Portable external batteries, or "power banks," as they are popularly called, are good for everyone, except for power output, and besides, they cannot charge laptops without USB Type-C and Power Delivery. You have to carry a charger with you, or even a replaceable battery. This did not suit me, so I decided to make the "bank" more powerful. And if the power allows – let the phones charge! Now I will tell you how to replicate this simple device.
I am a happy owner of an Asus K53 series laptop, which eats about 70 watts under load. Therefore, a wire with a 12V quick charge trigger and a boost board from Aliexpress will not help here – Quick Charge 3.0 quick charge gives a maximum of 12 V / 1.5 A (18 W) minus losses due to not 100% Converter efficiency. I don’t want to buy a more powerful “can” with Power Delivery. A set of replaceable batteries is not an option, a laptop with a charger already weighs more than 3 kg.
In short, I am an engineer with my mother, a brand new soldering iron with "Ali" came recently, there is a bag of 18650 cans in the box, and the BMS boards and the step-down module just arrived. It was decided to collect my battery, which I did, devoting three free evenings to this lesson.
I will briefly outline the requirements that I presented to my homemade product.
- Charging from a standard laptop PSU. Charging such a capacious battery via Micro-USB is masochism, there is not a single Type-C cable in the house, and it's not kosher to make a separate charger specifically for your homemade product, you want to be able to recharge from anything. The 220-volt socket requires a fairly large step-down and rectifier circuitry, which also needs cooling even when using a pulse transformer. Therefore, the input will be a standard 5.5 × 2.5 mm jack, where you can insert any charger with a voltage of 13 to 32 V.
- Large capacity for long battery life. Carrying a weighty thing with you that will sit down in twenty minutes is a mediocre pleasure. If you really do, then do it quite capacious, so that later you do not run your eyes around in search of an outlet.
- The output is a standard 5.5 x 2.5 mm plug. Of course, if you decide to repeat this homemade product and you have a different connector, it is easy to replace it with any other one.
- Button ‑ switch, for convenience and not to drain the battery charge by the idle current of the output converter and indicators.
- USB output with fast charging support for smartphone.
- High power output for normal laptop charging.
- If possible – adjustable output voltage for painless adjustment to another laptop if necessary.
Everything here is standard for this kind of gadgets. Battery cells themselves, BMS board, input buck and output boost converters, fast charge controller for USB output and consumables for the case.
With batteries, too, everything is traditional: lithium-ion cells of the size 18650 are firmly entrenched in the arsenal of homemade people and are not going to give up their positions. They are capacious, convenient for creating batteries, relatively inexpensive, and they can be collected for pennies (or even free) at service centers from dead laptop batteries. The capacity, in this case, leaves much to be desired, but it is an extremely affordable option for village rogues like me.
The BMS (Battery Management System) board is always needed if more than one cell is connected in series. The fact is that even batteries produced in the same batch have slightly different characteristics, in particular, capacity. When cells of different capacities are connected in series, they will be discharged unevenly, that is, at least one cell will go into overdischarge. And, as you know, overdischarge for lithium cans is not just bad, but very bad.
The opposite will happen when charging such a battery: less-capacious cells will charge faster, and further charging will lead to an uncontrolled combustion reaction, commonly called an explosion. Burning lithium batteries, scattering pieces of hellish unquenchable flame around, of course, look beautiful, but it's not safe, so let's not bring them to such a state, but just use the BMS.
Smart BMSs include a cell balancer and a protection circuit that will shut off the battery in case of overload, overcharge or overdischarge, and cost only a couple of bucks. However, you shouldn't save much on protection, so read the reviews before buying. There were boards in which the declared balancer did not work or was completely absent.
A downward DC-DC converter at the input is needed to obtain a stable operating voltage for charging batteries from an unknown (albeit obviously higher) input. Its important features are the presence of at least a small radiator, since all the input power will pass through it, and, optionally, the presence of a current limiter, so as not to kill the converter with a powerful current when charging dead batteries to zero.
The output DC-DC converter is what I decided to save on, and I bitterly regretted it, but more on that later. Its task is to provide the required stable output voltage from the voltage of our battery. Ideally, it should have protection against short circuits and overload, so that later you do not think about how to replace it. Well, at least a small radiator, from experience, will definitely not hurt.
Fast charging is a separate topic. As you know, it uses the data lines in the USB so that the device can negotiate the voltage and the charging current with the charger. The standard is simple, and you can even implement a charger yourself with its support, if your hands grow from the right place. But for me it was cheaper and easier to buy a ready-made board with a controller, the bonus received support not only for Quick Charge 3.0, but also for half a dozen other protocols.
Consumables are in the garage of any self-respecting techie. These are 5.5 × 2.5 sockets, wires, solder, plexiglass or plastic, a button, diodes and all that.
The socket, as you remember, has three contacts: one internal (+) and two external (-). I connected the external ones with a jumper to reduce the load. If you don't want problems, you should do the same.