Solar Light Shows Why Replacing NiMH With Li-Ion Is Dangerous

November 2, 2026 by Maya Posch 49 Comments

Recently [DiodeGoneWild] got his paws on a purportedly IP44-rated outdoor solar light with PIR motion sensor that also contains an 18650 Li-ion cell. Since it’s being sold in Czechia in a supermarket by a Czech company you’d expect it to meet at least some quality standards, but once he got inside the device a true horror show unfolded.

The best guess here is that someone took an originally cheap NiMH cell-based solar light circuit and simply replaced said NiMH cell with an – unprotected – Li-ion 18650 cell, as there is absolutely no charging management or cell protection. This is important as while NiMH will throttle charging current by rejecting it as heat the more it is in a charged state, Li-ion is not as tolerant.

Even ignoring that the ‘IP44’ enclosure has more gaps and holes in it than the average slice of Swiss cheese, the 5.5 V-rated PV solar panel happily tried charging the Li-ion cell at around 6 V. This raises the potential that on a particularly sunny day with an already charged Li-ion cell sitting at 4.2+ V, it’ll effectively grenade this cell.

During an outdoors test the 18650 cell was charged to 4.8 V before the sun vanished behind the building, which probably isn’t an amazing sign if you are expecting for this light to keep working for more than a few weeks. Or not violently explode/catch on fire on the side of your house for that matter.

Of course, treating a NiMH cell better than in this particular circuit will extend its lifespan as well, but the failure mode of that battery chemistry tends to be a bit less aggressive. Hence why people keep getting away with dodgy circuit designs like this.

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On The Wisdom Of Replacing A NiMH Module In A Prius Battery Pack

June 1, 2026 by Maya Posch 83 Comments

Old versus new Prius NiMH module. (Credit: HubNut, YouTube)

It’s possible to get a pretty good deal on used Toyota Prius cars, but as with all hybrid cars that also means a used battery pack and resulting issues. In the case of the Gen 2 Prius that [HubNut] recently acquired it was clear that its battery was effectively toast, with the engine running constantly and the car often giving up due to detected issues with the pack. After getting to an EV-focused garage for repairs, a spare NiMH module was used to replace a problematic module to bring it back to good health, while raising the question of how sensible such a repair is.

Certainly, compared to the average BEV where a much larger battery is generally integrated well into the frame, a Prius makes things very easy, with the compact battery readily accessible and removable from the trunk. It is also a very modular battery, with some elbow grease and bolt-twisting enough to disassemble it.

Even with that it still a high-voltage battery with all the associated risks, and as raised in the comments there’s a big question about putting a new(er) cell into a pack with more worn-out NiMH cells as generally the cells wear out fairly evenly. While this fix can give the pack some more life, the new cell won’t match the internal resistance and other parameters of the pack, leading to issues like voltage drift. Then there’s the issue that if one cell failed, others probably aren’t far behind, so this hack would soon become a regular ritual.

Much like swapping one bad 18650 Li-ion cell in a bigger battery, it’s probably a more sustainable solution to simply replace the entire battery at once, or at least replace all modules or cells to properly refurbish it. For [HubNut] this fix suffices because he suspects that this pack was already assembled from random modules, it’s an important consideration to make if you don’t enjoy ending up stranded during a trip.

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Boxie – A Gameboy-Esque Audio Player

May 11, 2025 by Matt Varian 8 Comments

This little audiobook player is a stellar example of the learning process behind a multifaceted project blending mechanical, electrical, and software design. [Mario] designed this audiobook player, dubbed Boxie, for his 3-year-old son to replace the often-used but flawed Toniebox.

The inspiration for Boxie was the Toniebox, a kid-friendly audiobook player. While functional, the Toniebox had drawbacks: it required internet connectivity, limited media selection, and had unreliable controls. Enter Boxie, a custom-built, standalone audiobook player free from web services, designed to address these issues with superior audio quality and toddler-friendly controls.

Boxie’s media is stored on microSD cards inserted into a slot on the device. To make this manageable for a toddler, he designed a PCB with a standard microSD card interface, ensuring easy swapping of audiobooks. The enclosure, crafted via 3D printing, is durable and compact, tailored for small hands.

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The Automatic Battery Charger You Never Knew You Needed

December 2, 2024 by Jenny List 47 Comments

When we saw [Max.K]’s automatic NiMh battery charger float past in the Hackaday tips line, it brought to mind a charger that might be automatic in the sense that any modern microcontroller based circuit would be; one which handles all the voltages and currents automatically. The reality is far cooler than that, a single-cell charger in which the automatic part comes in taking empty cells one by one from a hopper on its top surface and depositing them charged in a bin at the bottom.

Inside the case is a PCB with an RP2040 that controls the whole shop as well as the charger circuitry. A motorized cam with a battery shaped insert picks up a cell from the bin and moves it into the charger contacts, before dumping it into the bin when charged. What impresses us it how slick this device is, it feels like a product rather than a project, and really delivers on the promise of 3D printing. We’d want one on our bench, and after watching the video below the break, we think you will too.

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Old Prius Gets Upgraded Batteries

December 18, 2023 by Bryan Cockfield 24 Comments

So many of the batteries made today are lithium batteries of some sort, from mobile phones, laptops, and drones to electric cars and grid storage solutions. But this technology is relatively new; even as late as the 90s and early 00s the only widely-available batteries for things like power tools or the new hybrid vehicles coming on the market were nickel-metal hydride (NiMH). While it was good for the time, they don’t hold up to all of the advantages lithium has. There’s still plenty of hybrid vehicles on the road using these batteries, so if you’re driving an older Prius and want to give it a modern refresh, there’s a quick option to swap your old batteries.

Despite lithium technology being available for several decades, the switch to lithium for the Toyota Prius wasn’t instant, with many variants still using NiMH batteries as late as the 2020s largely because the NiMH batteries are less expensive and less maintenance-intensive than lithium batteries are. As these batteries lose capacity, the cars are still driveable but the advantages of the hybrid drivetrain won’t be as accessible anymore. The upgrade, from a company called Project Lithium, replaces these batteries with modern lithium technology that can improve the efficiency and performance of these cars even above their original capabilities since lithium batteries have more power density.

With the Toyota Prius being among the most reliable vehicles on the road thanks to the electric motor in the hybrid drivetrain taking a lot of stress off of the internal combustion engine, it’s often worth upgrading these old batteries to modern ones to squeeze every last mile from these workhorses as possible. With many of the replacement processes being almost as simple as lifting out an old battery and placing a new one in, it can be a no-brainer if that’s the only issue with the vehicle otherwise. This is also true of all-electric vehicles as well, although the process to replace the battery can be a little more involved.

Thanks to [JohnU] for the tip!

Reverse Engineering A Classic ThinkPad Battery

July 15, 2023 by Bryan Cockfield 14 Comments

The ThinkPad 701 is an iconic laptop series from the mid-90s and is still highly sought after today because of its famous butterfly keybaord. The laptop itself is tiny even by the standards of the time, so in order to fit a full-size keyboard IBM devised a mechanism where the keyboard splits and slides over itself to hide away as the screen is closed. But, like most 30-year-old laptops, the original batteries for these computers are well past their prime. [polymatt] takes us through all of the steps needed in order to recreate a battery from this era down to the last detail.

He starts by disassembling an old battery with extensive damage from the old, leaky batteries. The first part of the recreation is to measure the battery casing so a new one can be modeled and printed. The control boards for the batteries of these computers were not too sophisticated, so [polymatt] is able to use a logic analyzer with a working unit to duplicate its behavior on an ATtiny microcontroller. With that out of the way, a new PCB is created to host the cloned chip and a new battery pack, made out of 9 NiMH cells is put together.

[polymatt] wanted this build to be as authentic as possible, so he even goes as far as replicating the label on the underside of the battery. With everything put together he has a faithful recreation of this decades-old battery for a famous retro laptop. ThinkPads are popular laptops in general, too, due to their fairly high build quality (at least for their enterprise lineups) and comprehensive driver support especially for Linux and other open-source software projects like coreboot and libreboot.

Thanks to [Roman UA] for the tip!

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There’s Life In That Beard Trimmer Yet!

November 23, 2019 by Jenny List 33 Comments

You just can’t get a decent beard trimmer these days! At least that’s what [Peter Franck] found when his trusty Panasonic finally expired after a couple of decades and a few replacement batteries. The shaver’s PCB contained a mains-powered NiCd charger which had comprehensively released its magic smoke, and the expensive replacement trimmers he bought simply didn’t cut the mustard.

Many people would have given up in despair, but he persevered, and produced a custom replacement board containing a Maxim DS2710 single-sell NiMH charger, and an AA NiMH cell. It fits perfectly into the space vacated by the previous board, and takes its charge through a micro-USB socket on the edge of the PCB.

It’s interesting to note that NiMH-based projects have in recent years become a comparative rarity on these pages compared to ones using Li-ion or Li-poly cells. This is an inevitable progression on cost, size, and power density grounds, but it’s still worth knowing about projects using the older battery chemistry. He remarks that his razor is now future-proofed, but we’d probably have fitted a USB-C conector before making that assertion. Either way, it’s a neat piece of work that has achieved its aim of making an expired razor useful again. We’ve brought you another razor fix in the past, though a much less sophisticated one.