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| # | Post Title | Result Info | Date | User | Forum |
| Hammer Switch vs Trigger Geophone - Considerations | 14 Relevance | 2 years ago | Gretchen Schmauder | General Seismograph Info | |
| A seismograph with an active trigger input like the Geode Seismograph or ES-3000 Seismograph can be triggered many different ways. The most commonly used methods are with a trigger switch or a trigger geophone. Typically a trigger switch (known as a hammer switch) is attached to the handle of a sledgehammer near the striking end, so when the sledgehammer is hit against a striker plate to create an active source of energy, the piezoelectric crystal in the hammer switch is activated and the seismograph is triggered to record data along the preset parameters. A trigger geophone does this too, but it is placed near the source itself, and is more commonly used with larger energy sources like a propelled energy generator. If the seismograph isn't triggering with either a hammer switch or a trigger geophone, then the signal may be weak, so turning up the sensitivity could be a workable solution. If the sensitivity is set too high in SCS then false triggers might be encountered. In most situations having the sensitivity set to the middle works best. Depending on where the trigger geophone is it, there may be a difference between when it is triggered and when a hammer switch would have triggered. Especially in soft ground the trigger geophone signal may be delayed. In general the hammer trigger is a more reliable timing device. The differences in trigger time when using a trigger geophone could be due to things like not striking the center of the plate or differences in the strength of the impact. More trigger circuit information: The seismograph can be triggered by shorting the two input Pins A and B on the trigger connector of the seismograph. In fact, that is what the hammer switch does (contact closure device) when it impacts a striker plate. The inertia of the impact causes a momentary closure in the device, which in turn, triggers the Geode. There are no internal components that need to be added. Externally, you could construct trigger device or switch, if that is what you desire. If you were to measure the Pins on the Trigger connector on your seismograph (pin A +, Pin B -) you would see about 5VDC. The trigger circuit will sense a contact closure or a pulse. The Geode trigger input is capacitively coupled, with a 2mS time constant, to the midpoint of a resistive voltage divider. The voltage difference between the two ends of the divider constitute a voltage "window", which size is set by the trigger sensitivity parameter and can range from essentially zero at the highest sensitivity, to about +/- 2.5V at the lowest sensitivity. The Geode triggers (if enabled) if and when the coupled signal exceeds the window, in either direction. The signal, after the capacitor, is clamped by diodes to the range between the trigger signal ground and +5VDC. The trigger detector output is disabled when the system is disarmed, during a parameter change, and during a shot, up to the trigger hold-off time after the end of the shot. The trigger hold-off time is a parameter set by the user. Preceding the coupling capacitor (i.e., essentially the node accessible at Pin A of the external connector), there is a 3.3K-Ohm pull-up resistor to +5VDC (relative to Pin B). Also a fast transient suppressor clamps the input at about +/-14VDC. It is advised that the DC + AC level of any voltage applied to Pin A relative to Pin B be kept within the range of +/-7V, giving some margin of safety. If a DC voltage somewhat less than +5VDC is applied when the connector is first mated, the instrument may trigger at that moment. But, subsequently, because of the capacitive coupling, it will trigger on the next positive or negative going pulse that exceeds the window level. If the duration of the applied voltage pulse is less than the record length + delay time + hold-off time, then the Geode will effectively be ready to trigger on the same edge of another similar pulse. | |||||
| Geode SGOS Timing | 9 Relevance | 2 years ago | Gretchen Schmauder | Software | |
| The time associated with each data point in a SEG-2 data file generated by a Geode is related to the time of the “trigger” event which was instrumental in the production of the file and its content. The Trigger Master and Trigger Distribution The trigger event occurs at the Geode designated within the Controller software as the Trigger Master. Although all Geodes are capable of being Trigger Masters, there must be one and only one Trigger Master in any properly functioning Geode system. The Controller automatically takes care of this requirement when the designation is made by a user, and when the system is established at the time of Controller start-up based on a previous designation (or a default setting in the case of a “new survey”). All other Geodes in the system will have their Trigger Master circuit disabled. A trigger event can be initiated by an external electrical pulse provided to the trigger input connector of the Trigger Master Geode, or by a command sent via Ethernet from the Controller to the Trigger Master (usually for test purposes), but only when all conditions are satisfied to allow data recording. There is also a special trigger initiation situation, called “self-triggering” which will not be discussed further here. Upon acceptance of a trigger event, the Trigger Master will distribute the trigger signal to all Geodes in the system, itself included, via an RS-485 network that resides within the digital interconnect cabling. (Proper termination of this RS-485 network is automatically taken care of by the Controller.) The trigger signal is propagated through the cabling and Geodes at the nominal speed of 70% of the speed of light, or approximately 2.1x10^8 m/sec. The maximum distance of successful propagation depends on a number of factors such as the number of Geodes involved, the noise environment, the quality of the cables, and the acceptable amount of timing uncertainty for the particular application. Distances approaching or exceeding 1km should be given careful attention in this regard. In a 3-D Geode system involving LTUs, each LTU, unlike a Geode, will reconstruct the trigger signal before sending it on, effectively confining the maximum distance issue to each sub-network separated by LTUs. The penalty is an additional delay of about 100nS for each LTU in the route. The External Trigger Circuit The external trigger input is capacitively coupled, with a 2mS time constant, to the midpoint of a resistive voltage divider. The voltage difference between the two ends of the divider constitute a voltage "window", which size is set by the trigger sensitivity parameter and can range from essentially zero at the highest sensitivity, to about +/- 2.5V at the lowest sensitivity. The Geode will trigger (if enabled) if and when the coupled signal exceeds the window, in either direction (i.e., positive or negative going). The signal, after the capacitor, is clamped by diodes to the range between the trigger signal ground and +5VDC. The trigger detector output is disabled when the system is disarmed, during a parameter change, and during a shot, up to the trigger hold-off time after the end of the shot. The trigger hold-off time is a parameter set by the user. Preceding the coupling capacitor (i.e., essentially the node accessible at Pin A of the external connector), there is a 3.3K-Ohm pull-up resistor to +5VDC (relative to Pin B). Also a fast transient suppressor clamps the input at about +/-14VDC. It is advised that the DC + AC level of any voltage applied to Pin A relative to Pin B be kept within the range of +/-7V, giving some margin of safety. If a DC voltage somewhat less than +5VDC is applied when the connector is first mated, the instrument may trigger at that moment. But, subsequently, because of the capacitive coupling, it will trigger on the next positive or negative going pulse that exceeds the window level. If the duration of the applied voltage pulse is less than the record length + delay time + hold-off time, then the Geode will effectively be ready to trigger on the same edge of another similar pulse. Sub-sample Synchronization The Geode supports a sub-sample timing synchronization feature used for synchronizing the data acquisition after a trigger event to the distributed trigger signal, so that subsequent time points will be known to within 1/32 (~1/20 at the fastest two sampling rates) sample interval. It does this by increasing the sample interval at the trigger time by 0 to 31/32 of a sample interval in increments of 1/32, so that the first sample after the trigger would represent a time of one sample interval after the trigger event, with a tolerance within 1/32 of a sample interval. The following samples continue from there at the expected intervals. For example, with a selected sampling interval of ¼ mS and a recording delay of 0mS, the first sample in the recorded file for each channel would represent data at 250 to 258uS after the trigger event. This of course potentially introduces a small discontinuity at the time of the trigger, observable depending on the nature of the channel waveform(s). (The zero-phase anti-alias filter will smear the discontinuity into the nearby samples both before and after, consistent with the bandwidth of the filter.) Sub-sample synchronization can be disabled if it is deemed to be detrimental for the particular application, at the expense of losing the 1/32 interval timing accuracy. Timing Errors The principal errors in Geode timing are of two types: those associated with the trigger mechanism and which are static over the duration of the record, and those associated with the time base and which change over the duration of the record. Excluding the trigger propagation delay mentioned above, the trigger timing uncertainty is about 1uS. The known fixed errors have been lumped together and are reported in the SEG-2 file trace headers as channel SKEW. (The actual channel skew is zero, since all channels are effectively sampled simultaneously, but the SKEW value in the header is used as the only place permitting small timing corrections. Note that the SKEW value for every channel is identical.) If the size of this correction is important to the application, the SKEW value should be added to the calculated time points when the data is being processed. The Geode time base has a +/-15ppm stability over temperature (-20C to +70C) and component variations. Thus time drift relative to absolute time and relative to other Geodes is possible. (However, all channels within any Geode enclosure use the same time base, so there is no relative drift between channels in the same enclosure.) Therefore timing uncertainty increases from that existing at the time of the trigger until the time of the next trigger (or end of record). Special Timing Issues Involved with “Continuous” Recording “Continuous” recording is a method that allows unending 100% time coverage with recorded Geode data. It produces a series of time-overlapped records created by the use of a negative time delay set equal to the record length such that each record consists of completed history at the time of the trigger event. This technique circumvents the problem of data transmission overrunning data acquisition. The principle constraint is that the cycle time from trigger to trigger must always be less than the chosen record length. Otherwise, gaps rather than overlap would result. Commonly it is used with GPSderived triggering in order to provide time-stamping of each trigger event. Upon consideration of the above, it will become clear that the time-stamp associated with a particular trigger event will pertain to the data in the following record, not to the data in the record in which the time-stamp is written. This comes about because the trigger event ends the record. Because there is data overlap between records, the precise trigger point in the following record at which the time-stamp applies can be found by comparison of the data values at the end of the former record with those near the beginning of the subsequent record. The overlapping data will be exactly identical in both records (since they are read from the same memory location, twice). The earliest data in the subsequent record that goes beyond the data of the previous record is the data that is one sample interval (assuming sub-sample synchronization is enabled) past the time-stamp. Note well that this comparison must be made independently for at least one channel of each 8-channel Geode board set, because the discrete time at which data values are written to the memory buffer, relative to the trigger event, is a function of each individual board set in the Geode system. Correct GPS Time-Stamping There are differences between various GPS models that can affect accurate time stamping. The 1PPS signal from a GPS has a “timing edge” and return edge, of which only the former is the true whole-second edge. Some models use a rising edge as the timing edge, some the falling edge, and some have it selectable. Consult the GPS manual to determine the definition of its timing edge. As indicated earlier, the Geode can be triggered on either a rising or falling edge. It is important to insure that the Geode is being triggered on the proper edge in order to avoid timing that may be a fraction of a second off. This is expanded upon below. Some GPS units provide a very narrow timing pulse, others one that has a nearly 50/50 duty cycle. For the narrow pulse units, almost certainly it is the leading edge (rising or falling) that is the “timing edge”. This case can be easily handled by using the Geode Trigger Hold-off feature. If a 10-second cycle time is desired, set the Trigger Hold-off time to about 9.5 seconds. In this case, there is a very small chance that the very first trigger could occur on the wrong (trailing) edge, but from then on the leading edge will be used as the triggering edge. If the GPS provides a 50/50 duty cycle edge, and it is not alterable, then the Geode by itself could as easily start on the wrong edge as on the correct timing edge, and continue thusly until restarted. For this case, Geometrics can provide a Trigger Timing Interface Box (TTIB) that will correct the situation. The TTIB can be programmed to respond only to the correct edge (rising or falling), change the polarity if needed, and gate through only one of every N 1PPS pulses, where N is programmable. (The TTIB also incorporates an alarm system that can provide a remote alert if a record is missed.) Another potential issue comes from the variations between GPS models of the time that the serial time string (containing the time value of the associated 1PPS) is issued relative to the 1PPS itself. The Geode Controller attempts to pick the correct serial string based on a calculation involving the known record length, the PC times, and the trigger notification message from the Geodes. But if the GPS issues the serial string at an unusual time (and the time has been seen to vary somewhat with a given GPS unit) then it could pick up the incorrect time, off by 1 second. If rare, it can be subsequently detected and corrected during data processing, but if consistent it may not be easily detected. Again, the TTIB can accommodate the situation by only gating through to the Controller PC the string belonging to the gated-through 1PPS pulse. The Controller Serial Input Time Window can then safely be widened to 2 seconds (assuming the cycle time is more than 2 seconds) if need be, to expand the Controller’s search for the string around the calculated trigger time. | |||||
| MFAM 1 PPS signal input, 10 MHz reference input and synchronization | 5 Relevance | 2 years ago | Rui Zhang | Hardware | |
| The 1PPS pulse phase locks and synchronizes the sample interval to be in lock step with the GPS. Thus once locked there will always be 1000 samples per second, with the sample beginning time precisely lined up with the 1 PPS edge. The 10 MHz input is for a different function. This input phase locks the 40 MHz master reference oscillator to the incoming 10 MHz, which is usually a GPS disciplined reference oscillator or atomic clock (in other words exactly 10 MHz). This phase locks the 40 MHz reference oscillator to exactly 40 MHz. The 40 MHz oscillator is the time base reference for calculating the Larmor frequency, and therefor the magnetic field value. Even though the 40MHz oscillator is really good even without the 10 MHz input, there is some drift in the 40 MHz over time (mostly thermal drift and some aging). For some applications where they need to measure very low frequency and low amplitude changes in the magnetic field the 10 MHz input will allow drifts in the reference oscillator to be removed. Without that it would be impossible to distinguish between reference oscillator drifts and low frequency low amplitude changes in the magnetic field. The connector for the 10 MHz input is SMB RF connector from Molex. If you must simulate the 1 PPS signal in a GPS denied environment, please be aware of certain requirements of the 1 PPS signal. 1. The lock range for the 1 PPS input pulse is very narrow. The simulated 1 PPS signal must be within 100 ppm of an exact 1 Hz PPS signal (100us). 2. Timing jitter must be small (less than 0.5 ppm, 0.5us) too. If you are setting a GPIO Pin on a microcontroller, there may be some concern about timing jitter due to interrupt latency or other processor tasks delaying the I/O Pin toggle. Any rectangular waveform should work but the leading edge must be very close to 1 Hertz. It is the positive edge that specifies the 1 second rollover. | |||||
| How can I connect a GPS to my G-858 Magnetometer? | 5 Relevance | 2 years ago | Gretchen Schmauder | Hardware | |
| It is a fairly simple task to connect a GPS to a G-858 magnetometer. You can use the External I/O cable assembly and a null modem to connect the G-858 to most GPS receivers. Null modems are available from Geometrics or from local Radio Shack or computer stores. The Null modem Pin configuration for GPS receivers that have a data cable compatible with 9 Pin IBM PC COM ports has male Pins on both sides. The GPS should be set to output NMEA data that contains the $GPGGA sentence. Be sure to set the RS232 protocol to 9600 Baud, 8 Databits, 1 Stop Bit and No Parity. The G-858 must have its serial port set to the same baud rate as the GPS. You can use System Setup -> Com & Field Note String Setup -> Chat Mode to determine whether correct communications have been established. | |||||
| Magnetic Properties of Stainless Steel | 3 Relevance | 2 years ago | Gretchen Schmauder | General Magnetometer Info | |
| All of the 300 series stainless (austenitic) steel is considered to be weakly magnetic in its annealed state. These include the common 303, 304, and 316 alloys. 18-8 stainless is another name for 304 stainless (18% chromium and 8% nickel). Surprisingly it is the nickel content in the stainless that makes the steel less magnetic. Nickel usually makes things more magnetic in other metals. The 400 series are very magnetic (martensitic). They do not contain nickel. 17-4 stainless is a different class of stainless, and contains a little nickel (4%) but has other stuff in it that makes it magnetic. To make it even more confusing: All stainless steels, including the 300 series, become very magnetic when hardened. A soft ductile 316 stainless cotter Pin is not very magnetic, but a 316 stainless spring is highly magnetic. If it is stainless and "springy" it will most likely be very magnetic. | |||||
| I'm having trouble triggering my seismograph | 3 Relevance | 2 years ago | Gretchen Schmauder | General Seismograph Info | |
| Geometrics seismographs are designed to trigger on a contact closure, contact open, or signal input. The trigger circuit has protect from high voltages, but it is possible to damage the input circuit if voltages outside the specified range are connected directly to the input circuit. It is recommended that input signals, or voltages do not exceed + 10 volts. A typical voltage measurement using a hand held volt meter on Pins A (+) and B (-) of the 3 Pin trigger input connector will be 4.9 volts DC. Voltages less than 4.0 volts may indicate a problem with the trigger circuitry. Often times the unit will continue to operate and trigger, but should be serviced at the next opportunity. If the circuit has been damaged, typical problems will include false triggers, or failure to trigger. To verify the trigger function of the Seismograph, begin by removing the external connector from the trigger input, and short Pins "A" and "B" together on the trigger input connector of the seismograph. The unit should trigger each time the Pins touched as long as the interval between triggers is greater than the record time or the trigger hold off whichever is longer. It should not trigger unless these Pins are touched. If consistent triggering is achieved using this method, then attach the hammer switch directly to the seismograph. Do not put the hammer switch on a hammer yet. Tap the hammer switch cylinder on the edge of a table or other hard object and verify consistent triggering. Watch the stack count on the screen to confirm each tap of the hammer switch results in a trigger of seismograph. Check the trigger hold off setting and trigger sensitivity settings. Set the trigger hold off to 0.5 sec. and the trigger sensitivity to 50. If the seismograph is triggering correctly, insert any trigger extension cables between the hammer switch and the seismograph. Repeat the test to confirm consistent triggering. Then attach the hammer switch to the hammer, or other device used for triggering. Make sure the cylinder of the hammer switch is firmly taped to the handle, and the direction of motion is across the diameter of the cylinder. You can also attach a geophone or other signal producing devise at this point and verify proper triggering. If the unit does not properly trigger, contact support with the results of the tests above for assistance. | |||||
| Choosing the Right Lithium Polymer Battery for your MagArrow | 3 Relevance | 2 years ago | Gretchen Schmauder | Hardware | |
| The MagArrow uses a 3 cell Lithium Polymer battery to power the MagArrow during surveys. The two main requirements for the battery are that it must fit into the battery compartment, and it must be nonmagnetic. Non-Magnetic Batteries: Some types of Lithium Polymer batteries are extremely magnetic. This is because the cell-to-cell connections are made with nickel strips (nickel is extremely magnetic). This makes them unsuitable for use in the MagArrow since they will interfere with the background magnetic field that is being measured. Whether or not the batteries are magnetic is not something that appears on the data sheet, so it is important to choose batteries of a particular construction form factor that in practice has been shown to have a very low magnetic signature. Examples of this battery type will be shown below. There are many brand names for this battery type, and the brand names seems to change frequently. Evaluating the Magnetic Properties of a Battery: Batteries should be measured for magnetic signature before using them. This is especially true when trying a new battery brand just to be sure the battery is not going to affect the survey data. To perform this test you will need to start a survey with a stationary MagArrow pointing north-south on a nonmagnetic platform (wooden sawhorses, cardboard box, etc). Hold the battery to be tested immediately over the battery compartment and rotate it in all orientations. Download the data and look for variations in the magnetic field that correlate with the battery rotation. There shouldn't be any correlation above 1 nT peak to peak. Make sure the operator is nonmagnetic when doing this test (shoes, belts, watches, cell phones, keys, etc. can all corrupt the results). Battery Size and Shape: The correct batteries are rectangular in shape and measure roughly 105x34x24mm. They are made from 3 flat cells stacked up measuring 11.1 volts nominal. They should be between 1800 and 2200 mAh (milliamp-hour). Higher capacity batteries will not physically fit in the battery compartment. Lower capacity batteries will work, but with a reduced run time. One 1800 mAh battery will run the MagArrow for about two hours. The MagArrow power connector is XT-60 so the battery must match. There are other power connector types, but XT-60 is commonly used. The 4-pin balance port connector is a JST-XH4 connector (though this is standard on most batteries). Where to Find Batteries: If you are in an area that doesn't have strict controls on shipping Lithium Polymer batteries, then Amazon.com is a good source. Another good source is hobby stores, or anyplace that sells radio-controlled toy cars, boats, or airplanes. This is typically where this style of battery is used the most. What do the Battery Specifications Mean? 3S: This means it is a stack of three Li-Po cells Voltage: A fully charged 3 cell Li-Po battery measures 12.6 volts. A depleted battery will measure 9.6 volts. Thus, the voltage for this battery is typically labeled as 11.1 volt (the average of 12.6 and 9.6 volts. 35C (or any other "C" value): This is a rating on how much current can be safely drawn from the battery. To get the value in amps, take the milliamp-hour rating and divide by 1000 (to get amp-hours), and then multiply by the "C" value. For a 2200 mAh battery with a 35C rating multiply the 2.2 amp-hour capacity (2200 mAh / 1000) times the C value of 35, which gives a maximum discharge current of 77 amps. The MagArrow draws about 0.6 amps, so any C value is fine - even if is down to 0.5. Battery Chargers: Most battery chargers being sold now are universal chargers which support a variety of rechargeable battery chemistries and output connectors. They come in many sizes and shapes, but most of them operate identically because the internal circuitry is the same. Most chargers will charge at a much faster rate than the MagArrow discharges them, so you technically only need two batteries in the field. A nice feature to look for is the ability to power the charger off 12V as well as with AC power. This will allow charging in the field off a car battery. Be sure to charge in batteries in "Balanced Charge" mode using the battery balance JST-XH connector. This allows more charge current into cells that are more deeply discharged than the others and ensures that the battery gets all three cells completely charged. Battery Safety: Lithium Polymer batteries are small and light but store a tremendous amount of energy inside. This is good for running equipment for long periods of time between charges, but it also means that if something goes wrong and it releases all its energy at once it can be a serious fire hazard. Never charge a lithium battery unattended, charge only in a fireproof location. Batteries that are swollen or damaged should not be used. Dispose of these per local regulations. Be sure to follow all regulations for shipping or hand carrying Li-Po batteries. This may include packaging and labeling requirements, limiting the number of batteries, and discharging the batteries to 30% capacity before shipping. Do not discharge the battery below 9.6 volts (3.2 volts per cell). This damages the battery and could result in destructive decomposition and fire. If a battery that is discharged below a safe level is placed on the battery charger it will refuse to charge it. Batteries that are discharged below 9.6V should be removed from service and disposed of according to local regulations. To download a copy of this document as a PDF, click here. Some example batteries are shown below: | |||||
| Choosing the Right Lithium Polymer Battery for your MagEx | 3 Relevance | 2 years ago | Gretchen Schmauder | Hardware | |
| The MagEx uses a 3 cell Lithium Polymer battery to power the MagEx during surveys. The two main requirements for the battery are that it must fit into the battery compartment, and it must be nonmagnetic. Non-Magnetic Batteries: Some types of Lithium Polymer batteries are extremely magnetic. This is because the cell-to-cell connections are made with nickel strips (nickel is extremely magnetic). This makes them unsuitable for use in the MagEx since they will interfere with the background magnetic field that is being measured. Whether or not the batteries are magnetic is not something that appears on the data sheet, so it is important to choose batteries of a particular construction form factor that in practice has been shown to have a very low magnetic signature. Examples of this battery type will be shown below. There are many brand names for this battery type, and the brand names seems to change frequently. Evaluating the Magnetic Properties of a Battery: Batteries should be measured for magnetic signature before using them. This is especially true when trying a new battery brand just to be sure the battery is not going to affect the survey data. To perform this test you will need to start a survey with a stationary MagEx pointing north-south on a nonmagnetic platform (wooden sawhorses, cardboard box, etc). Hold the battery to be tested immediately over the battery compartment and rotate it in all orientations. Download the data and look for variations in the magnetic field that correlate with the battery rotation. There shouldn't be any correlation above 1 nT peak to peak. Make sure the operator is nonmagnetic when doing this test (shoes, belts, watches, cell phones, keys, etc. can all corrupt the results). Battery Size and Shape: The correct batteries are rectangular in shape and measure roughly 105x34x24mm. They are made from 3 flat cells stacked up measuring 11.1 volts nominal. They should be between 1800 and 6000 mAh (milliamp-hour). Higher capacity batteries will not physically fit in the battery compartment. Lower capacity batteries will work, but with a reduced run time. One 1800 mAh battery will run the MagEx for about two hours. The MagEx power connector is XT-60 so the battery must match. There are other power connector types, but XT-60 is commonly used. The 4-pin balance port connector is a JST-XH4 connector (though this is standard on most batteries). Where to Find Batteries: If you are in an area that doesn't have strict controls on shipping Lithium Polymer batteries, then Amazon.com is a good source. Another good source is hobby stores, or anyplace that sells radio-controlled toy cars, boats, or airplanes. This is typically where this style of battery is used the most. What do the Battery Specifications Mean? 3S: This means it is a stack of three Li-Po cells Voltage: A fully charged 3 cell Li-Po battery measures 12.6 volts. A depleted battery will measure 9.6 volts. Thus, the voltage for this battery is typically labeled as 11.1 volt (the average of 12.6 and 9.6 volts. 35C (or any other "C" value): This is a rating on how much current can be safely drawn from the battery. To get the value in amps, take the milliamp-hour rating and divide by 1000 (to get amp-hours), and then multiply by the "C" value. For a 2200 mAh battery with a 35C rating multiply the 2.2 amp-hour capacity (2200 mAh / 1000) times the C value of 35, which gives a maximum discharge current of 77 amps. The MagEx draws about 0.6 amps, so any C value is fine - even if is down to 0.5. Battery Chargers: Most battery chargers being sold now are universal chargers which support a variety of rechargeable battery chemistries and output connectors. They come in many sizes and shapes, but most of them operate identically because the internal circuitry is the same. Most chargers will charge at a much faster rate than the MagEx discharges them, so you technically only need two batteries in the field. A nice feature to look for is the ability to power the charger off 12V as well as with AC power. This will allow charging in the field off a car battery. Be sure to charge in batteries in "Balanced Charge" mode using the battery balance JST-XH connector. This allows more charge current into cells that are more deeply discharged than the others and ensures that the battery gets all three cells completely charged. Battery Safety: Lithium Polymer batteries are small and light but store a tremendous amount of energy inside. This is good for running equipment for long periods of time between charges, but it also means that if something goes wrong and it releases all its energy at once it can be a serious fire hazard. Never charge a lithium battery unattended, charge only in a fireproof location. Batteries that are swollen or damaged should not be used. Dispose of these per local regulations. Be sure to follow all regulations for shipping or hand carrying Li-Po batteries. This may include packaging and labeling requirements, limiting the number of batteries, and discharging the batteries to 30% capacity before shipping. Do not discharge the battery below 9.6 volts (3.2 volts per cell). This damages the battery and could result in destructive decomposition and fire. If a battery that is discharged below a safe level is placed on the battery charger it will refuse to charge it. Batteries that are discharged below 9.6V should be removed from service and disposed of according to local regulations. Some example batteries are shown below: | |||||
