Rugged Laptop Battery Pack

This article is the first of a series about the gadgets I use on the go, one year after I started to work remotely.

Motivation

The first thing I missed when starting the day on a café terrasse or on a train was a reliable source of power. Lowering the luminosity and selecting tasks that were not power hungry became annoying.

I wanted a battery pack:

• Able to charge my laptop and phone for at least one day of intensive usage.
• Rugged enough to be thrown in a backpack and used while being carried.
• Mains powered, with no adapter needed.

It actually took me some time to get started because I thought that the USB-C charging interface for the laptop would take time to design. When finally getting into it, I realized that off-the-shelf parts could do the job.

All the battery based USB-C chargers reference designs I found were targeting car adapters. I could just wire something with a cigarette lighter output and use a car adapter to power whatever I would like later!

Building

All the components could be used without being disassembled:

Which still looks impressive in the enclosure, I wouldn’t bring it to the airport 😅.

Battery

The main component is the battery. It is also the heavier, and I guess the most expensive. I salvaged mine from a batch of samples used to develop the Keecker robot. It is a 7S5P 264 Wh Li-ion battery, meaning it is expected to deliver 264 W during an hour, or 1 W for 264 hours!

This is six times the capacity of my laptop’s battery. According to the 12-inch MacBook Environmental Report, it consumes about 7 W with the display on. At best, this battery would provide 37 hours of usage.

It also means that I cannot carry the pack in a plane. For now the maximum allowed limit is around 100Wh…

This particular one was made by BMZ, I think it is pretty similar as bike or scooter ones. It already embeds a Battery Management System (BMS) and a fuse. The two-wire SMBUS allows to get data about the status and the health of the battery, like remaining capacity, voltage or output current. I do not use it in the project for now.

If you want to make a similar project with a Li-Ion battery, I highly recommend using a certified battery embedding a BMS, intended to be a replacement for a consumer electronics product. It will be more expensive, it will self-discharge faster and will be bulkier, but you will not fear a huge hazard while wiring it, charging it or carrying it. I did actually blow a fuse while soldering a connector on a similar battery at Keecker, it happens quite easily!

Power Adapter

I only found one matching my requirements:

• Suited for a 7S Li-ion battery (29.4 V output)
• Maximum output power above 30 W, allowing to use the battery pack as my laptop power adapter (note that more powerful laptops can consume up to 90 W)
• A plastic enclosure, preventing the mains to eventually get connected to the output
• Work at 110 V and 240 V to be able to use in other countries
• Expose a C8 connector to use in other countries (for now I found C7 cables in all of the places I went).
• Fit in the case and the battery I already had.

I guess you should look for an adapter, then a case to have more options…

Cigarette Lighter Socket and Adapters

I did not find any specification about what voltage to expect from a car power outlet. The nominal voltage is 12 V but it looks like the actual voltage can vary between 9 V and 15 V depending on the battery state of charge. The most restrictive adapter I have is the USB-C charger which accepts from 12 V to 18 V as input and consumes up to 60 W.

As my 7S Li-ion battery voltage was too high (19.6 V to 29.4 V), I had to use a step down conveter. It wastes a significant part of the battery power (not measured yet, around 15 % I guess).

A more efficient option would be to use a 4S Li-ion battery (ranging from 11.2 V to 16.8 V), directly connected to the output.

Case

Pelican cases are often used to carry sensitive equipment. They exist in a variety of sizes and can be moded easily. I managed to pack everything in a Pelican 1120 that I had from an older project!

Usability

I am very satisfied of it! I take the battery pack every time I plan to work remotely. As it weights 2.5 kg, it usually stays at the bottom of a backpack, output facing up. That way I can plug both my laptop and phone without making a mess on the table.

The main drawback is that I need to keep a way to access the bottom of the bag when it is packed. Since the mains input connector is not on the same side as the output, I cannot charge the pack while using it inside the bag.

At first I was worried about the cables being frequently disconnected or even damaged when the bag was more packed. No problem to date, even when using it to charge my phone while carrying the bag.

When charging and using the pack at the same time, the temperature inside the enclosure barely warms up. I guess the components are pretty efficient but I still haven’t properly measured that efficiency and the autonomy of the setup. I never actually ran out of power…

Improvements

Improvements for eventual next revisions:

• Cleanup the wiring.
• Get all connectors on the same side of the enclosure, allowing the pack to be used and charged at the same time more easily.
• Use a battery with a nominal voltage directly compatible with a car adapter, like a 4S Li-ion battery.
• Use some kind of holder for 18650 cells instead of a complete battery pack. It would allow to replace weak cells, or change the capacity depending on intended usage.

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