Building a rechargeable power supply

Sanyo recently released the Pedal Juice rechargeable battery pack for guitar pedals, which is a good idea. You won’t have to search for a mains outlet where you place the pedalboard on stage, and you won’t get any hum from AC ripple (more about that here), as the power source is a battery. But there are drawbacks, of course. For one, the cost – the Pedal Juice costs more than quite a few power supplies (even ones with isolated outputs, which it doesn’t have).

But if the battery pack idea tickles your fancy, you can easily put one together yourself. Here’s how to do it:

A little math

Rechargeable AA batteries are rated at 1.2 volts, rather than 1.5 volts, so by combining 8 of them in series (8×1.2), you should have a steady 9.6 volts – perfect for guitar pedals. And these days, NiMH rechargeable AA batteries can have a capacity of up to 2500mAh, which technically could power a 250mA pedalboard for 10 hours. In the real world, you will probably get less than that (you’ll have to recharge when the voltage drops below useable figures, rather than when the batteries are completely drained). But with a fairly small pedalboard, you should be able to get enough for a day’s playing, at least.

Also, get a quick charger (you can often get a combo pack with charger + 4 batteries). As the charger usually only holds 4 batteries at a time, you’ll have to run two cycles to charge all 8 batteries. You do not want to have a charger that needs 10-12 hours for each quartet. Inevitably, you will forget to keep the batteries charged, and end up having to panic-charge while you’re packing your stuff for a gig…

There is one caveat, though, and it is an important one. Remember that batteries are unregulated power sources, so their output voltage will vary with the load. Although the batteries say 1.2 volts on them, that is measured under load. With no load, or a low load (such as a smallish board of guitar pedals), you should expect to see closer to 12 volts from a bank of 8 rechargeable AA batteries. I don’t know exactly the formula for how the voltage drops under load, but suspect that even with a few 100+ mA digital pedals in the chain, the voltage would still be above 11 volts. So make sure your pedals can handle it – most can, but some will actually die if you go above as little as 10 volts.

If you only plan on powering a small board with, say, four or five pedals, you may want to find a battery holder for six AA batteries instead, or search for AA battery dummies (to be able to use 6 or 7 batteries in an 8-battery holder).  If 8 freshly charged batteries produce 12 volts with little or no load, 6 of them will be at exactly 9 volts while 7 would produce 10.5 volts.

Putting it together

To combine the  batteries, you need a battery holder. These come in various sizes, so look for one with that fits the number of AA batteries you want to use (normally 8, or possibly 6, as discussed above). At the top, there are connectors for a battery clip – the same type as you’ll find on a 9 volt battery. Those connectors are the output from the battery holder.

You also need a battery clip and a 5.5×2.1mm DC plug. Get a male plug if you’re connecting directly to a pedal, or want to use a daisy chain cable that starts with a female jack plug. If you’re connecting to a Boss daisy chain (the one that has all male plugs), get a female jack plug instead.

Now for some soldering. Connect the battery clip’s positive (red) wire to the sleeve of the DC plug, and the negative (black) to the center pin. Don’t forget to slide the plug cover over the wires before soldering – it happens all the time – and some shrink tubing over the wires and sleeve of the plug will also help relieve some stress from the wires.

There are ready-made battery clip -> DC plug cables out there (search for ”emergency battery connector”), but don’t confuse them with the converter cables you can use to run a battery-only pedal from a daisy chain. Those are wired ”backwards”, and you will end up with a center positive DC jack plug, unless you also add a reverse polarity converter cable to it.

Pros and cons

The advantages of this type of system was covered at the beginning of the article. The Sanyo unit has a power on/off button, but with the DIY method you’ll have to unplug it from the daisy chain to shut the power down. But the big disadvantage is – again – the cost. You do need to do a little math first, adding up the cost of the charger, batteries and parts, to see if it’s really worth it.

Also, keep in mind that even though the power from the battery pack has no ripple, it’s still a daisy chain setup. So you will still run the risk of ground loop hum and interaction between pedals – especially if some of your pedals are connected to the amp’s effects loop. If you have to have a second power source for the fx loop pedals, this type of setup will not be worth the effort. It will work best for a relatively small setup – I can definitely see myself using it for my small gig/rehearsal board, but not for the bigger setup.

Parts list

  • Battery charger and rechargeable AA batteries
  • Battery holder
  • Battery clip (for 9v batteries)
  • 5.5×2.1mm DC plug