Lithium-ion batteries’ means to ship a whole lot of energy from a small package deal have made them the go-to for makers and producers alike. It’s commonplace now to search out, say, microcontroller boards with integrated Li-ion chargers. Lithium-ion is so popular, in truth, that it’s simple to overlook that different battery applied sciences exist, even once they’re a greater match.
These worthy options embody detachable rechargeable nickel–metal hydride batteries. Whereas NiMH cells can’t be recharged as many instances as lithium-ion cells can and don’t supply the identical power density, they’re cheaper and in addition safer. No have to ship them in packing containers emblazoned with fire warning labels. The truth that NiMH cells ship decrease voltages than lithium has turn into much less of a problem because the voltage calls for of integrated circuits have fallen, with 3.3-volt and 1.8-V chips quickly displacing the ever-present 5-V customary of yesteryear.
Learn how to Make NiMH Simpler to Handle
A handful of 3D-printed components [bottom], a servo and display screen [middle] and a single printed circuit board [top] are all that’s wanted for the Spinc charger. James Provost
But it surely’s additionally true that recharging detachable batteries is usually a ache: You must load them right into a charger, which usually holds not more than 4 batteries at a time, and take care to place them in the best method. In any other case you get no charging at finest and irreversible harm to the cell and even overheating at worst.
To ease this ache level, I created the Spinc, a DIY system that expenses as much as seven NiMH AA batteries at a time and robotically figures out the polarity of every cell earlier than charging it; when completed, it drops the batteries right into a hopper. You’ll be able to verify on the charging standing through a show that doubles as a clock.
In my day job, I work on industrial autos as an EE in a midsize German firm. However I got here to this challenge as a result of a private curiosity in low-power electronics and after a fruitless try to reap the final dregs of energy from some nonrechargeable batteries. That failure—in a nutshell, my design wanted a buffer battery that needed to be recharged, which defeated the entire goal of the challenge—obtained me fascinated by rechargeable cells.
The toughest a part of creating the Spinc was a self-imposed problem. I wished the charger to be compact and intuitive to make use of. This meant spending a whole lot of time perfecting the mechanism that takes a battery from the highest of the charger, locations and holds it between two electrodes whereas charging, after which drops the battery out the underside earlier than resetting and grabbing the subsequent cell. Plenty of cautious iterations later, I had a set of seven 3D-printer information to create the components that assemble to kind the case and mechanism of the Spinc. To that, you simply want so as to add the show, servo motor, and a printed circuit board with all of the remaining parts, plus an infrared proximity sensor that detects when a battery has been inserted and is able to be positioned between the charging electrodes.
To permit the battery to be charged no matter which method it’s put into the charger, I used a basic H-bridge circuit, which is often used to let DC motors run in both course, with a number of modifications that permit it work at low voltages.
I made a decision to make use of a devoted built-in circuit to handle the precise charging, with thermistors to be careful for overheating. Whereas I may have used a microcontroller and written my very own software program to watch the battery, NiMH cells have a really flat charging curve, and it’s simple to overshoot the charging cycle. Utilizing an IC saved me from a whole lot of testing, and in addition gave me the power to make use of a fast-charging mode.
Nevertheless, it didn’t save me from some trial and error. The primary charger IC I used was attractively compact at 2 by 2 millimeters. Nevertheless, once I tried charging an AA battery, I found that the producer assumed the tip consumer could be charging two to 4 batteries at a time, all connected in collection. Charging only a single battery meant the IC’s linear voltage regulator needed to dissipate extra warmth than meant, and it rapidly burned the chip out. I ultimately discovered a considerably bigger (5.15 by 4.4 mm) charger IC that makes use of a switched-mode regulator and will deal with the job. Discovering that IC took some time, although, as the recognition of lithium batteries has lowered the provision of supporting parts for NiMH.
The charging curve of NiMH follows a gradual curve, with the battery’s voltage exhibiting a dip of simply 2 millivolts when absolutely charged, which is well overshot and will result in cell harm.James Provost
The charger IC is linked to an RP2040 microcontroller, which experiences the battery standing on an LCD show. The RP2040 can be chargeable for monitoring the proximity sensor and driving the servo that controls the inner feed mechanism, in addition to responding to the Spinc’s push-button controls, which allow you to set the clock and begin charging the batteries. All the system is powered through a USB-C socket.
Many of the electronics are surface-mounted on a PCB that has a big heart cutout to permit the batteries to go by means of it. I designed the PCB in KiCad and had it fabricated by JLCPCB. To make it simpler to manufacture the boards accurately, I didn’t attempt to specify 90-degree angles for the corners of the middle cutout. As an alternative, I went with rounded corners, that are simpler for the reducing tooling to comply with.
The result’s a desktop charger that’s as handy to make use of as attainable. A full set of PCB schematics, invoice of supplies, 3D printer information, and firmware information can be found from the project page on GitHub. I hope it helps be certain that everytime you want a freshly charged NiMH battery, there’s one proper at hand!
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