OP-Z Battery Replacement

This includes two ways to replace a teenage enginering OP-Z battery. The 3D printed version was developed fairly quickly with a tight deadline, so I was unable to fully refine it, but the end result works fine even though it looks a bit messy. The non-3D-printed version is much easier and requires fewer tools, but I was only able to find a significantly smaller battery that still fit, so it only has half the battery life.

There are Amazon US links to the less-common parts I used: these are not affiliate links, and I do not make anything from them. Replacements are almost certainly fine, as long as everything fits.

Do you need to replace it?

teenage engineering recommends that batteries that have been entirely depleted for some time may be able to be "kickstarted" by following these steps from their support page:

1. Connect OP–Z to a computer or usb charger, using the usb-c cable, and power it on.
2. While still connected through usb-c, turn the unit over, loosen the four yellow screws and remove the back panel.
3. Remove the battery from the OP–Z.
4. Press the screen index button ▭ on OP–Z.
5. A red led will light up, confirming no battery is present. this will reset the battery indicator.
6. Turn OP–Z off and reinsert the battery back in the unit.
7. Turn OP–Z on and you should be good to go. seal the back panel and charge the battery.
8. Repeat several times if it doesn't work the first time.

If that doesn't work and you are comfortable using a power supply to try "waking up" the battery, some people have reported success forcing a small amount of power (say, 4-5V) into the battery for a short time. Be careful when doing so, as doing it too much or for too long could also cause your battery to overheat, puff up, or potentially catch fire. The positive battery contact is nearest to the corner, and the negative is farthest from it. You do not need the middle contact to try to wake up the battery.

If either of those work, and you're happy with the battery life, you're done! If not, read on.

Safety

Working with batteries directly can be slightly dangerous, I cannot be held liable for battery fires or other damage. With that said, nothing described here should be particularly dangerous if you are careful to not short out or puncture the batteries, and only use batteries that have their own protection circuits (such as the ones I link to).

I would recommend working on a non-conductive surface that will not burn easily: I used a silicone mat for working with electronics and soldering, but a cutting board is fine.

While this replacement is relatively simple, you should know what to do if something goes wrong. If you puncture a battery or it starts smoking, if you feel you can safely move it outside (for example, by grabbing the board and quickly walking it to a driveway or other location unlikely to catch fire), you should do so. Otherwise, a home fire extinguisher rated for Class B fires (including a common household "ABC dry chemical" fire extinguisher) should put out any resulting fire. Do not use water to attempt to put out a lithium-ion battery fire. You should avoid breathing the smoke from lithium-ion battery fires as it is toxic, but a small whiff is not fatal.

You should pay attention to the first full charge of your replaced battery until the OP-Z indicates it is fully charged (the charging light will turn solid green and stop blinking): if the battery is flawed, it is most likely to have a problem while charging, and most likely to have a charging issue the first time it is charged. This is very uncommon, but it is better to be safe than sorry.

The original battery

There is an official replacement that is theoretically available at iFixit, but it is expensive ($60!) and has been out of stock for quite some time.

The original OP-Z battery is rated as a 740mAh / 2.74Wh battery. It is a plastic rectangle with stress reliefs on the corners and small metal plates on the top and bottom, wrapped in a sticker.

Inside is a small safety circuit with a 10k ohm thermistor built in to the terminal side of the frame, and a single 3.7V lithium-ion battery, marked as size 382758 (that is, 3.8mm height, 27mm depth, and 58mm width). The cell inside my battery was manufactured by EVE.

Method 1: Reusing the case (no printing, easy soldering)

If you have a fully dead OP-Z battery, you can reuse the existing battery frame to drop in a smaller battery and get a pretty easy-to-build battery. You'll need some wire cutters and/or wire strippers and a soldering iron, but the targets are big and forgiving.

Supplies:

1. Wire cutters
2. Wire strippers, if you're not comfortable stripping wires with a cutter
3. Soldering iron and a small amount of solder
4. Some thin, non-conductive tape (packing tape, gaffer's tape, anything that will last)
5. A new battery (small)
6. Connector compatible with the battery you purchased (optional, but recommended to allow the inner battery to be swapped)

You will need a relatively small battery to fit it in the existing frame. I was not able to find a 382758 battery, so I chose a 320mAh 402535 battery: this will give you only about half the battery life, but it works and it fits easily.

These steps are quite lengthy, but the full replacement can easily be done in half an hour if you have all the parts.

Steps:

1. Peel the existing sticker off of the existing battery: you can discard it.
2. Pry the metal sheets off the top and bottom of the battery: these have double-sided tape holding them to the battery cell, but you can safely tug against them. (Be very careful to not puncture the battery cell if you use anything to pry them up: use something like a credit card rather than a screwdriver.) Again, you can discard them. If your existing battery is "puffed up" (rounded), be particularly careful when peeling the metal off, as it has already been damaged: this may be safest to do outside, or avoid entirely and use "Method 2" below.
3. Gently pull the existing battery cell (the big grey thing) away from the short terminal side of the frame. We're just making some room, it is still attached.
4. Use your wire cutters to cut the existing battery cell's tabs off and remove it from the frame. You want to cut relatively close to the battery cell, just to give yourself extra room to solder to.
5. Tape over the old battery cell where you cut the tabs: you want to avoid letting the battery short out against anything conductive.
6. If you are using a connector, plug it in to the new battery cell and put everything into the frame to check the fit. You may wish to cut the leads from the connector down to make additional space. If you are not using a connector, pull the two wires going to the existing connector apart a bit to separate them about an inch, and cut off the existing connector.
7. Regardless of whether you are using a connector or not, you should now have two wires, a red and a black one, ready to connect to the frame. Be careful: these wires are "live", and you do not want to let them touch, as that will short out the battery, which will make it heat up and may lead to a fire. With that in mind, strip off a small amount of insulation from each wire: a few millimeters should be sufficient.
8. Solder the wires to the battery frame. The positive (red) wire should be soldered to the tab closest to the contacts on the outside of the frame, while the negative (black) wire should be soldered to the tab further from the contacts. See the pictures if you aren't sure: you don't want to flip those!
9. Test the battery in your OP-Z to ensure that it turns on. If it doesn't, double-check the wiring.
10. Wrap the frame in tape to hold everything in place. I would suggest one full wrap with a bit of overlap, but the frame already fits tightly, so extra tape may make it even more tight or not fit at all. Be careful not to tightly tape over the corners on the contact side of the battery, as they are keyed to fit the OP-Z.

After that, you're all done! Enjoy your working battery.

Method 2: Printing a new case (bigger battery, more fiddly)

If you want a bigger battery, or simply a backup, you can 3D print a replacement case and wire it up yourself. This is a bit trickier both because you need to have something for the OP-Z pogo pins to touch, and because you need to wire up a thermistor (or replacement).

In this case, the batteries I was able to find are not bare cells: they have their own protection circuitry. (That's the circuit board you can see under the amber-colored tape in the pictures.) As a result, I was not worried about putting a proper thermistor in: the thermistor is used by the OP-Z to ensure it's not charging the battery too fast, but the replacement battery will take care of that on its own. As a result, I just used a plain old resistor to make the OP-Z think things were at a normal temperature, and pass charging off to the actual battery. If you prefer, you can use an actual thermistor instead.

Supplies:

1. Wire cutters
2. Wire strippers, if you're not comfortable stripping wires with a cutter
3. Soldering iron and a small amount of solder
4. Some thin, non-conductive tape (packing tape, gaffer's tape, anything that will last)
5. Thin conductive metal, at least 4mm wide - I used copper tape (you could use wire instead, with some minor modifications to the 3D print)
6. A 10k ohm resistor (8-12k is likely fine - I used 12k) with wire leads
7. A new battery (slightly larger than the one above, nearly double the capacity)
8. Connector compatible with the battery you purchased (optional, but recommended to allow the inner battery to be swapped)
9. Hot glue (highly recommended) or some other non-conductive way to keep things in place (such as epoxy).

I used a 453048, 710mAh battery, which should give almost the same battery life as the original, with maybe $7-8 in parts.

Steps:

NOTE: The majority of pictures were taken before I realized the resistor was necessary for the OP-Z to charge the battery, and therefore show a longer piece of copper tape on the negative lead: yours should be about 2-3mm shorter, and should not cover the whole "channel" that it could fit in. If you accidentally make it too long, you can simply push the excess towards the bend with a pin or tweezers.

1. Print the 3D model. I used PLA, a 0.2mm layer height, and supports on the baseplate only. The STL file is oriented upside down: print it with the large flat side on the bottom. I also used a 0.6mm nozzle, but a 0.4mm nozzle might be cleaner. You do not need a perfect print as long as everything mostly fits: the short side for the terminals is the important part.
2. If you are using a connector, plug it in to the new battery cell and put everything into the frame to check the fit. You may wish to cut the leads from the connector down to make additional space. If you are not using a connector, pull the two wires going to the existing connector apart a bit to separate them about an inch, and cut off the existing connector.
3. Regardless of whether you are using a connector or not, you should now have two wires, a red and a black one, ready to connect to the frame. Be careful: these wires are "live", and you do not want to let them touch, as that will short out the battery, which will make it heat up and may lead to a fire. With that in mind, strip off a small amount of insulation from each wire: a few millimeters should be sufficient.
4. Cut two small pieces of copper tape, leaving the backing on. One piece should be 4x8mm, and one piece should be 4x6mm. (In the pictures, I had two 4x8mm pieces: if you cut them too big, you can probably use a pin to push them into the right spot, just make sure nothing shorts out.)
5. Lightly dent the copper tape 3mm from the end (I used my calipers, but a pen/pencil would do), so you have a 4x3mm rectangle as a target. At this point, you should have approximately the parts in picture A.
6. Solder the positive (red) lead to the 4x8mm strip of copper, inside the small rectangle you marked. You likely want to do this with the wire facing away from the rest of the copper tape, see picture B.
7. Similarly, solder the negative (black) lead to the 4x6mm strip of copper, inside the small rectangle. You likely want it oriented the same direction.
8. Bend the copper tape so that it will fit in a "U" type shape around the slots in the 3D print: bend it at the 3mm mark you made already, then again about another 2mm along, and you should be set.
9. Remove the tape backing and slide each piece in to the 3D print: this may be best accomplished using tweezers or a pin to position things correctly. The positive (red) tape should go in the corner, with the negative (black) tape further out. The black tape should only cover about half of the exposed area along the side of the battery. At this point, you should have something like picture B.
10. Apply a little hot glue to the wires where they are soldered, to keep them from moving: we want to make sure they can't touch and short out the battery. Avoid building up the hot glue above the height of the print, and if you expect to be able to swap out the battery, ensure you're only gluing down the wires from the connector, not gluing the battery in place.
11. This is the least-developed part of the print, as it came late in the design and I was rushed for time. Take your resistor and bend the leads so they look somewhat like the final product picture. (Resistors are not polarized: both sides are the same.) In particular, you want them to be a few millimeters apart, and long enough that the resistor can fit comfortably in the gap in the battery. Bend the ends so they can stick straight down into the sides of the frame (about 4mm long), and trim them using your wire cutter.
12. Stick one lead of the resistor into the corner of the bend by the negative lead's copper tape. This is just to keep it in place and in contact with the negative terminal from the battery. (You could alternatively solder it to the internal copper connector alongside the lead from the battery.)
13. Stick the other lead from the resistor on the other side of the channel by the negative lead. You want it to be positioned so that the middle pogo pin from the OP-Z will touch it, and it should not be touching the copper for the negative lead. (If it is, use a pin to reposition the copper tape, shoving it should be fine.) If they touch, no harm will occur to the battery, but the OP-Z will fail to charge the battery. If the lead is not able to be reached by the middle pogo pin from the OP-Z, the OP-Z will not recognize the battery, and will blink red instead of green when plugged in to power.
14. Optionally, wrap the new battery in tape, to keep things from falling out. (This is less critical than for the other battery, as the 3D print forms a tray to hold pieces in when removing the battery.)

And that's it. Somewhat laborious, and looks a bit sketchy, but it should work fine.

Improvements

Future refinements (please feel free to take these ideas!) would include a channel along the top of the 3D print for the resistor leads, small indents where the pogo pins press to guide the pins and resistor leads, and potentially a different mechanism for connecting to the pogo pins (channels for tinned / solid-core wire, as well as for the resistor? A thinner side and "windows" for pieces of conductive foil?).

The FreeCAD file for the battery frame is included. This text and the associated files are licensed CC BY 4.0. Please do not sell them without talking to me first.