Thanks to Gary - AAR6BL for forwarding the article
EMERGENCY BATTERY POWER – FIXED LOCATIONS
We should try to be as independent from commercial power for our communication needs as we can. It seems when we need it the most, such as during emergency operations, commercial power fails. There are several means of providing backup power to our radios and related equipment. The most common and least expensive source of emergency power is rechargeable batteries.
BATTERIES
Secondary or rechargeable batteries are routinely used to provide emergency backup power in our homes or fixed locations. Most of our communications equipment can operate from the 12VDC provided by these batteries. There are several different chemistry families of rechargeable batteries. Even within these families there are different types to choose from. The most commonly used are the lead acid types.
Within the lead acid family there are the flooded cell and sealed cell types. Any battery with a liquid electrolyte, such as a car battery, is referred to as a flooded cell battery. The car battery is known as an SLI (Starting, Lights, Ignition) type and is designed for short term, high current use such as starting the car. They are not designed for long term, low current use but will do in a pinch. The preferred type is known as “Deep Cycle”. These batteries are designed to provide long term, low current power. Don’t let the term “low current” fool you. Depending on their size, they can provide well over 20 amps for many, many hours.
The Ampere-Hour rating of a battery signifies the energy
available. This rating does not indicate the number of amps you can draw for one hour. Rather, this is known as the
20-hour rate. As an example, a 20 AH rated battery can provide 1 amp of current for 20 hours before reaching its fully
discharged point (10.5VDC under load). (The current being drawn multiplied by the time it is being drawn.) Much
higher current, such as 20, 30 or more amps could be drawn from this battery but for a much shorter time. The size
(AH rating) of the battery(s) required for your installation will depend on your operating style, amount of equipment
and time desired to operate on emergency power. At my location, I prefer to use several smaller batteries of equal AH
rating wired in parallel rather than a single, large battery. If one of the batteries should fail, I can remove it
from the pack and continue to operate. I can also remove one or more batteries and use them at a temporary location
should the need arise.
There are some guidelines to follow if you are going to use
flooded cell batteries inside your building. First and foremost, they should only be placed in a well-ventilated area
such as a large room. Do not place them in a closet, small storeroom or any other confined space. These batteries can
give off hydrogen gas during the charging process and can explode if the gas concentration is high enough and a source
if ignition is available. There should never be a source of ignition near these batteries during the charging process
regardless of location. These batteries should be kept in a protective housing since the sulfuric acid used as an
electrolyte can cause damage if spilled. Heavy-duty plastic cases are available for most size
batteries.
The preferred type of lead acid battery for emergency use is the SLA (Sealed Lead Acid). There are two basic types, Gel-Cell and AGM (Absorbed Glass Mat). The AGM is similar to the Gel-Cell with the exception that they are more efficient and expensive. SLAs do not exhibit the bad characteristics of the flooded cell batteries. They do not vent hydrogen while being charged properly. They are sealed and cannot leak sulfuric acid under normal usage. Please notice the emphasis on proper charging. This holds true for all types of rechargeable batteries. It is often said that “Rechargeable batteries seldom die of old age, they are usually killed by improper charging”. I will describe charging methods that will ensure a long, productive life for your batteries. These methods hold true for all types of lead acid batteries.
CHARGERS
A proper battery charger is the most important requirement for a long battery life. Just any old battery charger will not do. Before discussing specific chargers, I will share some general guidelines that hold true for all types of lead acid batteries. Always use a charger that is specific for the chemistry type and capacity of your battery. If you are going to float charge your battery, be sure to use a charger designed for that purpose. Some “trickle” chargers will do just that, they’ll trickle the electrolyte out of the battery! I will describe the two battery chargers I use for all types of lead acid batteries and the characteristics that make them ideal.
The multi-purpose charger I use is the model WM-1200A Computer Smart Speed Charger made by SCHUMACHER*. It has three (3) selectable charging rates; 2, 8 and 12 amps. To insure a long battery life, the rate should be set to 1/10C (one tenth the AH rating). A charge rate of up to1/4C can be used with care but is not recommended. This charger also has three (3) selectable battery types; Regular (car starting), Deep Cycle and AGM-GEL CELL. All types of lead acid batteries can be charged with this unit. At the completion of the charge cycle, it will automatically switch to a “float” mode. I keep a second charger connected to my batteries at all times, even while using the first one described. After reaching a fully charged state, I remove the first charger and allow the second charger to maintain a float charge. This second charger is a SCHUMACHER* model SE I-12 Mity Mite. It designed specifically to “float charge” all types of 12V lead acid batteries. It works in the following manner: when the battery(s) self-discharges to 12.8VDC, the charger turns on and supplies a current of 1.5 amps; as the battery(s) voltage approaches 13.8VDC, the current begins to decrease; when the battery(s) reaches 13.8VDC, the charger turns off and allows the battery(s) to self-discharge to 12.8VDC where the cycle starts over. This ensures no loss of electrolyte. I obtained both chargers from my neighborhood Wal-Mart. Newer models have replaced those listed above but they perform the same functions.
INVERTERS
So far we have addressed methods of using rechargeable
batteries to power our 12VDC equipment. We may also require a source of emergency 115VAC power for some of our
equipment. Computers and peripheral equipment (TVs, wireless telephones, home alarm systems, etc.) will only operate
on 115VAC power. I use several modified Uninterruptible Power Supplies (UPSs) to provide emergency 115VAC . These
UPSs have had their internal batteries removed and have been connected to much larger, higher capacity external
batteries.
They can provide many hours, not minutes, of AC power. I also
use several stand-alone inverters to provide additional AC power. As we know, inverters take DC power and convert it
to AC. I will describe the various types of inverters along with their advantages and disadvantages.
There are three (3) different types of inverters. The major difference is the type of AC waveform they produce. The oldest and least expensive is the “square wave” type. They should never be used to power any transformer-input device as they will greatly overheat the transformer and destroy it. They can be used to power any device rated as “AC/DC’, using a “switching” power supply and incandescent lighting, but produce a high level of Radio Frequency Interference (RFI). They are seldom found new but can still be found at some flea markets and swapfests.
The most commonly type found is the “modified sine wave”. It goes by many names, “modified sine wave”, “pseudo sine wave”, “quasi sine wave” and others. This type can be used to power most equipment found in the radio room requiring 60Hz, 115VAC and produces a moderate level of RFI. It may damage a few pieces of very power-sensitive equipment, but this is rare. It is moderately priced and readily available.The last type is the “true sine wave” inverter. It is not very common and is the most expensive, but produces a low level of RFI. I have not encountered a need for this type, but if I found a used one at a reasonable price, I would buy it. I have a power strip labeled “UPS” connected to each UPS and all of my critical equipment is plugged into them.
12VDC POWER DISTRIBUTION
Safety is paramount when distributing any form of power. Remember, “12VDC can burn a building down just as easily as 115VAC”. Fuse at the source and at the load. The use of Anderson PowerPoles* will provide safety, ease of operation and inter-connectability if using the ARES/RACES scheme. The ideal power transfer method between the AC power supply and the battery(s) should be automatic and instantaneous. There should be absolutely no interruption of power. Much of our modern, microprocessor-based equipment cannot tolerate even the shortest power interruption such as produced by transfer relays. Some of the equipment may reset or reboot at the slightest interruption of power.
The easiest (and cheapest) method is the use of a steering diode network. One can be built using two (2) diodes of proper ratings (25V-PRV, 30 Amp. minimum). Connect the cathodes of both diodes together. This junction will be the source of power for your equipment. Connect the AC power supply to one anode and the battery(s) to the other. Do not use regular silicon diodes as they have a nominal .7 volt forward biased voltage drop. This will waste power and produce problems that will be discussed in the next section. The preferred types are the Schottky diodes. They have a nominal .3 volt forward voltage drop, thereby having a lower loss and providing more power to the equipment. The afore-mentioned two diode network is available as a single package, three terminal device from many electronic parts suppliers. Be sure to use some form of heat sinking.
Use the proper size of wire to minimize the power loss. 12-gauge wire can be used to carry up to 25 amps for a reasonable run (10 to 20 feet). Larger wire should be used for higher currents and/or longer runs. We should strive to reduce power losses to a minimum within economic and practical reason.
DC TO DC CONVERTERS
Modern, microprocessor-based radio equipment is sensitive to low input voltage. Some of it will not operate properly at 12VDC or lower. Below the minimum useable voltage, HF rigs may exhibit problems such as SSB signals “FMing” and CW signals “chirping”. It is important to determine the minimum usable voltage for your equipment prior to depending on batteries for backup power. The easiest method is to check the input voltage specification in your equipment’s instruction/owners manual. I prefer to use a variable DC power supply and do empirical testing to determine the minimum usable voltage for each piece of equipment.
Fortunately, there are devices that can overcome this problem. DC to DC converters take voltages as low as 10.5 VDC and provide a constant 13.8VDC output. They are available with current ratings of 25 amps and higher. These devices will provide the maximum power capacity from the battery supply and eliminate the problems inherent with low supply voltages. Now is the time to set up a backup battery power source, not when you need it.
Bob Rodriguez – K5AUW
For comments, suggestions, help in setting up and/or answers
to questions, please e-mail me at, bobrod@flash.net
P.s. The only dumb question is the one you don’t ask!