Rescuing A Rescue Device With A DIY Battery Pack

September 3, 2026 by Tom Nardi 5 Comments

If you find yourself lost in the woods or adrift in the ocean, a GPS-enabled Personal Locator Beacon (PLB) is certainly a handy thing to have as it will pinpoint your location for rescuers. The one [Steve Jernigan] has is relatively affordable considering it might save your life one day, but it turns out that replacement batteries for the unit are extremely expensive and hard to come by because the manufacturer would rather you send the unit back to them for refurbishment. So naturally, he took it apart and figured out how to do it himself.

His DIY battery pack consists of a small custom PCB and pair of off-the-shelf 6 volt lithium manganese dioxide cells in a 3D printed enclosure that slots into the rear of the McMurdo PLB. In assembly video below, [Steve] shows how the pack goes together and demonstrates the use of a two-part potting compound in an effort to keep the elements at bay.

Before anyone says it — putting a homemade battery pack into a device which one day might be the only thing standing between you and death is probably not a great idea. [Steve] mentions in the description of this project that this is meant for backup purposes, and that you should really spring for a legitimate battery if there’s a chance you might actually need to be rescued at some point.

If you’re interested in battery pack hacking that doesn’t potentially risk life and limb, fixing the one that came with your cordless drill might be a better project to start with. Continue reading “Rescuing A Rescue Device With A DIY Battery Pack” →

Fixing A Dead Makita Battery With A 20 Cent Part

August 11, 2026 by Maya Posch 17 Comments

That's not what an NTC should be reading. (Credit: The Repair Forge, YouTube) — That’s not what an NTC should be reading. (Credit: The Repair Forge, YouTube)

It’s no real secret that battery packs for power tools aren’t the foremost when it comes to user serviceability, so if said battery pack suddenly stops charging outside of warranty, you generally just e-waste it. That’s what [The Repair Forge] could have done for the Makita battery pack in question, but instead it was opened up for a diagnosis and fix.

Rather than the charger throwing up an error with this specific battery, it would flash its red LED and run its fan, but never actually start the charging process. Apparently the charger seems to think that the battery is either too hot or cold to be charged, which already gives a big hint as to what might be wrong.

Using the open source PocketOBI tool it’s possible to query the battery, which showed that one of the internal thermistors reported the battery being at a chilly -30°C while the other a more reasonable 28°C. After popping open the pack and measuring the thermistors, the faulty one registered as infinite resistance thus confirming that it had failed.

By putting in a temporary resistor this diagnosis was confirmed, thus the next step will be to replace said thermistor. This same procedure was then used with a second battery, whose thermistor read a wild 64°C.

Overall it’s a pretty easy fix, using a 20 cent part, with the entirety of diagnosis to repair taking maybe ten minutes when using a tool like PocketOBI, itself based on the great Open Battery Information project that originally reverse-engineered the Makita battery protocol.

Continue reading “Fixing A Dead Makita Battery With A 20 Cent Part” →

A man's hand is shown adjusting a black Bakelite dial on the front panel of an instrument. The instrument is contained in a wooden box, and to the left of the box, a thermocouple is inserted into the flame of an alcohol burner.

Reading A Thermocouple With Mercury And A Potentiometer

August 5, 2026 by Aaron Beckendorf 2 Comments

If you’ve ever thought about the nomenclature of electrical components, potentiometer stands out as a strange name, etymologically suggesting something like a voltmeter. In fact, the component took its name from a voltage-measuring instrument also named the potentiometer. [Alnwlsn] recently took a look at one such device, which was integrated into a thermometer, and the Weston cell used to calibrate it.

The potentiometer (instrument) has a galvanometer at its heart. One side of the galvanometer is connected to the center lead of a potentiometer (component) which spans a voltage source; the other side is connected to a reference voltage. The potentiometer can be adjusted until no current flows through the galvanometer, at which point both sides match the reference voltage. The reference voltage source can then be replaced with some other source, which can then be measured relative to the reference by adjusting the potentiometer until both the voltages match. The reference voltage source is a Weston cell, which uses two mercury electrodes, one amalgamated with cadmium, to produce a stable 1.018 volt reference; despite being 74 years old, this particular cell still measured at 1.017 volts.

In this case, the potentiometer was made to measure the voltage produced by a thermocouple. After calibrating the potentiometer and connecting an iron-constantan thermocouple, [Alnwlsn] tested it with ice and boiling water, and in each case it proved accurate. In a more extreme test, it captured the temperature difference between the base and the tip of an alcohol flame.

For a bit more on the history of similar devices, check out the history of Weston Electrical Instruments.

Continue reading “Reading A Thermocouple With Mercury And A Potentiometer” →

Testing Hundreds Of Used LFP Cells Requires Some Automation

August 4, 2026 by Maya Posch 7 Comments

Although Li-ion cells have become a lot cheaper over the years, if you wish to buy hundreds of high-quality ones for that performance go-kart project, you may feel financially pressured into going for the option of stripping down years-old commercial battery packs instead.

While this is a financially sound option, you do have to figure out what the condition is of each cell before you happily stuff them into a new battery pack for said go-kart, as [Within Tolerance] recently did.

This is something that can be done manually, but for the 768 lithium iron phosphate (LFP) cells that were obtained for this project that’d be quite the tedious task. Hence it was decided to instead spend that time designing a system to automate this process, capable of charging, discharging, measuring and quantifying individual cells.

You can find the resulting Cell Goblin battery tester project on GitHub, which entails a custom PCB featuring an ESP32-S2 as the brains and associated software to monitor the process on a connected PC. Fortunately the issues on the PCB that are described in the video are claimed to be fixed in the repository version.

Using five of these dual-cell cell testers it was possible to run through the hundreds of cells with ten cells at a time. An internal resistance meter was also wired into the PC-based software via its UART. As of publication of the video the testing was still in progress, which gives some idea of how long it takes to work through those cells.

Continue reading “Testing Hundreds Of Used LFP Cells Requires Some Automation” →

Demonstrating LFP Battery Safety In Case Of BMS Failure

July 5, 2026 by Maya Posch 24 Comments

Generally, LiFePO₄ (LFP) batteries are quite safe and stable, but it’s still possible for something to go wrong, even something catastrophic, like the battery management system (BMS) developing a direct short. This is one of the tests required to be certified for the UL 2054 standard that targets household and portable battery safety. In a recent series of videos, [Will Prowse] demonstrates how a series of commercial batteries pass these tests, and how some still fail.

Continue reading “Demonstrating LFP Battery Safety In Case Of BMS Failure” →

Safely Using Old EV Batteries In Your Home Solar Setup

June 10, 2026 by Maya Posch 33 Comments

As straightforward as the concept of taking battery packs out of an old electric or hybrid car and reusing them for home power storage sounds, this thought process skips a few essential steps. As argued by [Ed] in a recent video based on his own experiences with high-voltage Nissan Leaf batteries in a home PV system, the main problem is that you’re taking a battery out of a larger system including a lot of the management hardware and software.

The referenced Battery Emulator project is an open source effort to create a suitable interface between these EV batteries, with the mentioned Nissan Leaf being just one example in the project Wiki, with the connection scheme shown in the top image. It’s also noted that the Leaf battery BMS is not designed to operate continuously, so they need to be restarted every day or so lest they become too inaccurate.

These and other things are all solid reasons why you have to be absolutely certain that you want to integrate these high-voltage battery packs into your 12 – 48V low-voltage DC system. You’re after all assuming all the responsibility of setting up a system that’s both safe and reliable, so having a good read through something like the Battery Emulator Wiki and sourcing first-hand experiences from the folk in this community would be a very wise first step.

Continue reading “Safely Using Old EV Batteries In Your Home Solar Setup” →

The Secret Wattcycle LFP Battery Downgrade

June 9, 2026 by Maya Posch 7 Comments

After recently reviewing a Wattcycle LFP battery sent over by the manufacturer, [Will Prowse] was made aware of some disturbing changes to the internals of batteries received by regular customers. Rather than the nice protected cables, thick solid metal busbars, braided wire and excellent build quality, the units that a regular customer – got as well as the one that [Will] bought off Amazon – all feature something more akin to what you’d find in a budget LFP battery, including a wide variety of LFP cells.

With these LFP batteries generally coming in fully opaque plastic cases, it’s really hard to tell what the internals look like without either going medieval on them or using less intrusive methods such as an X-ray machine. In this case more capable braided cables were replaced with regular cables that in a test showed a much higher voltage drop compared to the braided type.

Along with all the other changes between these batteries, this makes it impossible to rely on any reviews as a customer. [Will] notes that Wattcycle isn’t alone in doing this, and makes the case for more transparent cases for LFP batteries. After all, if you can see at a glance through the transparent case what the cables and wiring looks like, what BMS is installed and even what any LEDs on said BMS PCB are doing.

There are some LFP batteries with such a transparent case already, and with some smaller LFP batteries you can even pop the top off without having to resort to very permanent levels of violence, so this is not a problem without solutions. From a consumer perspective it definitely would be nice to see the internals as literal transparency from the manufacturer’s side, as well as an increased ability to monitor the battery for any thermal, leakage or other issues.

Continue reading “The Secret Wattcycle LFP Battery Downgrade” →