A DIY Charger That Makes NiMH Batteries Easier to Use

The ability of lithium-ion batteries to deliver a lot of power from a small package has made them a favorite for both makers and manufacturers. It is no longer unusual to find, say, microcontroller boards Integrated Li-ion chargerIn fact, lithium-ion is so popular that it’s easy to forget that other battery technologies exist, even if they are better suited.

These worthy options include removable rechargeable nickel-metal hydride Batteries. While NiMH cells cannot be recharged as often as lithium-ion cells and do not provide the same power density, they are also cheaper and safer. They do not need to be shipped in boxes labeled fire warning. The fact that NiMH cells provide lower voltage than lithium is no longer a problem as the voltage demands of integrated circuits have fallen, with 3.3-volt and 1.8-V chips rapidly displacing last year’s ubiquitous 5-V standard.

How to make NIMH easier to manage

A handful of 3D-printed parts (bottom), a servo and screen (middle) and a printed circuit board (top) are all that is needed for the Spink Charger. james provost

But it’s also true that recharging removable batteries can be a pain: You have to load them into a charger, which usually holds no more than four batteries at a time, and take care to place them correctly. Otherwise you will get no charging in the best case and irreversible damage to the cell and even overheating in the worst case.

To alleviate this problem, I created the Spink, a DIY device that charges seven NiMH AA batteries at a time and automatically detects the polarity of each cell before charging; When finished, it drops the batteries into the hopper. You can check the charging status through a display that also serves as a clock.

In my day job, I work on industrial vehicles as an EE in a medium-sized German company. But I came to this project because of a personal interest in low-power electronics and after a failed attempt to get the last bits of power from some non-rechargeable batteries. That failure – in short, my design required a buffer battery that had to be recharged, which defeated the entire purpose of the project – made me think about rechargeable cells.

The hardest part of creating Spink was the self-imposed challenge. I wanted the charger to be compact and intuitive to use. This means spending a lot of time perfecting the mechanism that takes the battery from the top of the charger, places it between two electrodes while charging, and then drops the battery down before resetting and grabbing the next cell. After many careful iterations, I had a set of seven 3D-printer files to create the parts that were assembled to create the Spinnaker’s case and mechanism. To this, you just need to add a printed circuit board with the display, servo motor and all the remaining components, as well as a infrared proximity sensor It detects when the battery has been inserted and is ready to be placed between the charging electrodes.

To allow the battery to charge regardless of which way it is inserted into the charger, I used the classic H-bridge circuitWhich is typically used to drive DC motors in either direction, with some modifications that allow it to operate at lower voltages.

I decided to use a dedicated integrated circuit to manage the actual charging thermistors To be careful about overheating. While I could use a microcontroller and write my own software to monitor the battery, NiMH cells have a very flat charging curve, and it is easy to overshoot the charging cycle. Using the IC saved me a lot of testing, and also gave me the ability to use the fast-charging mode.

However, this did not save me from some trial and error. The first charger IC I used was attractively compact, 2 by 2 millimeters. However, when I attempted to charge an AA battery, I discovered that the manufacturer had assumed the end user would be charging two to four batteries at a time, all connected in series. Charging only one battery means that the IC’s linear voltage regulator will have to dissipate more heat than expected, and this will quickly burn out the chip. Eventually I found something larger (5.15 by 4.4 mm) charger ic who uses one switched-mode regulator And could handle the work. However, it took a while to find ICs, as the popularity of lithium batteries has reduced the availability of supporting components for NiMH.

The charging curve of NiMH follows a slow curve, with the battery’s voltage only showing a 2 millivolt drop when fully charged, which is easily overshot and can cause cell damage.james provost

The charger IC is connected to a rp2040 Microcontroller, which reports battery status on an LCD display. The RP2040 is also responsible for monitoring the proximity sensor and driving the servos that control the internal feed mechanism, as well as reacting to the Spin’s push-button controls, which let you set the clock and start charging the battery. The entire device is powered via a USB-C socket.

Most electronics are surface mounted on a PCB that has a large center cutout to allow the battery to pass through it. i designed the pcb KiCad And it was created by JLCPCBTo make it easier to build the boards correctly, I did not attempt to specify 90-degree angles for the corners of the center cutout. Instead, I went with rounded corners, which are easier for the cutting tooling to adhere to.

The result is a desktop charger that is as convenient to use as possible. A complete set of PCB schematics, bills of materials, 3D printer files and firmware files are available here. Project page on GitHubI hope this helps to ensure that you have a battery available at a moment’s notice whenever you need a freshly charged NiMH battery!