Saturday, July 14, 2007

Building an external battery pack for portable electronic devices

This post will give you step by step directions for adapting an external battery pack to power a Zoom H-4 Handy Recorder using about $30 worth of supplies. For the most part, these instructions should work with pretty much any electronic device that can be powered by an AC adapter. With a little know-how, you could also get it to work with USB powered devices.

As I've mentioned before, I love my Zoom H-4... 99% of the time. There are two major problems with this unit:
  1. It does not work well with external microphones. This isn't a huge issue because the internal mics sound surprisingly good. Better, in fact than the EV RE-50 and minidisc recorder combo I had previously used. They provide a nice full, stereo sound with relatively little hiss.
  2. When running on battery power, you can hear a electronic noise in the recordings. It's very quiet. In fact, it's quieter than the hiss picked up by most recorders. But since it's rhythmic, it can be a bit more noticeable when you crank up the volume in playback.
So it's a vicious circle. The H-4 doesn't work well with external mics, but that's okay because I can just use the built-ins. But the built-ins record extra noise.

Since I usually record interviews on battery power, I didn't realize that the noise was caused by the batteries until I read up on the issue in the Zoom forums. It turns out there's two possible solutions (since Zoom ain't fixing it). You can void your warranty and crack open your unit to solder some parts together. Or you can build an external battery pack so that your unit is essentially always plugged in. I opted for the latter. As an added bonus, I should be able to record for 20+ hours on a single charge, as opposed to the 3-4 hours you could get with 2 AA batteries.

Here's a sample clip I recorded using battery power. You'll have to pump up the volume and listen with headphones on to hear, but there's a high pitch noise in the background.

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Now listen to this clip I recorded using my external batter pack.

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Here's how I built the battery pack.

Following the advice in this forum post, I went to Radio Shack and bought:
  1. 274-1569 Size M coaxial DC power plug (2-Pack) $3 (I DID NOT NEED THIS, SAVE THE $3)
  2. 23-445 Remote control car 9.6V battery pack connector repair kit $5
  3. 23-432 Remote control car battery pack and charger $20
Then I realized that the instructions assumed a bit more knowledge than I had. For the life of me I couldn't figure out how to get the Connector Repair Kit to fit into the Coaxial Plug. I eventually figured out it involved a soldering iron and decided to go the easy route. I decided to cannibalize the power plug that came with the Zoom H-4.

Here's a picture of the 9.6 volt remote control battery pack I used:

And here's the RC battery pack connector repair kit:

The eagle-eyed reader will note that the battery pack plugs into a charger via that white plastic doohickey. The repair kit features an identical doohickey. From now on, that will be the technical term.

In other words, you plug the batter pack into the charger to you know, charge the battery. Just unplug it and plug it into the battery repair kit to power whatever's on the other side. But instead of an RC car, we want a plug for your Zoom H-4 or other electronic device to be on the other side.

Cutting the wires

To do that, you need to cut off the tips of black and red wires and strip the cable. If you've got a pair of wire cutters, use those. I found a pair of needle nose pliers that could cut and strip wire, so I was all set.

Next comes the scarier part. I pulled out the power supply that came with my Zoom H-4 and stripped the cable close to the part that plugs into my audio recorder. Don't cut too close, because you want to have some wire to work with if you need to make a second cut.

When I was done, here's what I had:

Putting the pieces together

Now all you have to do is twist the cables together. In this case, I had a red and black cable from the doohickey, and two black cables from the power supply/plug. Luckily one of those black cables had a white stripe.

Through a little trial and error I determined that the striped wire should be twisted together with the black cable, and the plain black cable should be connected to the red cable.

That's it. If your battery pack is charged, you can plug the doohickey into your battery and the other end into your portable device, and you should have power.

Just to be on the safe side, I went and wrapped some electrical tape around each of the connections. And then I strapped the whole thing to my recorder. The result looked something like the nuclear power packs used in Ghostbusters.

Seeing as how the Zoom H-4 already looks a bit like a taser, I decided to do a bit more modding so as not to scare interview subjects any more than necessary.

I went back to my power plug and cut off enough wire to let me stick the battery pack in my pocket while holding the Zoom H-4 at arm's length. I then spliced this wire to the others using the same methods described above.


While this hack doesn't solve my external microphone issue, it does eliminate the buzzing noise on my recordings. I'd still like to find a good external mic that works well for settings where the internal mics are impractical. But now I'm happy with my Zoom H-4 99.5% of the time.

And these instructions should work for pretty much any device that uses a similar plug. Keep in mind that you'll want to use a battery pack that supplies a similar voltage to the power supply that came with your device. The Zoom H-4 comes with a 12V power supply, but the 9.6V battery pack seems to work just fine. There's little chance of frying your electronics by using a battery supply with a lower voltage than recommended, but I'd warn against using a higher voltage.

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Anonymous said...

hi brad, great job here! do you know if there are any other things to take note of when purchasing a battery... ie. keep it under 12v and what about the mAh rating? does it matter what its rated at?

your help would be appreciated.



Brad Linder said...

My understanding is that the voltage matters more than the mAh. Too many volts and you'll fry your unit.

FYI, my battery pack is 9.6 volts and 1000 mAh.

Josh said...

If you compare volts to the octane of gasoline, mAh would be how many gallons your gas tank holds.

If you know the wattage of your device you can figure out how long your batteries would last.

With Brad's pack: 9.6 volts x 1000 mAh = 9600 watt hours. Take the wattage of the H4 and divide it by 9600 and you get a rough run-time.

Anonymous said...

Thanks a mil Brad for the instructions... how daring to try this batt on yer unit. Question - why cut the barrel connectors off of the doohickey? They let you crimp the bare stripped wires together without twisting, exposed wires, or soldering. Perfect as is.

Lawrence said...

I decided I really liked the battery pack idea.. but instead of ruining my AC adapter I looked around through my old AC adapters from cordless phones etc. I found one that had a matching plug. The polarity is different but that is not a problem. I sorted out the polarity with a meter. Thanks for a great idea.. on more than one occasion the AA NMH batteries gave out too soon. Oh, and the battery pack I got at Radio Shack is 1600 MAH

Anonymous said...

Would something like the following work as a power-source??

???? or is there a reason for using 9.whatever volts battery, rather than a 12v to match the mains adapter supplied with the H4? what is the actual input operating voltage?

Tapeleg said...

I'm ready to do this. I found a pack with 2 battery packs (9.6 V and 2300 mAh) and a charge for $34 on Amazon. The thing should last forever. Toobad there isn't a good way to meter the power left in the battery packs.

It cracks me up that the unit runs off 2 AA's (3 volts) internal but 12 volts external. No wonder it has issues.

Perhaps a RE50N/D would be a little better with these mic preamps. Now we need a portable mic pre that doesn't suck.

kathy said...

we have 3v-24v external Lithium battery pack.Capcity from 7Wh to 266Wh.

We are looking to cooperate with ebay sellers.

my contact E-mail

Josephers said...

When I checked my Zoom H4 Unit, it said it took a 9V power source. Are you absolutely sure that yours uses a 12V pack? Because I think what's happening is that you're overfeeding your Zoom H4's by 0.6 volts.

Lawrence said...

The AC Adapter that came with my H-4 said it had an output of 12 volts.
The original idea of the Radio Shack kit and adaptor is the best idea put forth. I put together a battery pack as Brad recommended. It has worked flawlessly for many months now. Why try another route?

Josephers said...

The thing is, the AC adapter that came with MY Zoom H4 says that it has an output of 9V at 300mA. The same is said on my Zoom H4... and most times I don't hear that whining noise when on batteries.

Anonymous said...

Well Josephers, It could be that Zoom decided to change some things about the H-4 and one of them was to change the output voltage of the AC Adapter.

I assembled a battery pack as Brad suggested and it works great.. I can turn the recorder on while my band is playing and just let it go.. I have never run out of battery power.. and I do not recharge between every use either.,

pdubber said...

Thinking of making an external battery pack myself. My AC Adapter also says it has an output of 9V at 300mA.

I have measured the actual voltage and current from the adapter with a multimeter. I notice that the numbers are almost identical to those that I get when I measure a standard PP3 (AKA "transistor") 9V battery.

I'm no electronics expert, but what would be wrong with simply hooking up a PP3 battery to a standard PP3 snap that is wired to the proper size DC connector?

Of course, I don't want to blow up my recorder. Any thoughts would be appreciated.

BJ said...

You legend! I've been fumbling around with rechargeable NiMH AA's for ages; they wouldn't last long at all recording on location, and I had to carry a bag-full to share with my lectro radio mics.

A 9.6v 2000mAh batt took 2 hours to charge with a 1A delta charger and took approx 24hrs to drain fully. That's with normal everyday use: switching on/ off, recording at 48khz, but the rest of the time leaving on/rec armed. Plus that high pitched noise is now not present in my recordings. Extremely happy all round!!!

Anonymous said...


Graham Riches said...

@ Brad..This is a better solution, you can use standard batteries as well as Rechargeble. check this link:


Anonymous said...


Josephers said...

The thing is, the AC adapter that came with MY Zoom H4 says that it has an output of 9V at 300mA. The same is said on my Zoom H4... and most times I don't hear that whining noise when on batteries.

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