop from 12.7V (100%) to 11.58V (20%) and back to 12.7V (100%) (Table 4.4.1). For a 24 volt system, simply multiply the voltage by 2, and for a 48 volt system, multiply by 4. All 12, 24, and 48V lead acid batteries have the same voltage-level-to-charge ratio. Batteries that are not specified as deep cycle may be rated for discharge cycles as low as 20-50% discharge. Battery life can be extended by shortening the discharge percentage. A battery that is only cycled to 50% will last twice as long as a battery with a charge cycle of 80%. Different manufacturers and different battery models are rated for a certain number of discharge cycles and discharge depths.Determining how deep a battery is discharged is fairly easy. Lead acid batteries have a near linear discharge rate. This means that their charge level is linearly proportional to their voltage level. As seen from the table below, a lead acid battery which measures 11.75V has been discharged 70%.
| State of Charge | 12 Volt battery |
| 100% | 12.7 |
| 90% | 12.5 |
| 80% | 12.42 |
| 70% | 12.32 |
| 60% | 12.20 |
| 50% | 12.06 |
| 40% | 11.9 |
| 30% | 11.75 |
| 20% | 11.58 |
| 10% | 11.31 |
| 0% | 10.5 |
Table 4.4.1
Temperature Effects
Lead acid battery life is mostly determined by the number of charge cycles and the depth of discharge cycles. However, another effect on battery life is storage temperature. A battery used at 50oF will last twice as many discharge cycles as a battery used at 80oF (Figure 4.4.1a).
Figure 4.4.1a: Concorde Lifeline battery chart:
http://www.lifelinebatteries.com/
While battery life decreases as temperature increases, battery capacity increases as temperature increases (Figure 4.4.1b). This is why a car battery will sometimes not start on a cold winter morning even though it started fine the afternoon before.
Figure 4.4.1b: Concorde Lifeline battery chart
http://www.lifelinebatteries.com/
The temperature effects on battery capacity and battery life is why charge controllers are best if they have temperature compensated outputs.
Flooded: Wet cell batteries
Wet cell batteries typically have the longest service life and fewest failures of lead acid battery technologies. They are also capable of handling more frequent and deeper discharge cycles and offer a higher battery capacity for the same price and size. Additionally the electrolyte that is vented during charging can be replenished; typically through a small 1/2inch hole in the top casing of the battery. The electrolyte to refill these batteries is de-ionized water. As they are not sealed they must be kept in an upright position. These batteries are ideal for high power applications that need to be able to last extreme charge cycles, and can also be accessed for maintenance. Flooded wet cell batteries are the only lead acid battery technology that requires maintenance. Letting battery become dry can permanently damage its capacity and ability to hold a charge.
VRLA: Valve Regulated Lead Acid batteries
VRLA batteries are classified as maintenance-free lead acid batteries that incorporate a valve regulation system. This regulation system uses a safety valve which allows venting of gas buildup during battery charge and discharge. They are also referred to as SLA, or sealed lead acid batteries, which is a misnomer as they are not entirely sealed. The construction of VRLA batteries allows them to be oriented in any direction (unlike flooded lead acid batteries that must be kept upright to avoid spills), which is how they developed the sealed name. They are best kept in a constant state of float charge and last longer with partial discharges than with full deep discharges of battery capacity. It is also important that the maximum charge current of the battery is not exceeded during charging; as it will cause the battery to overheat (the battery will feel hot to the touch). This heat will eventually cause the battery to leak gases that will damage electronics around the battery as well as permanently damage the batteries ability to hold a charge. The maximum charge current is typically based on the battery capacity and can be found in the battery manufacturer's specification sheet.
Absorbed Glass Mat
AGM batteries are manufactured with fiberglass mats that are filled with battery electrolyte and leaded battery plates. As the lead is supported by the fibertglass mats and does not have to support itslef inside of the electrolyte, this construction allows a purer form of lead to be used. This allows higher battery capacities to be stored in physically smaller battery dimensions.
Gel Cell
Gel cell batteries are manufactured with gel electrolyte and lead battery plates. Gel cell batteries are typically small in size and capacity as the lead plates must support themselves inside of the battery electrolyte. Gel cells have many disadvantages when compared to AGM batteries such as: low maximum charge rates (C/20), susceptible to freeze damage, overcharging can easily create voids in the gel causing permanent damage, and hot climates cause voids in the gel to occur naturally.
Starter, lighter, ignition car batteries
The type of lead acid battery that is found in automobiles, called starter batteries or SLI, are not recommended for environmental monitoring systems. These batteries are designed to supply an enormous amount of current when a car ignition is ignited and then be constantly charged for the remaining period of usage. They are constructed of large lead sponges inside of electrolyte which gives a large lead surface area. This gives the battery the ability to produce a large amount of source current. However, starter batteries will begin to fail if they are continuously discharged. The lead foam will begin to decay and fall to the bottom of the electrolyte inside of the battery, causing permanent damage over time. Starter batteries are typically rated for 20% or less charge cycles.
Marine Batteries
Marine batteries are a hybrid of starter batteries and deep cycle batteries. They can supply the enormous source currents of starter batteries and can also be regularly discharged without damaging the battery. They are constructed of large lead sponges inside of electrolyte, just like starter batteries, however the sponge is much coarser and made of material closer in construction to the solid lead plates used in VRLA batteries. Marine batteries can come in either AGM or Gel Cell designs and are best used with discharge cycles of 50%
Enclosure Vents
Whenever a lead acid battery is installed inside of a sealed enclosure, it is important that an enclosure vent is on the enclosure. This allows gases expelled during valve regulation to filter into the open air. Gas buildup inside of an enclosure risks permanent damage to the battery and other components inside of the enclosure, explosion, and at a minimum will emit a foul odor when the enclosure is opened.
Recycling
Due to the type of metals contained in batteries, they should never be disposed of. 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.
More information, including drop off sites, can be found here:http://www.rbrc.org/
For recycling batteries in large quantities, companies such as Inmetco and Toxco are two of the largest battery recycling companies in North America.
More information can be found here:< a href="http://www.inmetco.com">http://www.inmetco.com and:< a href="http://www.toxco.com">http://www.toxco.com
Charging Source
There are two main charging sources for an environmental monitoring system power source: solar panels and battery chargers. Solar panels convert energy from the sun's rays into useable electric current. Battery chargers convert AC power from an electrical outlet into useable electric current. This current is used to continuously charge a battery connected to the charger output.
Solar Panels
Solar panels are used in remote applications where AC power is inaccessible. Photovoltaic cells form the body of a solar panel or module. For environmental monitoring applications, a solar panel typically consists of three components; the solar panel, a charge controller, and a cable to connect the solar panel to the power source it is charging.
Charge controllers
Solar charge controllers, sometimes referred to as solar regulators, regulate the voltage output from a solar panel to provide a steady voltage to the battery it is charging. An unregulated 12V solar panel can output as higher than 20V in addition to have large current swings based on the position of the sun, which will damage batteries.
A solar charge controller will regulate the voltage and current to a steady level, typically 14 to 14.5V at a steady current, for optimal battery charging.
There are three kinds of solar charge controllers:
1. Simple (referred to as 1 or 2 stage controllers)
2. PWM - Pulse Width Modulation (referred to as 3 stage controllers)
3. MPPT - Maximum Power Point Tracking
Simple controllers use circuitry to disconnect the solar panel charge from the battery once the battery has reached a certain voltage level. This type of charge controller is highly reliable as it consists of only a few components. They are also fairly inexpensive due to their construction and limited functionality. However, they are not very efficient, and do not take into account variables other than voltage that affect how well a battery is charged, and how long a battery lasts, such as temperature.
PWM (Pulse Width Modulation) controllers are the most common solar charger in the market. This type of charge controller sends short pulses of current to the battery rapidly at a frequency determined by the charge state of the battery. If the battery is fully charged, a PWM charger will only pulse every few seconds with a short burst of current. When the battery is mostly discharged a PWM charger will pulse very quickly and for long periods of time so that there is almost a continuous amount of charging current sent to the battery. A PWM charger checks the charge state of the battery in between each pulse and adjusts the next pulse accordingly. MPPT (Maximum Power Point Tracking) controllers are the most expensive and feature rich solar charger. They can provide as much as 15-30% more power to a battery than a typical PWM controller by calculating the solar panel voltage that will produce maximum current at the current moment. MPPT controller's use the same pulse techniques as PWM controllers to charge a battery. However, they are more efficient because they take into account what the solar panel can output as well as evaluates the charge state of the battery.
Additionally some charge controllers incorporate features such as LVD (low voltage disconnect), or Load, which turns off the voltage to the data logger and sensors when the battery drops below to a predetermined voltage level. Discharging batteries beyond a critical low voltage can damage the batteries. This feature is not required for data loggers that have their own built in LVD (for example, shutting down telemetry modems when the battery is drained to 11.5V and shutting down the data logger and sensors when the battery is drained to 10.7V) and can sometimes be very problematic with deep discharge batteries where it is not uncommon for battery voltages to reach voltage levels between 11-12V. An important specification to take into account when using a charge controller in a system is the controller current drain. Solar controllers typically have a current draw of 2-10mA which needs to be added into the average system current drain calculation. For installation it is recommended that the charge controller be kept at the same temperature as the battery it is charging, as some charge controller outputs are temperature compensated to take into account the charging needs of the battery at the current temperature. At colder temperatures a slightly higher voltage is ideal for battery charging, while at warmer temperatures a lower battery voltage is required.
Sizing solar panels
The required size of the solar panel is determined by the amount of charging current a system requires and the amount of time it has to charge.Solar panel current> ((system Ahr/day) x 1.2 )/(peak sun hours)The number 1.2 is used to account for solar panel system efficiency loss from the solar charger (nearly 25%). The peak sun hours is an average of the sun hours available for solar charging. This number can be determined by checking a solar insolation map, which will give a good indication of the average peak sun hours (Figure 5.1.2). Even though the sun may be visible for 14 hours in a particular day, there may only be the equivalent of 4-6 hours of full sun. Because of the angle the sun reflects light; the most productive charging hours are between 9AM and 3PM. Additionally, during winter months, the peak sun hours can be between 0-2 hours in some regions of the United States. December and January typically have the lowest solar insolation while June and July have the highest. It is best to use the average solar radiation of winter months when calculating solar panel size. Otherwise the solar panel may only be able to keep the system charged during the summer. To check solar insolation data for a particular month and panel mounting visit the National Renewable Energy Laboratory (NREL - Figure 5.1.2).
Figure 5.1.2:http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/
For example, a data logger located in the Great Lakes region powering a wind sensor and communicating with a cellular modem without
using power conservation techniques has an average system current draw of 66.1mA, and would need at least 67mA (0.067A, as 1mA = 0.001A) of
solar charging.
634.56mA = ((66.1mA* 24hrs)*1.2)/3 hours.
The typical current supply of 12V solar panels are:
5W - 0.39A
10W - 0.59A
20W - 1.19A
20W - 1.78A
So a 20W solar panel would be required for adequate solar charging for this system. See the examples of system Ahr/day calculations in 3.1: Data Loggers and Sensors section of this document for calculating system current draw.
Installation
Solar panels should be angled to receive maximum incident solar radiation over the course of a year. When located in the northern hemisphere, the orientation should be facing south. When located in the southern hemisphere, the panel orientation should be facing north. The tilt angle for your site can be found in the following table.
Avoid shaded areas and keep the panel clear of debris and dirt. Doing so, as well as using the correct orientation will maximize battery life and keep your system up and running year round. During winter months and the fall, falling debris such as leaves or snow can cover the panel which will block UV light from reaching the photovoltaic cells. If a solar panel is installed facing the wrong direction or with the wrong tilt, it may provide minimal or worse: no charging at all to your battery system.
Blocking Diode
A blocking diode is not required for power sources charged from a single solar panel or charge controller. However, if multiple solar panels are used a diode should be connected in series with each solar panel to protect each module from sending reverse current flow through a solar panel which can cause thermal destruction of the solar cells.
Battery Chargers
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 AC power is available.
Float Chargers
Float chargers are used for charging lead acid batteries. Battery manufacturer's typically rate their lead acid batteries rate as either C/4, C/8 or C/20, etc, where C is the amp hour of the battery and the divisor number is used as the battery's C rating in the maximum current rating calculation.
Maximum battery current rating = battery's AHr / battery's C rating
Most flooded lead acid batteries are rated at C/8; most gel cell lead acid batteries are rated at C/20, and most AGM lead acid batteries are rated at C/4. For example, an 8.5A/Hr AGM lead acid battery has a maximum current rating of 2.125A (8.5AHr / 4 = 2.125A), and should not be used with a charger that outputs more than 2.125A. Exceeding the maximum current rating will damage the battery and shorten its usable life. It is always safer to use a charger with a current rating that is well below the maximum current rating. The lower the current of the charger, the longer it will take to charge the battery which is usually acceptable in continuously charged battery applications.
A quick calculation to determine the time to recharge a completely discharged battery is:
Hours for full recharge = battery's AHr rating / charger's current rating in Amps * 1.25
For example, and 8.5AHr battery with an 800mA charger would take about 13.3 hours to fully recharge.
There are three main stages in a lead acid battery charge: (1) bulk, (2) absorption, and (3) float.
(1) Bulk Stage:
During the bulk stage the battery charger sends a constant charging current to the battery. This stage is normally the first 80% of a battery recharge.
(2) Absorption
During the absorption stage the charging current is gradually decreased until the battery if fully charged. This stage is normally the last 20% of a battery recharge. At the end of this stage the charging current is decreased to less than 2% of the battery's AHr capacity. For example, an 8.5AHr battery will have an end charging current of 170mA or less (8.5AHr*2% = 170mA). For wet batteries, a bubbling sound (gassing) usually starts at 80-90% of a full charge and is normal.
(3) Float
During the final stage, the float stage, the current is decreased to less than 1% of the battery's AHr capacity and can be held indefinitely to keep a battery continuously charged.
Conclusion
Hopefully this section has given all the information needed to make an informative selection for an environmental monitoring system's power and charging source. If you have any questions or comments, or would like any additional information, please contact the author via email: mattk@nexsens.com
Resources
http://data.energizer.com
http://www.greenbatteries.com/
http://www.ua.nws.noaa.gov/factsheet.htm
http://www.nexsens.com
http://www.ibexmfg.com/
http://www.batteryuniversity.com/
http://www.eveready.com/
http://www.solar-electric.com/
http://www.morningstarcorp.com/
http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/atlas/