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Be Prepared: Build Your Own Backup Battery

Be Prepared: Build Your Own Backup Battery

The battery is the heart of the system. The most economical technology is lead acid -- the technology used in car batteries. However, basic car batteries aren't suitable for our purposes, because they expel gasses during charging, so they can't be used inside the home. We use a version of the car battery called "Advanced Glass Mat" It is designed for deep discharge cycles and is gas- and leak-proof.

By Patrick Nelson
12/20/12 5:00 AM PT

"Normal connected lifestyle to resume in due course," could have been the tag line recently in New York and New Jersey. If there was one lesson learned from Hurricane Sandy, it was that even the most sophisticated of urban areas can experience an unexpected loss of essential services.

One way to prepare for the kind of interruption millions experienced during and after Sandy is to build a simple, home backup power system for charging phones in the event of an extended power outage.

Functioning communications can create a sense of normalcy, while providing emergency information and connecting communities. I've written about how social networks were utilized during Sandy. I've also written about how to capture remote mobile network signals.

I've built a basic power system for my home in earthquake-anticipating Southern California. Here's how to do it.

Rethink the Technologies

Historically, the technology used for emergency backup power has been the gasoline generator -- good for fridges, but overkill for gadgets that require less oomph.

Generators are noisy and smelly, and electronics-sensitive versions are expensive. My backup power system, which uses a battery, cost about a third. It won't run a fridge, but it will charge phones, and mine can also be used as an emergency car starter.

Step 1: Calculate Your Needs

Select a 55-amp hour or greater battery. Amp Hours, or aH, is a measure of the number of amps -- or the juice -- that the battery can provide over a period of time.

A 55-aH battery theoretically provides one amp for 55 hours, or 55 amps for one hour. For chemical reasons -- and without going into detail -- these numbers are rough, but they give you a ballpark idea of what to expect.

A smartphone charger supplies about one amp to the phone. Therefore, the 55-aH battery will theoretically charge the phone for 55 hours. In fact, you shouldn't run your battery down past 50 percent, so make that 27 hours of charging.

So, if your phone takes two hours to charge and lasts you a day, your new battery pack, based on a 55-aH battery, will provide 13 charges, or 13 days of use -- roughly.

Step 2: Gather the Parts

Parts include a battery, battery wall charger, cigarette lighter adapter, voltmeter and micro USB automobile charger.

The Battery

This is the heart of the system. The most economical technology is lead acid -- the technology used in car batteries. However, basic car batteries aren't suitable for our purposes, because they expel gasses during charging, so they can't be used inside the home.

For our purposes, we use a version of the car battery -- also used in marine environments -- called "Advanced Glass Mat," or an AGM battery. It is designed for deep discharge cycles -- unlike basic car -- and is gas- and leak-proof.

I use the 55-aH 12 volt Optima 8014-045-FFP YellowTop Group 34/78 Deep Cycle Battery (about US$185 on Amazon).

Wall Battery Charger

The purpose of the wall charger is to top off the battery periodically.

Look for an electronic charger that's compatible with AGM battery technology -- classic lead acid "floating" car battery chargers aren't suitable.

I use the Schumacher SC-600A SpeedCharge High Frequency Battery Charger (about $34 from Amazon).

This charger only provides six amps, and consequently takes overnight to fully charge the 55-aH battery from 50 percent. However, it's cheaper than the 10-amp charger from the same manufacturer -- we're not running an auto shop business here.

Battery Clip-on Cigarette Lighter Adapter

This replicates the in-car cigarette lighter port and is attached by a two-foot wire to clips that attach to the car battery -- its' similar to how jump-start clamps attach.

I use the Roadpro 12V Battery Clip-On and Cigarette Lighter Adapter (about $6 at Amazon).

USB Car Charger

This acts as the interface between the phone's USB charging cable and the cigarette lighter port. It steps the voltage down from 12 volts to smartphone-friendly five volts and provides one amp.

Any like-charger will do, but verify that it provides an amp by identifying any labeled specifications. Many cheap ones are half-an-amp -- OK, but they'll take longer to charge your phone.

The Voltmeter

This displays the condition of the battery when you plug it into the cigarette lighter adapter and lets you know how much juice is available. Voltage is used to gauge capacity.

I use the Equus 3721 Battery and Charging System Monitor (about $15 at Amazon).

Step 3: Assemble the Parts

Place the battery on a firm surface. Remove the caps that protect the top terminals and attach the electronic charger by clipping the red lead to the battery pole labeled "Plus," or "+," and then clip the black lead to the corresponding negative, or "-" labeled pole.

Plug the electronic charger in to the wall and select the battery type (12V) and charge (six amps). Allow the battery, likely shipped 50 percent, to charge for seven to eight hours. An LED indicator will display when charged.

Then remove the plug from the wall and unclip in reverse order -- black first, then red.

Warning: Always wear safety glasses when working with batteries. AGM batteries, while safer than classic car batteries, are still full of acid and contain electricity. Keep everything away from kids.

Step 4: Attach the Port

Attach the clip-on cigarette lighter port -- again, following the same order of attachment as in the previous steps. Then insert the USB adapter as you would in a car. Attach the phone, and you'll see the battery charge indicator on the phone become active.

Allow the phone to charge as it normally does.

Step 5: Check Voltage

Periodically insert the voltmeter into the cigarette lighter adapter when in use to check voltage.

Tip: A fully charged 12-volt AGM battery will measure 13.1 volts. A deeply discharged battery will measure 10.5 volts or below. Never let the voltage drop below 12.4 volts. Fully charge the battery if it falls below 12.4 volts.

Step 6: Fully Charge

Fully charge after using the battery pack, and then check for a voltage drop below 12.4 volts every three-to-six months while in storage.

In that case, fully charge again.

Want to Ask a Tech Question?

Is there a piece of tech you'd like to know how to operate properly? Is there a gadget that's got you confounded? Please send your tech questions to me, and I'll try to answer as many as possible in this column.

And use the Talkback feature below to add your comments!


Patrick Nelson has been a professional writer since 1992. He was editor and publisher of the music industry trade publication Producer Report and has written for a number of technology blogs. Nelson studied design at Hornsey Art School and wrote the cult-classic novel Sprawlism. His introduction to technology was as a nomadic talent scout in the eighties, where regular scrabbling around under hotel room beds was necessary to connect modems with alligator clips to hotel telephone wiring to get a fax out. He tasted down and dirty technology, and never looked back.


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