A Power Case for a FOSS Phone

The Problem

I want to use a FOSS phone running Debian as my daily driver and not use a non-free OS on my phone. GrapheneOS [1] is a FOSS rebuild of Android, it’s a great project and I commend them for what they are doing. But I want a system where I have access to all the bits and where I can ssh in to it and manage it in the same way as all other Linux systems. Android is free software but it’s a closed design and the phones are expected to be appliances not full peers on the network. The Android native terminal emulator (AKA Linux development environment) is a nice feature, a VM with a default of 1GB of RAM but it’s still separate from the main OS.

KDE Connect [2] is a system for KDE talking to phones, it is a great project and allows convenient interaction between a Linux PC and a phone. But in addition to that I’d like the option to ssh to my phone to send an SMS or have one sent from a cron job. I blogged about using the basic functions of Kdeconnect from the command-line [3].

I have had ongoing issues with the battery not lasting long enough on the PinePhonePro (PPP) and the Librem 5 (L5) which makes them unusable for my purposes. For a phone running Droidian I can have an open ssh session (with keep-alive packets) running overnight on Wifi without any problems while for the PPP or L5 an hour of reading an ebook (the least demanding use of a phone) will use most of the battery.

I sent my FuriPhone back for warranty repair and it’s been almost a month with no follow up, so I need to have another option.

The Aim

The aim of this post is to develop a rough design for a battery case for either a L5 or a PPP, I have both phones and they are both capable of doing what I want apart from battery life. Also designing a case that basically works for both phones and has two variations of the CAD file for 3D printing is good to allow collaboration with people who use either phone. Protecting the phone from damage when dropped is also a core feature.

I investigated commercial options. There are some reports of cases for other phones working, someone reported a PPP working well with a battery case for a Samsung S20 Ultra. The OtterBox uniVERSE Case for Samsung Galaxy XCover Pro is reported to fit the PPP so probably battery cases for the XCover Pro will also work. There are also universal power cases which have spring clips to fit a wide range of phone sizes, but they are much larger than other options and probably increase the risk of damage if the phone is dropped.

At this stage I’m planning out the rough specs of a case that can be 3D printed to protect a phone while being easy to change the cells. The process of changing cells should be quick, easy, and not risk breaking anything. The hardware required should be affordable ($100AU plus some 3D printing seems reasonable) and should not require significant skills to assemble. After recent experiments with Thinkpad repair I’ve determined that I am not good at electronics by today’s standards so I plan to avoid anything difficult in that regard.

Battery Options

There are phone cases with batteries built in for the more common phones. A quick search on AliExpress turned up options for recent Pixel phones. They aren’t as common as they used to be as Android phones and iPhones generally have good battery life nowadays. Of course there aren’t such options for less common phones like the L5 and PPP.

It is cheap and easy to buy a portable battery for charging phones, but that’s a separate bulky device and unless the connector is inside a protective case it leaves the phone vulnerable to catastrophic damage if dropped.

So can a suitable battery case be designed and 3D printed?

Cheap Batteries

According to Wikipedia the 18650 LiIon cell is the most commonly used LiIon battery, that is 18mm in diameter and 65mm in length. For a phone case thinner batteries would be more convenient but economies of scale make the 18650 cheap to buy new and commonly available on the second hand market. The current prices on AliExpress (in Australian dollars) are about $8 per cell when buying 10 at a time. Most of the cells on AliExpress don’t have wires attached so they can be swapped in a carrier the way AA batteries are used, but the cells are symmetrical (no bump for positive) so the user has to take care of polarity. Chargers are around $22 for a 4 cell charger or $52 for a 12 cell charger.

I’ve seen prices as low as $1 per cell on the second hand market, I haven’t investigated the amount of usable capacity in such cells. Probably the prices for chargers I’ve found aren’t the best available but they are good enough to start the design. 10 cells and a 4 cell charger is about $100. That’s about 130Wh while a 74Wh USB-C battery pack costs about $50. So the price per Wh is OK.

I have read about an attempt to do this for the L5 with two of the batteries designed for the L5 in a case attached to it, so you have 3*L5 batteries. It’s an interesting approach but those batteries have poor value for money when compared to 18650 ($29US for 4500mAh compared to $8 for 3500mAh) and they also don’t ship them outside the US at this time. The 18650 batteries are available everywhere cheaply and have multiple uses.

How They Fit

According to my measurements the PPP is 76mm wide and the L5 is 73mm wide. That’s wide enough for 3*18mm or 4*18mm cells side by side arranged parallel to the long side of the phone or one 65mm long cell going across the phone with the necessary spring and wires.

The L5 is about 151mm long and the PPP is about 5mm longer. So that means that for just the cells we could have 2 cells in a row in parallel to the long side of the phone giving a 2*3 or 2*4 array or we could have a row of 8 cells parallel to the short side of the phone.

A quick search for controllers for a battery of LiIon cells that outputs USB-PD turned up one of the smaller ones as 40*28mm, here is the AliExpress page [4]. So that means we could have a maximum of about 4 cells and the controller while leaving space for the camera. Looking at other similar items it seems that most of them take a large portion of the 73mm width available and take a bit less than 30mm height. The price range for DC-DC voltage converters ranges from about $3 to $20, the one I linked to is currently $8.

The mass of 18650 cells is around 45g so 4 of them would be about 180g before counting the weight of the case and electronics. A L5 weighs 262g and a PPP weighs 220g so 180g of battery will make a significant difference, not an impossible difference but holding a PPP while reading an ebook is already annoying. Maybe the case could be designed to be easier to hold, a loop of wool attached to the top could be used to suspend the phone from one finger while the rest of the hand just keeps it steady.

In a default configuration the L5 has a 4500mAh 3.8V battery and the PPP has a 3000mAh 3.8V battery. The 18650 cells have up to 4000mAh with 3500mAh being common. So 4 such cells could multiply the battery life of a PPP by a factor of 5 or a L5 by 4. That would not be enough to last for a whole day without charging so they need to be easy to swap.

A 3 cell battery seems like a good option, 135g of batteries so the project overall wouldn’t even double the weight of the phone with battery case. As it doesn’t seem possible to put in enough cells to last a day of reasonable use (running a web browser, checking email, reading wikipedia, and having some IM systems checking for notifications) there doesn’t seem to be a benefit in trying to stuff the maximum number of cells in. Going to 4 or 5 cells doesn’t provide much benefit over 3 while increasing the weight and making the design more difficult. The modules for DC Voltage conversion often have configuration for the number of cells to be used so it wouldn’t be difficult to print a new case and reconfigure the Voltage converter.

Design

A case needs to provide protection as well as hold the batteries. The lack of cases for the L5 and PPP is a problem even without the extra weight of a battery pack increasing the kinetic energy of falling phone. Ideally a drop from 1M on to concrete would not cause any damage to the phone, destruction of the case is OK as it would be 3D printed and can be rebuilt easily.

I think that a case where the phone slides in from the top would be best. To change cells one would slide the phone out and the cells would be immediately accessible with no divider between the phone and the cells. Sliding the phone in would hold the cells against the back of the case and as there is a gap of about 3mm on each side of the PPP between the display area and the edge of the phone there’s plenty of space for the case to wrap around it at the sides and bottom. At the top there would have to be some sort of plastic clip. The PPP only has buttons on the top right so the case can be snug on all sides apart from the top right. The L5 has switches at the top left and buttons at the top right so whatever clips on to the top would really need some strength to cover the fall flat on face case.

The USB C plug would need to be held in place well enough to allow the phone to easily slide on to it but also be weak enough that it would move if a drop forced the phone out of the case.

Thingiverse has a design for a L5 case [5] (with several derivatives) and a design for a PPP case [6]. Those cases could be used as starting points and then changed to store batteries and the voltage change circuit board. I have just learned that it’s possible to use rubberised material in a 3D printer which is apparently what those designs are for. If a case was printed with rubberised material the top could just stick in place after being pushed in.

Potential for Excessive Excitement

Having an exploding device in your pocket would be the exciting in a bad way. My initial idea was to have multiple 18650 cells in series. The problem is that if the cells have different capacity levels then one can run out before the rest and get to a deep discharge state which at a minimum causes damage to the cell.

One way of minimising risk is to have only a single cell in use at one time, here is a converter for a single cell to 5V [7]. It is possible to run multiple cells in parallel by simply wiring them together, but it’s recommended to make sure that the voltage difference between cells is less than 0.2V to avoid high current when connecting.

The arrangement of cells is often referred to as nS or nP to refer to n cells in series or parallel. Aliexpress has a range of 1S to 6S devices. Some of the 2S devices have a connector for the mid point which permits separate voltage monitoring for both cells but there doesn’t seem to be anything equivalent for 3S or more. They also don’t appear to have anything to specifically manage 2P or 3P arrangements at the moment (they had a 3P board on sale when I looked into this last year).

Conclusion

This is a bigger project than I expected when I first started investigating it. I’ve played with FreeCAD which seems OK but will take a lot of learning and practice. Then I need to do more investigation into the DC-DC converters to find one that works well and is unlikely to start a fire.

I will look into other phone options as well and keep chasing Furilabs people about my phone replacement.

• [1] https://grapheneos.org/
• [2] https://kdeconnect.kde.org/
• [3] https://etbe.coker.com.au/2026/10/06/kdeconnect-dbus/
• [4] https://www.aliexpress.com/item/1005009329156591.html
• [5] https://www.thingiverse.com/thing:4961260
• [6] https://www.thingiverse.com/thing:4933237
• [7] https://www.aliexpress.com/item/1005007555665949.html