Lithium-ion batteries’ ability to deliver a lot of power from a small package have made them the go-to for makers and manufacturers alike. It’s not unusual now to find, say, microcontroller boards with integrated Li-ion chargers. Lithium-ion is so popular, in fact, that it’s easy to forget that other battery technologies exist, even when they’re a better fit.
These worthy alternatives include removable rechargeable nickel–metal hydride batteries. While NiMH cells can’t be recharged as many times as lithium-ion cells can and don’t offer the same power density, they’re cheaper and also safer. No need to ship them in boxes emblazoned with fire warning labels. The fact that NiMH cells deliver lower voltages than lithium has become less of an issue as the voltage demands of integrated circuits have fallen, with 3.3-volt and 1.8-V chips rapidly displacing the ubiquitous 5-V standard of yesteryear.
How to Make NiMH Easier to Manage
A handful of 3D-printed parts [bottom], a servo and screen [middle] and a single printed circuit board [top] are all that’s needed for the Spinc 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 typically holds no more than four batteries at a time, and take care to put them in the right way. Otherwise you get no charging at best and irreversible damage to the cell and even overheating at worst.
To ease this pain point, I created the Spinc, a DIY device that charges up to seven NiMH AA batteries at a time and automatically figures out the polarity of each cell before charging it; when finished, it drops the batteries into a hopper. You can check on the charging status via a display that doubles as a clock.
In my day job, I work on industrial vehicles as an EE in a midsize German company. But I came to this project due to a personal interest in low-power electronics and after a fruitless attempt to harvest the last dregs of power from some nonrechargeable batteries. That failure—in a nutshell, my design needed a buffer battery that had to be recharged, which defeated the whole purpose of the project—got me thinking about rechargeable cells.
The hardest part of creating the Spinc was a self-imposed challenge. I wanted the charger to be compact and intuitive to use. This meant spending a lot of time perfecting the mechanism that takes a battery from the top of the charger, places and holds it between two electrodes while charging, and then drops the battery out the bottom before resetting and grabbing the next cell. A lot of careful iterations later, I had a set of seven 3D-printer files to create the parts that assemble to form the case and mechanism of the Spinc. To that, you just need to add the display, servo motor, and a printed circuit board with all the remaining components, plus an infrared proximity sensor that detects when a battery has been inserted and is ready to be placed between the charging electrodes.
To allow the battery to be charged regardless of which way it’s put into the charger, I used a classic H-bridge circuit, which is normally used to let DC motors run in either direction, with a few modifications that let it work at low voltages.
I decided to use a dedicated integrated circuit to manage the actual charging, with thermistors to watch out for overheating. While I could have used a microcontroller and written my own software to monitor the battery, NiMH cells have a very flat charging curve, and it’s easy to overshoot the charging cycle. Using an IC saved me from a lot of testing, and also gave me the ability to use a fast-charging mode.
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