DIY portable battery source

The Goal

I don't need this often, but I have several portable gizmos that go with me when I travel. Annoying each take a different votlage to charge and of course each has their own silly connector (someone really should standardize these connections).

So I wanted to build an on-the-go recharge source for these. Several parts I found had variable output voltages, but many of those parts also required picking out several supporting components, with more wiring connections to make, etc, so getting 2 fixed outputs seemed the simpler course to take.

I happen to have a 12V charger for my scuba light batteries, so I went and bought the smallest 12V sealed lead acid battery I could find. Its 12V, 1.3Ah, so with an efficient voltage conversion, it should provide at least 2 charges for the Axim, and 3-5 for the cell phone and iPod. Long enough to get me thru any long day of flying/traveling, or a weekend of camping.

You can also easily produce your input voltage from 8 AA rechargeable batteries (figure roughly 9.6V for 8 1.2V AA NiMHs), wired in series. You can get these up to ~2.5 Ah now, so that's better than the 12V. Note:The DC/DC converter I used requires 10V as Vin-min, so you'd need 1.25V per AA to make it work (in which case the 12 SLA is a safer bet).

Here is the circuit I need to build:

I will use a USB/Dell charging cable I purchased off of Ebay, hence my desire to use USB as the connector with the correct voltage.

Parts list

On the parts, I'm not trying to cop out, but I was able to cannabalize most everything I needed off of spare parts that I already had laying around. The biggest help was a dead USB hub that I had, which gave me 4 USB female connectors, 5 100 uF caps, a DC female connector, an enclosure, and a circuit board to use as a building platform.. and some LEDs that already were wired to +V with pullups.

Buying all of this piece by piece would make this impractical in terms of cost (vs. buying a pre-fab "Powerbank with selectable outputs). If someone knows of cost-effective sources for the connector, email me and I'll add links.

Plus hacking this out on your own is just fun.


The parts.... my otherwise useless USB hub (top and circuit board removed from enclosure), the 5V regulator (on top of its packaging).

I picked the 5V converter b/c it would accept 9-28V in (I was planning to give it 10-12V) and gives a rock-solid 5V out, with 2A continuous (the Dell Axim can take up to 1.5A. The standard LM7805 that many 5V sources use is limited to 0.5A and is not an efficient way to down-convert power.. the switching regulators can be >85% efficient). This is a ~$10 part, but you can get this, or equivalent parts from other suppliers as samples. You are not going to find this part in your local Radio Shack.

So the parts were free, but I decided it was easier to remove them from the board rather than hack the circuit board in place.. so I desoldered the USB connectors and the caps (use solder wick to remove the solder). (since I wanted different output voltages on each connector, and the circuit board was laid out to tie all those pins together, which would have been a pain to work around).

I also turned the converter upside down and soldered the 100uF cap directly to pins 2 & 3 (see the data sheet for recommended supporting components).

I then glued the USB connectors down to the board in the same place, upside down (what we call "deadbugging" (ie. bug on its back, feet up in the air)) which would make soldering easier. I put the TI part in the open space in the middle of the board (leaving room to place the 6V part later).

I also connected 12V to the top side of the LED pull-up resistor (it was there, why not add a power-is-connected sanity check?)
So now power it connected, the LED is on, the board fits back in its case with the USB connectors aligned to their holes (albeit they are upside down now). Time to make a quick check with the multimeter to verify I have produced +5V and not -5V.
Close it all back up...

Its hard to see, but the Axim is connected to the 5V output and the power applet says it is charging. Looks like a success.