SARC Events

SARC Events

SARC Courses
Course Information
Field Day


A Long Lasting Battery Backup For Your HT

A Communicator Reprise: April 2011

This is day five of a SEPAR callout and you're at home. Your hand-held radio (HT) battery is out of juice and you have no means of charging it. What you now wish you had is a substantial, fully charged battery to keep your HT operational for the couple of weeks that Hydro is going to take to restore the power.

Where Can I Get One Real Cheap?

Organizations which replace all their Uninterruptible Power Supply (UPS) batteries on a scheduled basis will often give them away. These batteries are typically the Sealed Lead Acid (SLA) type which means no gases escape them (provided you don't overcharge them) and so no particular safety precautions are required re: handling or ventilation as well as no need to add water from time to time. Clubs are often given a number of these batteries which are then made available to members.

What Does It Cost To Keep My HT Going For A Couple Of Weeks?

The battery has to have sufficient capacity. Battery capacity is specified in Ampere-hours, abbreviated Ah. A very common UPS battery is rated at 12 Volts and 7 Ah. What this means is that, theoretically, the battery will be able to deliver: The battery voltage has to match the voltage expected by your HT. Most HTs will work fine with a 12 Volt battery but you will need to check your HT documentation to be sure. For example, some iCom HTs cannot be powered from an external source. Charged - Yes. Powered - No.

How Does This 7 Amp-Hours Translate Into Operating Time?

Let's suppose that in order to ration your battery current consumption you listen for a total of 5 hours per day and that your total transmit time in a day is 1/4 hour.
For my old Kenwood TH28A, the current drain would be as follows:

  • Receiver current drain is 0.075 A (75 mA), loud so others can hear;
  • Transmit current drain is 0.100 A (100 mA), Low power;
To calculate the total ampere hours used in one day:
  • Receive amp hours per day = 0.075 A x 5.00 hrs = 0.375 Ah;
  • Transmit amp hours per day = 0.1 A x 0.25 hrs = 0.025 Ah; 
  • Total amp hours per day = 0.400 Ah

So, number of days you'll be able to use your HT using one of these 7 amp-hour batteries is 7 Ah / 0.400 Ah per day = 17.5 days. If you were to listen and transmit for twice as many hours in the day the battery would last for about 9 days. This is based on the Kenwood TH-28A. The current drains for your HT will undoubtedly be different. Still, this shows that one of these batteries can run an HT for quite a number of days.

OK, I'm Convinced I Need One Of These... What Else Do I Need?

  • A means of charging the battery (BEFORE the power goes off);
  • A cable with a fuse in the middle and appropriate connectors on the ends for connecting the battery to your HT.
The Battery Charger — Doesn't have to be expensive. You will encounter two different types, Trickle charger and "Smart" charger. The very inexpensive trickle chargers will continue to supply a small amount of current to the battery even when it is fully charged. As this is typically more current than needed to keep the battery fully charged, the battery lifetime will be reduced. For this reason, once the battery is fully charged it should be disconnected from the charger. As batteries lose their charge over a period of time, even when not used, the battery should be reconnected to the charger after three months.
The Smart chargers will supply charging current to the battery until the battery is fully charged. At this point, the Smart charger reduces its output voltage so as to supply just enough current to keep the battery fully charged. Consequently, there is no need to disconnect the charger and it will maintain the battery in the fully charged state indefinitely without overcharging it. RP Electronics in Vancouver sells a very suitable Smart charger for about $30. Part # FC-1212B. It charges 6 and 12V batteries. Charging current is selectable. 

Cable - Wire size and type - #18 AWG 2 conductor stranded wire. One red conductor and one black, max 20 feet
Fuse holder - you want an in-line fuse holder with a 5 amp fuse. Put this in series with the red conductor. The fuse is necessary to protect the wire and battery from short circuits. These batteries store a lot of energy so if you short circuit the terminals there's a good chance the battery will blow up and spray sulfuric acid all over the place. So... don't leave out the fuse.
Battery connector - Different batteries have different style terminals so it depends on what battery you get. Most of the ones SARC gets have two metal tabs 1/4" wide and about 3/8" long. A very suitable connector for this is the Pico fully insulated female Quick Connect #1765. It does a good job of covering the metal tab so that if you drop a screwdriver across the tabs it won't short circuit the battery and melt a hole in the screwdriver shaft.
HT DC power connector - Different HTs also have different style power connectors. You will probably need to take the HT with you when purchasing this connector. Some suppliers sell the power connectors with leads attached. This is very desirable as it means you don't have to solder the leads on yourself. As these connectors are quite small, soldering to them is a bit tricky. You MUST make sure that you know which wire is to go to the positive (Red) terminal of the battery and which to the negative (Black) terminal. If you get this wrong you will probably cause serious damage to your HT.
Once your cable is assembled (complete with fuse), you connect it to the battery. Make sure you don’t mix up the positive (RED) and negative (BLACK) connections, plug the other end into the DC In socket of your HT, and you're in business.

How To make This Even More Useful

It would be nice if this same battery/cable combination could be used with another HT or if your HT could be plugged into a different battery. There is a way to do this using Anderson Power Pole connectors. These connectors are used by emergency communicators all over North America so that any radio can be connected to any power source, regardless of what kind of connector is on the radio or power source. To modify your cable to do this, simply cut it in half, install Power Pole (PP) connectors on the free ends and plug them into each other. 

For installation instructions see NOTE The red and black connectors MUST be oriented with respect to each other as shown in the instructions. If you orient yours differently, they won't mate with anyone else's gear. 
To operate your radio from a different power source, separate  the PP connectors on your cable from each other and plug the radio end into the PP connector of the new source. Similarly, to use your battery with a different radio, unplug the PP connectors on your cable and plug the battery end into the PP connector of the new radio.

What If I Don't Have The Necessary Skills Or Tools To Do This?

If there's enough interest, SARC can put on a workshop session during which you make your cable.

Battery Disposal

At some point, your battery won't hold a charge anymore. Any recycler which accepts standard lead-acid batteries (like the one in your car) should accept it - You may even get some money back for it but you shouldn't just chuck it into the garbage.

Additional Information

Technical overview of the batteries we typically get:

Local Parts Suppliers

  • Battery charger - RP Electronics
  • Wire - RP Electronics, Main Electronics
  • Connectors - Canadian Tire, RP Electronics, Main Electronics, Lee‘s Electronics
  • Fuse holders - Canadian Tire, RP Electronics, Main Electronics, Lee‘s Electronics
  • Power Poles - MRO Electrical Supply

 eBay and Amazon have these parts on-line.

No comments:

Post a Comment


The July-August 2024 SARC Communicator

Hello summer... With another big Summer issue. The July-August 2024 Communicator, digital periodical of Surrey Amateur Radio Communications ...

The Most Viewed...