Rebuilding a Duo Battery

First, a warning, this procedure will require skill with a soldering iron, prior knowledge of simple electronics and a safe place to store small parts. Furthermore, be prepared to have your Duo battery out of action for a few days whilst you do this work, and take plenty of rest breaks. Ideally, you should read through this guide first before doing anything and drop me a line if you want more info.

Electrical short-ciruits from batteries can generate large currents, and possibly, high temperatures - they can start fires and cause burns. If you open and modify your Duo battery then I assume no responsibility for any data loss or damage that may arise from your use of this document. Note, this guide applies to a type II NiMH battery, the other types of Duo battery may differ internally and the instructions may not be fully applicable.

Opening the battery

The case of the battery can be opened with a sharp knife. I used a surgical scalpel to make multiple deep cuts along the seams along the three accessible faces of the battery case wall, as shown below. Make many light passes with the knife, rather than trying to slice your way through in one go.

Take particular care when cutting along the face that houses the five recessed electrical contacts because a membrane circuit lurks but a millimetre or so behind the plastic, and it is easily cut and damaged. This is exactly what I managed to do. (sigh)

With the three case sides separated, begin to flex the case until the cells can be released from within the battery case. Then bend the case open (by 10mm) and closed till the remaining face, which is covered by the sliding latch, fails through fatigue along its glue line. You should now have a cleanly snapped case, a set of cells wired together.

Now, with any luck, you will not have sliced across the membrane circuit alluded to earlier. If that is the case, then lucky you. Skip the next part.


That wasn't smart was it? So you've damaged the all-important connections between the cells and the connectors...

Ho hum. Have a good rummage in your Box of Useful Materials - you are looking for some sheet brass or copper.

Stare hard at the guts of the duo battery - the various components from a Type II battery have the following layout.

If, like I, you've completely severed the contact pads from the foil circuit, you'll need to reattach them to a brand new circuit using the original components. You could try to repair the cut tracks if you like, but the currents involved (100s of mA) are sufficiently high for me to doubt whether conductive paint would be of any use. So I chose the 'back-to-basics' approach and remade the whole circuit. By the way, if you are intrigued as to what the transistor-like component is, have a look at this link to the Maxim catalogue. Well, it impressed me - it's a dedicated 64 bit serial code store that allows the battery charging circuit to know what type of battery it is talking to.
(Thanks Genki!)

From a convenient sheet of brass (say, 100 micron thickness) I snipped out five links to take the place of the printed wiring on the flexible circuit. I trimmed them to leave a fair amount of space between the tracks - there's no point in trying to copy the original tracks exactly, as the original design had little clearance between tracks. There ought to be no problems if you use a more generous spacing. After some time you may have something that looks like this:

Now have a good rummage around on the Net to find Nickle-metal hydride (NiMH) "4/5 A" cells with the right price and charge capacity for your needs. Each cell has a terminal voltage of 1.2V and you will need to buy 10 of them to complete one Type II battery.

I found a company in the US (Advanced Battery Systems) who supplied tagged Sanyo 2150 mAh cells, which ought to put the finished battery in the Type III-and-a-bit category. Now, if you've actually read this far, you'll realise that perhaps the smartest thing to do first is to order the cells, and then to start slicing your battery case open. But hindsight is a marvellous thing and till I had the case open I wasn't sure of the type of cell needed.

Before the cells arrived I made some measurements of the current drawn by the Duo in various operating states so as to get some appreciation of what lifespan the replacement cells may offer.

"How long?"

A Duo 250, when powered from an external power supply with the Duo's battery withdrawn, will draw the following current at various driving voltages. As you can see, start-up is a power-hungry process, and once booted the Duo will run with lower applied voltages - interesting, but of not much use to the battery refurbisher.

Voltage provided (V)Current drawn (mA)Comment
12.005Will not turn on
12.511Will not turn on - screen flashes
13.025Will not turn on - screen does not flash as fast
13.529Will not turn on - no screen flashing
14.030Will not turn on
14.1520Boots (mean current while booting = 500 mA)

Of more interest is the current drain under particular conditions. I don't know what bus voltage the DC-DC convertor delivers, so cannot directly measure the current when operating from the battery pack alone, but when powered from an external power supply the Duo draws the following currents.

ConditionCurrent drain (mA)
Minimum drain at 15V (backlight off disk spun down)130
Maximum drain at 15V (backlight max, disk spinning up)740 ish peak
Disk spinning at 15 V drive voltage + screen backlight off200
Disk spinning at 15 V drive voltage + screen backlight on260

If we assume a 100% efficient converter (ha!) then with a 12 V supply (ie the battery pack) the currents would be 163mA, 930 mA, 250 mA, and 325 mA respectively.

Let's assume that the average current drain at 12V is then around 200mA, then the 2150 mAh Sanyo cells ought to give around 10 hours of operation.


More likely, we ought to have about 5 hours of operation.

"Make Room! Make Room!"

(time passes) The cells arrived, safe and sound, and after a few hours work, I had them soldered up into the circuit shown earlier, and sitting fairly comfortably in the battery case halves. I put a thin coating of solder over the pads to raise their mating surface a little (that's why they're silvery in a following image) as there's a known problem with Duo batteries making less-than 100% firm contact with the charging prongs inside the Powerbook. The next image shows the cells without the circuitry from the three middle pads.

It pays to take care when making your joints between the cells, pre-bend the tags to minimize the space that will result between the cells and use a minimum of solder. There should be an absolute minimum of 'play' between the cells when you've finished, otherwise the case halves simply won't fit together again.

Now, as I had to remake the foil circuit, I had to solder the contact pads to the brass strips and ensure that they were correctly positioned so that they protruded through the five holes in the case - with any luck you'll not need to click on the following for clues.

We're nearly there now. With a dab of your favoured epoxy or impact adhesive, glue the strips of the new circuit (if you've damaged the original) to a thin sheet of card so that the relative positions of the cells and contact pads allow the pads to rest without strain in the case's contact holes. Place the centre three contact pads and their retaining strip of card into the appropriate case half, and fit the cells in place. I used a spot of silicone elastomer to stick the thermistor and cells in place.

If, like I, you doubt your work sufficiently, you'll want to run five wires from the Duo (unscrew and angle-up the keyboard) to the corresponding pads of the battery with its case still open. Then, you can watch and feel the cells charging, noting if anything is getting overly warm.

Okay - the next step is to fully charge and discharge the battery a few times, in doing this I used Jeremy Kezer's Battery Amnesia, and after the initial charge of only 1 hour, I was rewarded with an Amnesia discharge time of over 100 (screen dump) minutes! In contrast, my original battery managed about 5 minutes (hand drawn red line in link in last sentence) when being drained by Battery Amnesia before being exhausted.

So - it works, and I've now got a battery that is at least twenty times better than the one which would give me about 10 minutes of real-life use. Once the cells have been 'run-in' (charge/discharge a few times) I believe that I'll have many many hours of real-life use, maybe not 10, but certainly more than 3.

Postscript from 2006. I sold my Duo many years ago, it did indeed give me around 3 hours of real-life use (mostly typing) and I've made a PDF of this page with the large images embedded. Download it from the icon below.