Lithium-ion The batteries' ability to provide more power in a small package has made them popular among manufacturers and manufacturers. Nowadays you can often find, say, microcontroller boards with built-in chargers for lithium-ion batteries. Lithium-ion so popularIn fact, it's easy to forget that other battery technologies exist, even if they are better suited.
These worthy alternatives include removable batteries. nickel metal hydride batteries. While NiMH cells cannot be charged as many times as lithium-ion cells, they do not provide the same effect. power densitythey are cheaper and safer. No need to send them in boxes decorated fire warning signs. The fact that NiMH cells produce lower voltages than lithium cells has become less of a problem since the voltage requires integrated circuits fell: 3.3 V and 1.8 V chips quickly replaced the ubiquitous 5 V standard of yesteryear.
How to Simplify NiMH Management
Several 3D printed parts [bottom]servo and screen [middle] and one printed circuit board [top] That's all there is to the Spinc charger. James Provost
But it's also true that recharging removable batteries can be cumbersome: you'll have to load them into the charger, which usually holds no more than four batteries at a time, and take care to seat them correctly. Otherwise, you will get, at best, no charging, and at worst, irreversible damage to the battery, and in the worst case, even overheating.
To alleviate this problem I created Spinc, DIY a device that charges up to seven NiMH AA batteries simultaneously and automatically detects the polarity of each cell before charging it; when completed, it dumps the batteries into a bunker. You can check the charging status through the display, which functions as a clock.
In my main job, I work on electrical equipment for industrial vehicles in a medium-sized German company. But I came to this project because of my personal interest in low-power electronics and after unsuccessful attempts to collect the last remaining energy from non-rechargeable batteries. This failure (in short, my design required a buffer battery that had to be recharged, defeating the whole purpose of the design) got me thinking about rechargeable cells.
The hardest part of creating Spinc was the volunteer task. I wanted the charger to be compact and intuitive to use. This meant spending a lot of time perfecting the mechanism that takes the battery from the top of the charger, places and holds it between two electrodes while charging, and then ejects the battery from below before dumping and grabbing the next cell. After many careful iterations, I had a set of seven 3D printer files to create the parts that assemble into the Spinc body and mechanism. To this you just need to add the display, servo motor and PCB with all the other components as well infrared proximity sensor which detects when the battery is inserted and ready to be placed between the charging electrodes.
To ensure that the battery could charge no matter which way it was inserted into the charger, I used a classic H-bridge circuitwhich is usually used to allow DC motors work in any direction, with some modifications allowing operation at low voltage.
I decided to use a custom integrated circuit to control the actual charging. thermistors watch for overheating. While I could use a microcontroller and write my own software to monitor the battery, NiMH cells have a very flat charge curve and it's easy to skip the charge cycle. Using the chip saved me a lot of testing and also gave me the opportunity to use fast charging mode.
However, this did not save me from trial and error. The first charger chip I used was an attractively compact 2 by 2 millimeters. However, when I tried to charge the AA battery, I discovered that the manufacturer intended the end user to charge two to four batteries at a time, connected in series. Charging just one battery meant that the IC's linear voltage regulator had to dissipate more heat than intended, and this quickly burned the IC. In the end I found a slightly larger one (5.15 by 4.4 mm) charger IC which uses switching regulator and could do the job. However, it took some time for this chip to be discovered due to the popularity lithium batteries has reduced the availability of ancillary components for NiMH.
The NiMH charging curve has a slow curve: the battery voltage drops by only 2 millivolts when fully charged, which is easy to exceed and can damage the cell.James Provost
The charger chip is connected to RP2040 microcontroller that reports battery status to LCD display display. The RP2040 is also responsible for monitoring the proximity sensor and controlling the servo that controls the internal feed mechanism, as well as responding to the Spinc push-button controls that allow you to set the clock and begin charging the batteries. The entire device is powered via USB-C.
Most of the electronics are mounted on a circuit board with a large cutout in the center through which the batteries pass. I designed the PCB in Kikad and ordered it to be made JLCPCB. To make it easier to fabricate the boards correctly, I didn't try to specify 90 degree angles for the corners of the center cutout. Instead, I chose rounded corners that are easier for the cutting tool to follow.
The result is a desktop charger that is as easy to use as possible. Complete set of PCB schematics, specification, 3D printer files and firmware files available from project page on GitHub. I hope this helps ensure that whenever you need a freshly charged NiMH battery, you'll always have one on hand!
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