Safely Using Old EV Batteries In Your Home Solar Setup

June 10, 2026 by Maya Posch 31 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.

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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.

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Using Brand New NiMH Cells After Sitting 12 Years Unused

June 5, 2026 by Maya Posch 26 Comments

You know your batteries are old when their labels have faded. (Credit: DiodeGoneWild, YouTube)

After finding a pack of NiMH rechargeable cells that had never been used since buying them in 2014, [DiodeGoneWild] decided to test whether they could be tossed or not. After previously testing different brand cells that had gone high internal resistance after only about five years, he wasn’t expecting much. Amazingly, the batteries not only recovered, but seems to be not that much worse off for wear.

Three of the four precharged cells still held some voltage and happily charged back up to their rated 2,000 mAh capacity basically with the first cycle. One of them read 0V initially, but was revived using the typical manual charging approach involving a bench power supply. After a few charge-discharge cycles only the deep discharged cell showed some noticeable degradation with slightly reduced capacity, but all of them read healthy internal resistance values.

What this mostly shows is that not all NiMH cells are made the same, with the Tronic ones that previously failed after a few years doing much worse than these Activ Energy cells which are apparently sold primarily at Aldi stores. Overall NiMH is a pretty robust battery chemistry, so it’s always worth it to try reviving a cell before tossing it.

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Testing LFP Battery Failure Modes With Overcharging

May 28, 2026 by Maya Posch 33 Comments

As great as batteries are, it’s essential to understand their risks and how to keep them from going spicy. Recently there has been a bit of a fuss about the dangers of LiFePO₄ (LFP) batteries after someone’s dedicated LFP battery shed got shredded into matchsticks by a hydrogen explosion, following said LFP batteries having a thermal event. The thing about the LFP chemistry is that if it suffers such a thermal event, it generates hydrogen gas, which is one of the most explosion-happy gases known to man. This is demonstrated in a recent video by [Will Prowse].

To kick things off, a single prismatic LFP cell is overcharged for half an hour after it was already at 100% state of charge. This ultimately pops the vent as the cell begins to release hydrogen gas into the aquarium that the cell was placed in. Using a spark generator it’s then attempted to ignite the gas, which initially takes a bit as enough hydrogen has to collect first.

Once there’s ignition, however, it happily keeps burning as more and more hydrogen pours out of the by now bulging cell’s vent. If any other LFP cells had been nearby these too would be at risk of suffering thermal runaway, showing how just one bad LFP cell is enough to potentially set an LFP battery bank ablaze.

In a commercial setting you will have precautions such as hydrogen sensors, ventilation and spark generators to deal with any generated hydrogen gas, as well as blow-out panels in case things end up going squirrely in a hurry.

While a benefit of LFP chemistry is that it does not generate its own oxygen as with other lithium-ion chemistries, hydrogen gas is a major problem due to how incredibly volatile it is. It’s not just a headache with battery storage, but also in the nuclear power sector, where zirconium fuel rod cladding can very efficiently turn steam into hydrogen and oxygen. This was the reason why some of Fukushima Daiichi’s buildings suffered detonations, with the nuclear plant operator opting to not install recommended hydrogen gas mitigation systems.

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A ZInc Air Battery You Can Make Yourself

May 26, 2026 by Jenny List 9 Comments

Zinc air batteries have been a familiar sight for decades in the world of photography, where they provided an environmentally less dangerous alternative to mercury cells. They operate by the oxidation of metallic zinc using air, and the zinc comes in the form of a paste spread between two electrodes. Can their astounding energy density be harnessed for something useful? [ZollerLab] has designed a zinc air battery to find out, and is using it to power a rudimentary model car.

The video below is in German so you’ll have to enable translated subtitles if you’re an Anglophone, and it’s very long. But it goes into extreme detail on the chemistry, construction, and constraints of a zinc-air battery, and describes the system in this design. It’s a stack arrangement, in which the cells are held together on threaded rods, and pushed into each other with springs.

We think the car model is intended to demonstrate that this battery chemistry might one day be used in automotive applications. It’s not such a far-fetched idea given the low cost, relatively low environmental footprint, and high energy density, indeed we’ve heard of similar experiments with aluminium primary cells. But in this case we can see it provides the hacker with another route for their experiments, and that’s no bad thing.

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ChargeCap Helps Your Batteries Last Longer By Limiting Charge Level

May 14, 2026 by Donald Papp 22 Comments

If you want to maximize the life of your lithium-ion batteries, proper storage voltage is critical. That is, don’t store them empty, and don’t store them completely full either. “Almost fully charged” is a sweet spot for occasional-use devices. Sadly, this is easier said than done. While many devices use integrated rechargeable batteries these days, most provide no method of limiting charge level. That’s where [DaverDavid]’s ChargeCap comes in.

By sampling charge current and disconnecting when it drops to 50 percent of peak, charging is reliably stopped when the target device is 80 to 90 percent charged, regardless of cell count or capacity.

ChargeCap sits between a USB charger and target device, disconnecting when it detects that recharging is 80 to 90 percent complete. This is particularly useful for maximizing the cell life of devices that see only intermittent use.

The way ChargeCap does this is clever, and relies on the fact that all lithium-ion charging curves look the same regardless of cell capacity or cell count. Charge current remains at pretty much the same level for most of the charging process, but tapers off quickly (and in a linear fashion) as cells approach their maximum capacity. That’s because charging a battery is a lot like blowing up a balloon: the first breaths are easy, but once the balloon fills out, every breath needs to push harder than the last.

ChargeCap works by sampling the peak charge current at the beginning of the charge cycle, then detecting when it drops below 50 percent of peak, at which point charging is stopped. The result is a device that reliably charges to 80 to 90 percent of capacity, and no more. ChargeCap uses an ESP32-C3 and a small OLED display that, as a nice touch, inverts colors to signal charge completion. Design files and code are at the GitHub repository.

Lithium-ion cells are fantastic devices, so flesh out your knowledge by reading [Arya Voronova]’s primer on designing them into your own projects, or a more in-depth explanation of how they work.

Teardown: ChargeTab Emergency Phone Charger

May 14, 2026 by Tom Nardi 59 Comments

If you own a modern smartphone, there’s an excellent chance that its battery has run dangerously low on you at least a few times. Murphy’s Law dictates that this will naturally occur at the worst possible moment, say when you need to make an important phone call or when you’re lost and need to navigate home.

With this in mind, it’s not hard to see how a product like the ChargeTab would have a certain appeal. A small $10 USD device that you can keep in the car or pack in a bag that’s always available to charge your phone in an emergency.

Because it’s not meant to be used regularly — indeed it may never get used at all — it’s not completely unreasonable that such a device would only be good for one or two charges before its spent and must be replaced. It’s a bit like keeping a road flare in the car; it’s unlikely you’ll ever use the thing, but if you do, it only needs to work once.

But then what? According to ChargeTab, once the gadget has depleted its internal ~3,000 mAh battery it cannot be recharged and is no longer usable. Now to be fair, they specifically tell you to not throw it in the trash. They’ll send you a free return label to ship it back to them, at which point it will be refurbished and put back into circulation. The company argues that this recycling program, combined with the fact that the batteries inside the ChargeTabs were supposedly diverted from landfills in the first place, makes their entire operation eco-friendly.

Yet here we have a pair of ChargeTabs that were thrown in the regular garbage and would have taken a one-way trip to the local landfill if it wasn’t for the fact that I habitually dig through garbage cans like a raccoon. So let’s take a look at what’s inside one of these emergency phone chargers and if the idea is as green as the company claims.

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