Battery Back-up for Your Shack

Steve Blankinship, AG4SO

Article from the West Mountain Radio Summer 2013 Newsletter


In this project we will learn a few basics of safely installing a battery backup for D.C. powered equipment in your ham shack. Technology has evolved in both battery technology and controlling the system. While it may appear to be straight forward to add a battery backup for emergency radio operation in the ham shack there are several considerations the foremost being safety.

Objective:
Provide backup D.C. power to communications equipment with automatic switching between power supply and battery while float charging the battery to maintain a state of readiness for emergencies.

Project Requirements:
We will need a regulated power supply, battery with safety fuses, proper sized wiring, switching circuit between the battery and primary power supply, and a regulator to provide a float charge to the battery. Since I am powering more than one piece of equipment I also need a fused distribution panel; Universal connectors and the ability for quick exchange of equipment in the event of a failure or upgrade. For this I am also incorporating Anderson Powerpole^® connectors.

This project while straight forward for the experienced ham may be full of unknowns for the inexperienced. Let's look at some basics for the various items we will need for the project.

Power Supply: Which should I use?
There are several types of power supplies on the market from general purpose to specialized units.

For this project we will need a fixed voltage with sufficient current capacity for the project. If you are looking at surplus power supplies make sure it is a 60 cycle power supply, the wrong frequency power supply such as a 400Hz type used in avionics can damage equipment real fast. A 50Hz will not operate well either.

We need 12V right? Well not exactly, in the specifications portion of manual for your equipment you should find the recommended operating voltage. This is normally the lowest voltage for the radio to operate correctly. Modern D.C. powered ham radios operate at 13.8V +/- 10 to 15 percent depending on the manufacture. Most mobile transmitters will not produce full RF power at 12V and can become unstable below 12V. Never go above 14V it can damage equipment.

Let's look at two of the A.C. to D.C. power supplies common to hams today: Analog: Consist basically a transformer, rectifier, regulator and filter; they are the time tested, heavy beast most of us have used for years. They are commonly used for fixed voltage applications and heavy duty operations such as repeaters.

Switching: A switching power supply generates a square wave, samples its output voltage then compares it to a reference voltage to regulate its output. It contains a good filter to eliminate internally generated spurious signals. This type of power supply is lighter since it does not contain the heavy transformer traditional analog power supplies rely on. This makes it particle for GO Kits for use when AC power is available. Be careful not to remove the cover unless you are qualified, these power supplies contain dangerous voltages inside.

Voltage and current needs:
Research the manuals of the various pieces of equipment you plan to operate with this backup system. Plan for worse case which is 34 Amps in this case, realistically I will not be transmitting on two radios at one time. This system will handle 35 Amps maximum from the power supply. I am also installing a 50 Amp safety fuse on the battery in case of a catastrophic short, we will discuss why as we talk about fusing the battery for safety.

Batteries:
Warning: These batteries contain highly corrosive sulfuric acid, and can explode if mistreated.

Always use caution working with high current batteries, use safety glasses, rubber gloves when handling acid and always safety goggles when making electrical connections. Keep battery in a plastic container in the shack in case of an acid leak. Never make the last connection not the battery, this could draw a spark and cause an explosion in the presence of vented gasses. Ensure you are properly charging the battery; overcharging charging can cause the battery to overheat and explode. Never short a battery it can explode.

Which type of battery to use? The standard automotive or marine led-acid was the choice for many hams since it was rugged and cheep. It required outside ventilation when it charged and regular checking of the acid/ water level.

Today's technology has made improvements; today's choice is the Valve Regulated Led-Acid Battery. (VRLA) The VRLA battery is commonly known as a sealed battery; they do not require regular service of the water/acid and vent far less gas than the old style flooded lead-acid battery. Among these are the Gel Cell and AGM batteries. They are designed for use in confined and poorly vented spaces.

In a lead-acid battery the internal reaction breaks down water, oxygen is produced by the positive plates, in a conventional flooded-led- battery it escapes into air through the vents along with hydrogen gas. This depletion of gases on is the reason for water loss.

NOTE: Use distilled water only when servicing flooded batteries, impieties in faucet water will contaminate the cells and shorten the batteries life.

VRLA batteries are really recombinant batteries this means the oxygen generated by the positive plates will primarily re-combine with the hydrogen on the negative plates reducing water loss.
Gel-Cell: The electrolyte is mixed with silica dust to form a gel reducing the possibility of a spill
AGM: the AGM or Absorbed Glass Mat battery has its electrolyte impregnated in a fiber glass mat separator installed between the led plates.
Though these batteries are called "sealed" they always include a pressure relief valve for safety. Unlike the old style flooded battery they cannot spill their electrolyte if inverted or knocked over. VRLA batteries also contain much less acid than the flooded battery, hence their other name "Acid Starved". Check out this "AGM / Gel Cell Battery Capacity Calculator" for a fast determination of AGM & Gel-Cell battery capacity vs. load. http://www.westmountainradio.com/capacity_calculator.php

Charging:
The requirement for charge voltage varies by battery type; some chargers are intelligent and can determine the type of battery you are using, and the proper charge state needed. Some need to be manually programmed and others are dedicated to the particular battery type. For AGM 14.5V is typically for a charge and for Gel 14.1V is the norm. Either way when floating a battery, the output voltage should be maintained around 13.8V to the equipment being powered. This is why we need a charging circuit that includes a regulator.

Any deep cycle AGM or Gel-Cell that meets our voltage and current requirements will be suitable for our needs. They are available from several sources local, Ham fest and on line.

I am using a HR12-350 AGM by Energy Storage Technologies. The battery is a storage pull from a company that services UPS systems for major cooperation's. They can only keep spare batteries on the shelf for a limited time than has to discard them even if never used. Most cities have these companies and they are often willing to donate these batteries to local hams for EMCOM purposes if asked nicely. You can also purchase suitable batteries from West Mountain Radio: http://www.westmountainradio.com/batteries

Controller/Regulator:
Basically all you need to switch between a battery and power supply is a pair of switching diodes of proper voltage and sufficient current capacity configured in to an OR gate. We also want to properly charge the battery. For this project I chose the Super PWRgate PG40S.

Distribution:
There are many ways to distribute the output of the power gate to the radios. Building your own cables using Powerpole^® connectors is not hard; if you are not familiar with them take a look at this link for information on how it is done. http://www.westmountainradio.com/videos

For correctly installing Powerpole^® connectors with ease consider the PWRcrimp Crimping Tool. An inexpensive tool that works well. I have purchased additional dyes for use crimping RF connectors making it a more valuable addition to my Powerpole^® connector kit. http://www.westmountainradio.com/PWRcrimp

I also selected the RIGrunner 4012 it has a fused 40A input and fused outputs varying from 20A to 1A. I can add additional equipment and can increase the capacity of any output by changeling the fuse sizes including the primary just keep in mind the maximum current limit of the system. For the radio connections do not double fuse by using fused factory power cables doing so will increase the delay time in protecting a circuit. Instead purchase or build non fused power cables.

Wiring:
It is important to have to proper size and type of wire for the current draw and voltage. In the US we typically use the American Wire Gauge or AWG for measuring wire size. Look on the jacket for the AWG # for example 8AWG this is the wire size. If you are trying to determine the wire gauge physically do not include the wires insulating jacket as part of the wire size. The insulation jacket thickness can vary by manufacture and intended application. The safest way is to identify by the jacket markings or the spool if one.

Should we use stranded or solid wire?
If a wire is made up of several small strands of wire instead of one large wire it has a larger surface area allowing the capability to carry more power than a solid wire. This also results in less heat dissipation of the power in the wire. It also allows more surface contact with a connection in comparison to a solid wire. It is more flexible than solid and therefore fewer tendencies for breakage from movement. This is why good automotive battery jumper cables and wielding cables are commonly made of fine stranded wire. Some stranded wire will have less and larger strands depending on the intended capability of the wire to handle a given load and physical size. In our application we are using red/black zip cord. Beware of discount wire that has fewer strands.

West Mountain Radio is a good source for quality zip cord. See their site recommendations of wire size with Powerpole^® connectors. When using a wire capacity chart remember automotive and other D.C. wire is stranded and home and A.C. wiring is typically solid. Keep in mind the length of the wire run as well as the current load and of course voltage.

Here is a very useful calculator to aid in determining the correct stranded wire size for a 12v D.C. system. http://www.westmountainradio.com/cable

I have found in the ham world the size of your factory power cable will vary typically eight to ten feet in length and be of varying gauge depending on the radios current needs.

Starting to put it together:
It is good to start any installation by performing a survey of the area to be used, locate the power receptacle and identify where to locate the best places for the components such as battery, distribution panel and Super PWRgate PG40S to be installed. How long will wiring runs need to be and how you will keep them away from feet and so on. Keep in mind accessibility for maintenance, especially the battery and fuses. Use the calculator above to ascertain the correct wire sizes. I need a 3' wire between the PWRgate PG40S and battery then between the power supply and PWRgate PG40S then to the RIGrunner 4012. #16 AWG is sufficient. The manual for the Super PWRgate PG40S recommends #10 AWG for the battery wire which would give you plenty of capacity for the 40 amps it is capable of and could encounter in a recharge of the battery. If in doubt use a heavier gauge wire for safety and efficiency.

I need a six foot wire between each radio and the power gate, #14 is sufficient. For a computer Interface #24 AWG was correct for one half amps on a six foot run. Remember you can always go up in size but never go down below the load rated size for safety sakes.

I mounted the PWRgate PG40S on the top of a plastic marine battery box and the RIGrunner 4012 under my radio bench. Be sure to not create a potential short with in the box between the battery and hardware. The same goes for any hardware that might rub or puncture the battery case. There are pre-assembled versions of DC Battery Back-up Boxes available by following this link. http://www.westmountainradio.com/dc-power

Fusing the Battery for Safety:
I chose to use a standard automotive ATC type blade fuse for the battery. These are easy to obtain and DC rated. Resettable D.C. breakers can be used instead of fuses they are however expensive, your choice.

CAUTION: Use only D.C. rated circuit breakers.

Everything that is true for fusing A.C. circuits is true in D.C. circuits; but with the addition of one important consideration. D.C. voltages do not go through zero volts twice a cycle like A.C. voltages. What this means is they will not break a D.C. circuit as easily as an AC circuit. What happens is the current wants to keep flowing and sustain the arc that exists when a fuse blows. A fuse will sometimes have two voltage ratings, one for A.C. current, and one for D.C. current. The D.C. current voltage rating is typically lower for these dual rated fuses. Not only does the voltage not go through zero volts, but the inductive load will want to keep the current flowing whenever the circuit is opened.

An overload is an over current condition where the current exceeds the normal full load capability of the circuit and no short-circuit present. A momentary overload condition (known as in rush currents) often occur when a circuit is first initialized due to capacitors charging and/or motor-start up. This is especially common when you start up your vehicle with the radio on.

A short circuit is when a low-resistive path is suddenly created which will cause the circuit current to increase as the circuit resistance is decreased. When this occurs the current can exceed 1000 times the normal current of the circuit. When this happens to a battery it rapidly overheats, swells and can explode and or catch fire. Even if it holds the case integrity it often damages the plates beyond further use.

Remember you want to fuse above the maximum current you expect from the battery but still protect it from a dead short. I chose to use a 50 amp fuse however a 75 amp would work just as well.

Hint: keep spare fuses handy for all the values used in the system. It would be a bad day to blow one during an emergency or training net.

Conclusion:
I measured at the battery input as well as the output of the Super PWRgate PG40S. There is a good device for testing and recording parameters from West Mountain Radio. I would suggest: PWRcheck. It handles 8 display modes including voltage, current flow in either direction, wattage or amp-hours measuring 0V to 60V, 40A continuous load. I measured voltage and current at with all equipment on in receive mode as well as transmit first with individual transceivers one at a time then with both transceivers simultaneously operating in full power transmit in to dummy loads, the V/U radios in FM and the HF rig in AM mode and modulating the audio. The voltage held at 13.11V and worse case for the current was 21.6A. Add the computer interface the power demands at a half of an amp, the system is well within or needs.

Like any project this has been fun, remember such projects can be a learning experience while adding to your resources. Technology is always evolving so there is always something new to learn. I hope you found this useful.

Have fun and be safe.


Powerpole^® is a registered trademark of Anderson Power Products.