** Battery Systems For Emergency Communications ** John Bartone, K4KXK, MSEE Date: Tue, 03 Mar 98 17:42:13 GMT From: Ed.Harris@p3006.airnsun.pcbuddy.com (Ed Harris) To: sarinfo@mindlink.net Subject: Battery Power for Communications Presentation for ARES Institute, June 13, 1998 American Red Cross National Headquarters, Falls Church, Virginia Radio amateurs can't learn all they need to know about battery power for emergency communications just from books or lectures. Practical experience is needed to apply the theory with common sense. The following won't make you an expert, but provides enough basic knowledge to "keep you out of trouble," explain the basics of battery- based DC power systems and enable you to plan adequate auxiliary power for your communications equipment. Batteries store DC electricity in a relatively portable form, but are only a temporary source of power unless you have a sustainable charging means which is not dependent upon AC mains. Photovoltaic systems are an important element of that sustainable DC power supply, when combined with a properly designed battery bank and charge controller. Attention to POLARITY is always important, but is especially so in DC systems! Whenever using battery power, the correct connection sequence is important to avoid sparking or damage to system components. Just because the battery is lower than your house current does not mean it is harmless. An arc caused by wiring a connection in the wrong order may ignite hydrogen given off by a battery, causing an explosion! High current flows at low voltages can still be lethal. Always disconnect all circuits before working on any power system and always follow the correct re-connection sequence: 1 positive connection to battery 2 positive connection to load 3 negative connection to battery 4 negative connection to load Typical 12-volt lead-acid batteries have a voltage of about 14 volts when fully charged and 11 volts when fully discharged. Because most of our communication equipment doesn't operate properly below about 11.5 volts, you can't exceed the depth of discharge which depletes the battery voltage under load to below that figure. Battery systems are current limited and their capacity is finite. Oversized loads or excessive duty cycle cause rapid depletion of our battery capacity, therefore, our battery systems must be sized to match the load, or else they cannot supply the required current demends needed. Cold Cranking Amps (CCA). Used to rate starting batteries represent the current that battery can provide continuously for 30 seconds at 0 degs. F before cell voltage is depleted to 1.7V per cell, at which point it is deeply and fully discharged. For MCA or Marine Cranking Amps, the measurement is taken at 32 degs. F. Cranking amps tell little about how long a battery can run your transmitter. Starting batteries do not perform well for communications because they are designed for a short period of high load, and not for long periods of slow discharge. More useful performance measurements for a "comm" battery are amp-hour capacity and depth of discharge (DoD). Amp-hour capacity is the rated current available over time, normally measured at 80 degs. F. For instance, 200ah equals 10 amps for 20 hrs. Lower temperatures reduce capacity significantly! DoD is the percentage of battery capacity available during a charge-discharge cycle. Engine starting batteries are designed for 20% DoD, gel cells 25%, "deep cycle" marine batteries from 50% to 80% and aviation grade flooded nicads 100%. A C20 discharge rate means that a battery is discharged at a rate of 1/20 of its total capacity, such as a 200ah battery discharged at 10 amps, continuous duty. A 100w HF SSB radio, normally operates at 25% duty cycle, has a current drain of 20 amps on transmit and requires a 100 ah battery not exceed a C20 discharge rate. More rapid rates of discharge, such as using a battery that is marginally sized for the load, reduce available capacity and the number of charge- discharge cycles the battery can provide. A 30 ah wheelchair gel battery is well balanced when powering portable VHF at 25% duty cycle, 10w PEP transmit, requiring 6 amps, approximating a C20 rate of discharge. Powering the same transceiver at 50 watts requires 10 amps, which is a mildly oversized load, approaching, but not quite C10 discharge. The common rule of thumb to approximate a C20 discharge rate is one amp-hour per PEP watt. This is recommended as minimum design practice for 24-hour auxiliary power in emergency communications systems. Estimate the amp- hour capacity required to run your station for 24 hours by summing all of the loads: transmit current times total operating time times duty cycle, plus receive current with squelch open times standby time and repeat for each piece of equipment, HF rig, VHF mobile, VHF amplifier, TNC, PC, emergency lighting, coffee pot, etc. Then multiply the total loads by a 150% safety factor. If you are too lazy to do that, use the " 1 amp-hour per PEP watt" rule of thumb as a minimum for each 24 hours. You must also pay attention to how the performance ratings are measured, because not all batteries are equal in terms of design, quality, capacity or construction. Lead-acid batteries are most common and consist of lead alloy grid plates coated with lead oxide paste which are immersed in a solution of sulfuric acid. During manufacture the plates are subjected to a "forming" charge which causes the paste on the positive grid plates to convert to lead dioxide. The paste on the negative plates converts to "sponge" lead. Both materials are highly porous, allowing the electrolyte to freely penetrate the plates. Plates are alternated in the battery, with porous, nonconductive separators between them, or with each positive plate surrounded by an envelope, open at the top. A group of negative and positive plates with their separators makes up an element. When immersed in electrolyte, an element makes up a battery "cell." In lead acid batteries each cell is nominally 2 volts, which are connected in series to increase voltage. A 12-volt battery contains six cells. Larger or more plates increase amp- hour capacity, but not voltage. Thicker or fewer plates per cell allow more cycles and longer life for the battery. The lower the antimony content in the plates, the lower the internal resistance and the less resistant the battery is to charging. Less antimony also reduces water consumption through electrolysis. However, pure lead has low strength and may break during transportation or service operations requiring removal of the battery. More antimony allows deeper discharge without damage to the plates and longer service life. The plates in most automotive batteries are 2-3% antimony and deep cycle batteries 5-6% Sb. Calcium or strontium are used in sealed lead-acid batteries, which offer the same benefits and drawbacks as antimony, but reduce self discharge when the battery is stored without being used. The DoD recommended for Pb-CA batteries is no more than 25%. Cells in lead-acid batteries are vented to permit hydrogen and oxygen to escape during charging and to provide an opening for replacing water lost due to electrolysis. Open caps are common in flooded batteries, but some caps are of flame arrester type to prevent a flame outside the battery from entering the cell. "Recombinant" caps contain a catalyst which causes hydrogen and oxygen liberated during charging to recombine into water, reducing the need to replace water lost from the battery. These are highly recommended for stationary batteries in seasonal equipment left for extended periods on maintenance level float chargers or used in photovoltaic systems. The percentage of acid in battery electrolyte is measured by its specific gravity (Sg). Only batteries which use acid electrolyte can use specific gravity as a measurement of the state of charge. A hydrometer is used to measure how much the electrolyte weighs compared to an equal quantity of water. The greater the state of charge, the higher the specific gravity of the electrolyte. The lower the state of charge, the weaker the acid and the lighter the electrolyte. Differences in acid density are measured by the float in a hydrometer, which rises higher in an electrolyte sample of high Sg than in one with a lower Sg. Measuring Sg of a wet, lead-acid battery during discharge is a good indicator of the state of charge. A fully charged battery has an Sg of 1.265 grams per cubic centimeter, at 75% charge 1.225, 50% charge 1.19 and fully discharged 1.120. During charging of a flooded battery Sg lags the charge state because complete mixing of the electrolyte does not occur until gassing commences near the end of the charge cycle. Because of the uncertainty of mixing, this measurement on a fully charged battery is a better indicator of the health of a cell. Therefore, Sg is not the absolute measure of capacity but should be considered in combination with load testing and open circuit voltage. Lead-acid batteries accept only about 1/10 of the charging at 30 degs. F which they will accept at 80 degs. F. Correct charging current for lead-acid batteries at normal ambient temperature is between 1/10 and 1/20 of battery capacity. When not in service, all lead-acid batteries self- discharge at rate of about 5% per month. The rate of self discharge increases with the temperature. If a lead-acid battery is left in a deeply discharged condition for a time it becomes "sulfated" as sulphur in the acid combines with lead from the plates to form lead sulphate. Auxiliary batteries should be connected to a charge controller to provides a regulated, low-level current to compensate for self discharge and protect against sulfation, but also require regular test and inspection including replacement of lost electrolyte. If water is lost during charging and not replaced, the process of sulfation is accelerated if the plates are partially exposed to air. "Treeing" is a short circuit occuring between positive and negative plates, caused by misalignment of the plates and separators. This may be a manufacturing defect or induced by rough handling. "Mossing" is caused by circulating electrolyte bringing particulate matter to the tops of the plates can also cause a short. Sealed, flooded (wet) lead-acid batteries are called "maintenance free" and experience less self-discharge because they contain lead-calcium or lead-strontium plates to reduce water loss. The "best" ones have catalytic recombiners to reduce water loss and sealed, valve regulated vents and can tolerate the same temperatures as unsealed batteries. Because Sg cannot be readily measured, some sealed-wet batteries are provided with a captive float hydrometer in the electrolyte. Sealed-wet batteries are common for automotive starting, but should not be discharged below 25% or their life is dramatically shortened. If you power a transceiver directly from a car battery, run the engine ten minutes out of every hour to keep the battery charged. Sealed lead-acid (SLA) batteries with stabilized or "starved" electrolyte include gel cells and absorbed glass mat (AGM) types, which are valve-regulated and completely sealed. Since there is no free liquid electrolyte to spill, the battery can be used safely in any position. SLAs are, therefore, much safer than flooded types for indoor use and around sensitive equipment which would be damaged by acid spills or escaping corrosive fumes. Any sealed battery will vent if overcharged to the point of excessive gassing, because the valves are designed to purge extreme pressure building up inside the battery case. Always follow manufacturer's charging recommendations. Self discharge of gel cells is minimized by storing them in moderately cool areas of 5 to 15 degs. C. Gel cells are NOT deep cycle. A DoD of greater than 25% significantly reduces their life. They must never be used below -20 degs. C, in the engine compartment of vehicles or in uses subjecting them to temperatures above 50 degs. C. Aviation-grade AGMs are deep cycle and can be quickly-recharged with no current limit, provide a broad operating temperature range. Their extreme depth of discharge equals flooded nicads, but with virtually no maintenance and low life cycle cost. New aviation grade AGMs are substantially more expensive than flooded deep cycle batteries of equal capacity, but marine or emergency vehicle grade AGMs such as Lifeline or Optima are not prohibitively expensive and are highly recommended as auxiliary battery power for emergency communications. Nickel cadmium (NiCd) batteries have a physical structure resembling lead-acid batteries, but use nickel hydroxide for the positive plates, cadmium oxide for the negative plates and a potassium hydroxide electrolyte. Cell voltage of a typical NiCd is 1.2 volt, rather than 2 volts per cell as for a lead-acid. NiCds can survive freezing and thawing without any affect on performance and are less affected by high temperatures. The self-discharge rate of NiCds is 3-6% per month. NiCds are not subject to sulfation. Large industrial or aviation NiCds can be totally discharged without damage and their ability to accept charging is independent of the temperature. Their lower maintenance cost and long life makes them a logical choice for repeater back-up systems in remote or dangerous locations. However, large NiCds cannot be tested as accurately as a "wet" lead-acid battery, because the specific gravity of the electrolyte in NiCd batteries does not change with different states of charge, so if constant charge monitoring is necessary, NiCds may not be the best choice. Small NiCds of less than an amp-hour, such as used in low power VHF or UHF hand-held transceivers require care to avoid prolonged deep discharge or over charging. The use of properly sized wire and appropriate connections is important in DC power systems. DC polarity must be maintained throughout the system, as must color code conventions of wire insulation: positive-red, negative-black and equipment ground-green or bare, identical to the DC wire color conventions in automobiles. There are additional NEC requirements for large DC power wiring systems. Wire gages are much larger than are typical in AC systems, because undersized wiring causes excessive voltage drops which result in loss of available power which causes some loads to work poorly, or not at all. If too small a wire gage is used between a charge controller and the battery, the voltage drop measured during full charging rates will reduce the regulation set-point the battery is charged to and reduce the capacity and life cycle of the battery. To minimize the effects of voltage drop on your equipment, keep cable runs as short as possible. For instance, in a 12-volt system with 10 amp load, such as a 2-meter mobile transceiver, the AWG #14 wire normally provided with the rig results in a 5% voltage drop over 11ft. AWG #10 has a 2% drop over the same distance, and 5% drop over 18 ft. If you must extend the battery leads to trunk-mount a transceiver, use larger gage wire for as much of the distance as possible. All splice connections must be secure and able to withstand vibration, moisture and corrosion. Splices of wire up to AWG#8 should be overlapped for a length not less than five times their diameter, spiral wrapped at least three turns, soldered, covered with shrink wrap or electrical tape and then waterproofed. Larger gage wires should be similarly overlapped and connected with split bolts, soldered, covered with electrical tape and waterproofed. Batteries are connected in series to increase voltage or in parallel to increase their amp-hour capacity. These interconnected groups of batteries are called "battery banks." Even when partially charged, an interconnected battery bank can deliver sufficient voltage and current to arc weld! Always be careful around battery banks. Keep sparks and other ignition sources away at all times. Never allow tools to fall onto terminals or connections. Never permit construction or use of shelves above the batteries. Battery banks must always be adequately vented. When paralleling batteries, reduce the effects of voltage drops which cause unequal resistances between parallel branches, so that all batteries in the system operate at an equal current and voltage level. This is done by using the same length of cable from each battery terminal to a central junction point. Positive and negative do not necessarily have to be of the same length. This eliminates uneven voltage drop between batteries. The battery cable size is calculated based upon the peak charging and load current demands of the system multiplied by the resistance of the wire. Marine grade or multi strand welding cable of AWG #8 or larger is recommended. Most battery problems are caused by oversized loads or equipment operating at excessive duty cycle for too long. When batteries are in a low state of charge, it is necessary to check the load as well as the batteries and charging system. The four ways to determine the charge state of lead-acid batteries, in declining order of accuracy are: 1 hydrometer/refractometer 2 actual equipment load test 3 artificial load test 4 open circuit voltage When using a hydrometer you are working with strong acid. Wear eye and face protection and rubber gloves. Have baking soda and plenty of fresh water ready to neutralize spills. To use a hydrometer, squeeze the bulb while the inlet tube is still above the electrolyte level. Then lower the hydrometer into the electrolyte and slowly release the bulb to draw in the electrolyte. At the first cell being checked, fill and drain the hydrometer three times before removing a sample. This brings the hydrometer to the same temperature as the electrolyte. Take a sample and allow the bulb to fully expand. The sample must be large enough to completely support the float. Hold the hydrometer straight up and down, so that the float does not touch the sides, top or bottom of the tube. Look straight across the electrolyte level to read the float. Ignore the curve of the electrolyte on the sides of the hydrometer. Be careful not to drop the hydrometer or allow acid to drip out of it. After reading the hydrometer, to empty it slowly squeeze the bulb again with the inlet inside the cell, but just above the electrolyte level to reduce risk of spills. Record the Sg of each cell on a work sheet. After use, rinse the hydrometer with fresh water at least five times to flush out any acid. Allow it to dry completely before using it again. Temperature compensation is required for batteries not at 80 degs. F. Use an accurate glass thermometer and immerse only the thermometer bulb into the acid, leave it for 5 minutes, read it and then rinse in clear water. For every 10 degs. F above 80 degs. F a factor of 0.004 must be added. Subtract the same factor for each 10 degs. below 80 degs. F. As an example, if a battery at 30 degs. F has an Sg of 1.240, the battery is 50 degs. below the standard, so the compensation is subtracted from the specific gravity. The compensation to be subtracted is .004 x 5 = .020; so 1.24 - .020 = 1.220. A refractometer is the most accurate way to measure electrolyte Sg, requires only a small amount of fluid and is automatically temperature corrected. Refractometers are generally compact and rugged and are highly recommended for ongoing maintenance of large auxiliary power systems. To test the no-load, open circuit voltage an accurate DC voltmeter is required. Operate the equipment loads from the batteries for five minutes to remove any surface charge the battery plates may have. Turn off the loads and disconnect the batteries from the rest of the system. Now measure the voltage across the terminals of every battery. This open circuit voltage without any load being applied is a good preliminary indicator of the state of charge. The open circuit voltage of a fully charged 12-volt battery is over 12.72 volts, whereas it is about 12.6 at 75%, 12.48 at 50%, and 12.12 at 25%. To perform a load test, reconnect the system after recording the no-load voltage, but leave float chargers, charge controllers or solar arrays disconnected. Operate the equipment at its normal duty cycle for one hour, then disconnect batteries and measure battery voltages again. If any battery indicates a voltage of 10 percent higher or lower than others, it should be serviced or replaced. An automotive load tester, if available, can be used, although you must be aware that this places a large artificial load on the batteries for a short time rather than waiting for a small load to slowly discharge the batteries. Because at faster rates of discharge, available capacity is reduced, this test is not as reliable for communication use as a load test done at the normal system duty cycle and rate of discharge. Accurate trouble shooting requires that all batteries in a bank and individual cells of unsealed, wet-type batteries be numbered. Recording a system history identifies patterns and trends and is a great time saver for others who may service your system in an emergency, because they can focus first on the most frequent problems and can anticipate the proper tools and materials to bring. Battery systems which are not used on a regular basis must be checked in the spring and fall, at minimum. Monthly is recommended. First disconnect all loads. If battery tops are wet or dirty, remember that fluid on top of the battery is highly acid electrolyte! Clean battery tops with a cloth or brush and a baking soda and water solution. Rinse with clean water and dry with a clean cloth. Remove the caps from all cells, check the electrolyte level of every cell in every battery and add distilled water to the fill line on the battery, or ½" above the top of the plates. Determine the battery's state of charge with the hydrometer. Discolored or odorous electrolyte is usually indicative of contamination caused by adding other than distilled water, resulting in battery failure. Inadequate charging without adding water can result in lead sulfate shorts between the plates, cracked partitions between cells and leakage which require the battery to be replaced. Never hammer cable connections onto terminal posts as this breaks fragile spot welds between terminal posts and plates, causing shorts. Check to ensure that all caps are in good condition, replace and tighten them securely by hand. Tighten battery tie- downs to hold batteries securely, but no so tight as to distort the case. Batteries that will not accept a charge have been harmed by being left at a low state of charge for too long, physical damage, or overcharging and should be replaced. Inspect and repair all corroded, loose or burnt connections and blown fuses. Large cartridge fuses don't look different when they are blown, so remove these and check continuity with an ohmmeter. A blown fuse shows an infinite reading, whereas a zero reading means it is still intact. Always determine why a fuse blew before replacing it. Proceed logically and check the most obvious things. Check for excessive voltage drop at the load. Knowing what failed is necessary to avoid repeating the condition that caused the failure. If the same fuse blows again, don't consider a system operational until everything has been checked out. Trust your intuition, use your common sense and above all, always be careful. ---- 73 de K4KXK. -------------------------------------