Friday, March 14, 2008
Power Supplies and Batteries for Hams
Why have Battery backup? How much Power Supply or Battery for the Station? What connectors?
One of the items found in most ham shacks is a battery or a DC power supply. The reason for this is simple enough: most of our equipment is meant to be used in a mobile environment and that means a DC. power supply in the 12.5 v to 13.5 v range. Even a lot of our base equipment has the provision to be run from a DC. Supply.
How big a power supply to get has been the subject of some heated discussions on the air, so how do you pick a supply for the home? Past experience is that we tend to outgrow an existing supply. If you stay in this hobby long you keep on adding equipment that runs off DC and soon you are popping fuses. Start by looking at the equipment in the shack and adding up the currents in amps. Lets say that the Radio, Linear Amp and TNC add up to about 7 amps. The general rule of thumb is to double that number, in this case 14 amps, and then look for a supply in that range. In this case a 15 amp supply should serve you well and leave you room to change equipment around at a later date. Don't buy the biggest supply you can afford. If you have too little load on a supply, they don't regulate well. Picking a supply for mid range seems to be the
best compromise. Supplies on the surplus market have their hazards. Computer Power Supplies often use switching type regulators. These can generate harmonic interference on your radio. Supplies meant for the audio industry can break regulation when they get in a RF Field.
Since a lot of us want Battery Backup we tend to sooner or later get a battery that will run the station. For those of us who work the "Thons" (Walkathon, Bikeathon, or Runathon) or other Public Service Events the ideas of a battery for these events become very attractive. Batteries tend to become a ten ton liability after a few of these events. We tend to buy something like a Boat or RV battery to run a handi-talkie for 4 to 5 hours. You are never stationed near the car and lugging this battery all over the place gets old after an event or two How do you pick a battery that is the best comprise between weight and length of service? Batteries are rated in "AMP HOURS" meaning that 25 Amp Hour Battery can deliver 25 amps to a load continuously for 1 hour. Most radios only draw maximum current when transmitting so the specifications on the radio are broken up into Transmit and Receive current draw. A mobile radio (for example: 35 W output) draws about 10 amps when transmitting and 0.7 amps when receiving.
A 25 Amp hour battery would last 2.5 hours continuous transmit (25/10=2.5) or 35.7 hours receive only (25/0.7=35.714285) for most of these events. Unless you are the Net Control Station (NCS), this battery is much too large. When trying to size a battery for these
events, figure on a 30% transmit and 70 % receive cycle. A small battery such as a 1.2 Amp hour will power a 5 Watt handi-talkie for about 4.7 hours in receive and 18 minutes in transmit (These numbers assume a transmit current of 1.3 amps and a receive current of .250 amps.): more than ample for a public service event. Connectors for power sources vary widely from automotive lighter plugs to Banana Plugs and Jones Connectors and everything in between. The ARRL Field Service recommends the 2 pin **Molex connector. Regardless of the Battery or Power Supply you choose please have in addition to your connector, an adapter with standard RACES Connectors on your DC source. This lesson was recently learned the hard way this spring during a midwest tornado. Operators who were not in Emergency Service Organization such as RACES or ARES brought their equipment in to help. Some of this equipment could not be used without extensive modifications and there just wasn't time to tinker with it at the time.
Batteries and power supplies have become as much a part of the shack as Radio and antennas. Having the ability to help in a pinch is a great asset. Even if it's not a major emergency, being able to bring up the net when the lights go out in your neighborhood is a great comfort to you and your family.
**NOTE: Since the writing of this article the norm now is to use the Anderson Powerpole connectors in wiring for EMCOMM situations. See the link on this Blog re. the Powerpoles.
Posted by R. Small at 7:10 AM