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After getting into drone racing (where you need to take good manual care of your batteries because they're basically a bunch of cells soldered together), I realized that every battery we have everywhere is just some standard cells in some standard configuration.

With that knowledge, you can replace the battery in any gadget (small ones are usually 3.7V, or 1S), if you know how to care for it. Replacing the battery in a MiniDisk like this, I would maybe not trust the internal MiniDisk charger and charge by hand, but it should otherwise work identically to the factory battery.

This might work if you're the kind of person to continue manually checking and caring for the battery pack, but it's not good advice for the average hobbyist.

The crucial missing detail is that battery packs are assembled from identically matched cells, which age together as a unit. As soon as you replace a single cell with a new cell of the exact same model, or a new cell of a different capacity and charge characteristics, that cell will charge differently than the rest of the battery pack. This results in an unbalanced charge situation.

If the battery pack has a battery management system with leads in between each cell to balance the pack, this might work. The BMS can drain off excess charge from the unbalanced cells and convert it to heat.

If the battery pack does not have a BMS, the unbalanced charge situation can lead to pack failure, with varying degrees of smoke and flame depending on the battery chemistry.

When in doubt, play it safe and just get a new battery pack.

On a tangent, but... do you think it's possible to build a battery that discharges as a pack (in series), but charges each cell individually?

I imagine it will require some creative wiring, but charging efficiency should then go through the roof, right? I image I went e-biking and returned to my car to recharge - having individually chargeable cell should reduce the overall charge time a lot.

A battery management system handles this.

You still charge the pack in series, but the BMS will discharge cells that have a higher voltage than others. This results in all cells having similar charge levels.

It doesn't change the charge time, though. You're putting the same amount of energy into the pack either way. 14.8V @ 100mA applied to 4 cells in series is the same energy as 3.7V @ 100mA applied to 4 cells individually. Can't escape the laws of physics.

If power efficiency is critical (solar applications, for example), the BMS might have fancy switching circuitry to re-route energy from overcharged cells to undercharged cells. This is expensive, though, so most BMSs just use resistors to burn off extra charge from overcharged cells.

> You still charge the pack in series

I understand that's how it's done, but can we charge faster if we DECIDE NOT to charge in series? I take it the answer is no?

Imagine a series of 14 cells. I could charge them as a whole, but there is normally some limit on how much power I can pump into it. Now suppose that I disassembled the series and now I have 14 cells that I can charge independently before re-assembling them back. If I did that, would I gain anything? If nothing else this should prevent unbalanced charging, reducing overall heat of the pack and allowing higher current overall?

I can see that some packs advertise 5a charge current and other packs of the same capacity offer 10a or even 15a. Wonder why is there such difference.

It would be marginally faster (best case probably a percent or two) for a much more expensive charging circuit. The reason for this is that you can discharge batteries much faster than you can charge them.

Also, it seems like you're imagining that you can charge batteries with more power if you charge them in parallel instead of series. This isn't true since you're just trading a higher voltage/lower current for a lower voltage/higher current.

Absolutely, and this is not unheard of in custom e-vehicle wiring.

You have a discharge circuit, in series, and a charge circuit, wired separately to each cell. You're only using one at a time, as long as everything is properly grounded they don't get in each other's way.

It's not really about charge time though: any vehicle battery is going to discharge faster than it charges, so it's practical to just dump a bunch of current across the pack if you're charging using the same circuit.

What it does, is takes good care of each individual cell, and if you log some metrics you can detect underperforming cells and replace them.

I will say that for most applications, charging off the series wiring, and adding an overvoltage module that shuts off charge to each cell when it's topped off, is going to be much simpler and good enough.

But it can be done.

Not a problem, just expensive. Wires running to each individual would have to be larger than ones used just for sensing or small charge bleeding.

I was considering a design where there was a PCB on every cell with an individual charger, but it's just more cost and parts to break, plus no redundancy (unlike having two big chargers in parallel).

I don't think it's very expensive. Nearly all hobbyist RC drone batteries that are have 2 or more cells in series have a big discharge lead with all the cells in series, plus a smaller parallel "balance port" that provides access to each individual cell. Balance ports are used for charging, and can also be used in flight to wirelessly report the voltage of each cell. There are plenty of common (and apparently somewhat reliable) balancing chargers for under $30.

A popular $22 balancing charger for hobby RC drones: https://hobbyking.com/en_us/turnigy-accucel-6-50w-6a-balance...

A $15 sensor that plugs into the balance port and sends telemetry data about each cell: https://alofthobbies.com/frsky-sp-flvs-smart-port-lipo-volta...

A similar sensor that beeps really loudly when any cell falls to 3.5 volts (these are very popular for people without telemetry systems): https://outofdarts.com/products/lipo-voltage-alarm-1-mini-si...

An article explaining balance ports in RC aircraft batteries: http://www.tjinguytech.com/charging-how-tos/balance-connecto...

What problem are you trying to solve? Wasting a tiny bit of power in resistors?
I’m not sure what you mean. I believe the problem is that battery cells wired in series will tend to discharge at slightly different rates due to slight differences in their composition from their manufacture, and that cells can be damaged when their voltage goes outside a certain minimum or maximum range, so it’s important to monitor each cell individually while discharging and charging.
How about a middle ground then?

Let's make a few 3-series batteries and charge them. When done charging we stack four of them together to get a single 12-series to power an e-bike. Would that allow for much faster charging?

I guess any fast charge pack has this, cars in particular. Pushing large currents thru strings of high impedance empty batteries must be dangerous and energy inefficient, while switching them to parallel configuration would make a world of difference. I'd be surprised if there weren't power routing circuits already available from major IC manufacturers.
The only difference between charging them in series or parallel is that the Volts/Current ratio changes.

The actual power required does not change..

Doesn’t it improve efficiency? Squeezing electrons through a string of exhausted batteries require current over a longer time and therefore more losses.
I was actually wondering how balance chargers work, do they really burn off excess voltage with resistors? I always assumed they applied extra voltage to charge the low cells, rather than discharge the high ones.
Yes, usually they use ADCs to read the voltage in between each cell in the series. If individual cells have higher voltage than other cells, a resistor for that cell is switched on to burn off excess charge.

More advanced controllers can use flyback transformers to move charge from one cell to another. This is vastly more expensive than just using a resistor, though, so it's only used in applications where energy conservation is key, like solar projects or where heat is a constraint. The LTC3300 is a good example: https://www.analog.com/en/products/ltc3300-1.html#

Balance chargers for small packs, like you'd have for a radio controlled aircraft, have separate wires for the positive and negative plus in between each cell. So, a 6-series configuration would have 7 wires and the charger uses them to charge each cell separately to the correct voltage.

The problem with that is that each wire has to be able to handle the full charge current, but at a low voltage. So, if you're dealing with, say, an electric car with dozens or hundreds of cells and thick copper cables the size of garden hoses it's no longer practical. Instead, you charge it the simple way (by applying a large voltage to the whole series string through a positive and negative lead) and use a battery management system that measures voltages between cells and drains any high-voltage cells gradually through a resistor. It's kind of wasteful, but it's fairly simple and if your batteries are well balanced the BMS shouldn't have to do much at all.

Using the high cells to charge the low cells would be a nice feature; I'm not aware of any EV BMS that does that, but I'm not an expert and I'm really only aware of what's going on with DIY conversions. I don't know what the state of the art is for BMSs in commercially manufactured vehicles.

generally in these situations you'd be replacing all the cells at once, so they are reasonably matched. Nobody tears apart a battery and resolders everything just for one cell, that's a waste of time.

and if you're really concerned about it, you can match cells for impedence yourself, but it's generally not a problem if you replace all the cells at once with cells of the same kind/batch/etc.

Even though cells may exist with the same specs, it's hjighly recommended to create a battery pack from a single supplier and standardize on that. For example when I used to do professional laptop repair, I discovered the '18650' is only a size rather than a catch-all for all 12V cells, and isn't recommended to daisy-chain the various good ones together if they only share the same voltage. There's over a dozen different colors that indicate these batteries were built by various suppliers with use cases such as fast discharge rate, higher peak voltage, greater capacity, or a cost-effective battery that survives fewer discharge cycles. Creating a pack with mismatched batteries could cause the load balancer or device to operate outside of spec- don't cheap out on your power source!
I don't disagree, and building battery packs for battlebots was a fun experiential way to learn this, but it is important to match chemistries. Chargers for nickel-metal-hydride batteries won't correctly charge lithium-ion batteries, or worse nickel-cadmium batteries. Not to mention that some battery packs (the ones in the Thinkpad come to mind) have a "battery monitor IC" actually built into the battery pack so it helps to start with an existing pack and then "re-manufacture" it with cells rather than just wire together the same number of cells.

Perhaps to most critical element in these "smart" batteries is the temperature sensor which dials back the current when they start getting hot to avoid thermal runaway. (source of MANY battery fires in laptops from counterfeit battery packs).

As for matching the cells, in our battlebot we would take each "unit" (which in our case was 20 NiMH batteries) and condition it by draining it and charging it in cycles while monitoring voltages to level out the batteries and "sync" their charge levels. We still ended up with a couple of fires but that was because we were charging them right at the limit of what is reasonable to do because the time between bouts could be pretty short.

> I would maybe not trust the internal MiniDisk charger

Why wouldn't you? It has same voltage and even same capacity (weird considering evolution, but maybe choice was deliberate). I dunno, but maybe MiniDisk charger even has temperature sensor.

Do you just reuse the battery management system (bms) that’s already in the cell?
No the batteries he's referring to (4-6 cell LiPo's) don't have any electronics in them. They are as bare bones as you can get for the racing factor (lightweight and more capacity).
For many people integrating batteries into their projects, this is true and you can simply take most off-the-shelf cells with the right voltage profile and put them into some series/parallel config.

At least with lithium-ion technologies, for larger applications, this ignores a lot of the degradation phenomena and electrochemistry of the battery.

Even off the shelf 9v batteries are typically 6x1.5v AAAA cells in series. (i.e. they actually are batteries)

They can also be stacks of plates though, so buy a sample of a particular brand before you decide bulk-buying them is a great/cheap source of cells!

How does one can get into drone racing? Where should I start?
You buy a cheap controller (RadioMaster TX16S is the best value for money right now, at $120ish, but you can do quite well with a $30 FlySky), some goggles ($40 for some Eachine EV800D, I forget the exact model number), $150ish for a low-end starting 5" quad, and $30 for a 4S battery.

I might also recommend getting a tiny whoop instead, those are very small drones for inside the house, they are super fun and you can fly them whenever you want, vs having to make a trip to a suitable location.

That should get you started. Nowadays I much prefer acrobatic wings, though, I made a Flitetest Versa from Depron foam for like $30 and it's amazing fun.

  very small drones for inside the house, they are super fun and you can fly them whenever you want
House yes, anywhere with a fire suppression system _NO_. The little glass bulbs used in most of them don't react well to having a drone bump into them. Buddy of mine had this happen where he worked not long ago, you really don't want to be that person...
I'm not having any luck finding the TX16S for that price, any suggestions?
Don't order from Banggood. It takes weeks to arrive and is likely to be a knockoff. There are several US retailers like RaceDayQuads.com, GetFPV.com, etc.
As far as I'm concerned this is one of the main killer apps for 3D printers - making battery packs for vintage electronics. It's great to breath new life into old equipment which just needs a shot of lithium to be revived. I'm not a huge fan of Thingiverse but I still try to curate a couple of battery related channels there:

https://www.thingiverse.com/dosman33/collections/two-way-rad... https://www.thingiverse.com/dosman33/collections/power-tool-...

I also released a universal battery charging cradle system I call the Gadget Hamper to help people out who design new battery packs: https://github.com/dosman33/Gadget-Hamper

I've thought of this but people also would need to use something durable, e.g. carbon-fiber-reinforced nylon.

You definitely don't want your battery pack shattering if you drop it.

The 3D printer world today seems to have an unfortunate love affair with PLA which is for both temperature and durability reasons a very bad choice for battery pack enclosures, and a bad choice for just about everything else. I'm really not sure how to get the word out there that PLA is outdated and should be deprecated for just about every 3D print use case. (Personally I use PETG For just about everything, but for a battery pack I'd probably want something carbon-fiber reinforced.)

I think the larger concern should be fire safety. Normal 3d printer filaments don't have the fire retardants required by UL/etc for power electronics. I've experienced power supply and battery failures that were contained by their enclosures enough times in my life to take this very seriously. Had any of those cases been wrapped in common PLA/PTEG/ABS from a 3d printer, i'm sure it would have been much worse.

So, if your printing anything that has a likelyhood of catching fire you should probably be doing it with something like: https://www.3dxtech.com/flame-retardant-filaments/firewire-f... or just wrapping that part of the design in sheet metal.

You also have to realize things like this have to take scale into consideration. No one is going to do mass manufacturing of battery and electronics enclosures with 3D printing, this is a prototyping and hobby level manufacturing system. So the odds of injury and damage are a lot less with this as you automatically self-select down to a tiny fraction of the market share, and only a tiny fraction yet of those will ever experience an explosive outcome from a battery.

Also I'm sure that most of this market violates someones IP so again, this ensures that this type of thing (custom making battery enclosures) will never reach a critical mass where worrying about the world burning down due to non-fireretardent plastics is something to be concerned about.

But if a person is very concerned about this issue then certainly there are options out there.

How does this flame retardant ABS work in the event of a lithium battery fire? Are there flame retardant PETG? Are they any better than just printing with normal filament and spraying flame retardant on it?

I've been hesitant to use lithium batteries in home robotics projects, but I can't find charging and power mux boards for NiMH, so that has been a huge blocker in making robots that charge and operate at the same time.

Also I wonder if it is possible to put mini pressurized CO2 balls inside the battery enclosure such that if a fire happens they explode and release CO2, putting out the fire?

> Also I wonder if it is possible to put mini pressurized CO2 balls inside the battery enclosure such that if a fire happens they explode and release CO2, putting out the fire?

No, that may momentarily put out the flames but as long as the thermal runaway condition is still present it will reignite. The only way to stop a battery fire, which is actually a metal fire, is to permanently suffocate it - usually by dumping heaps of salt, cement or salt on it which melts and so removes oxygen from the fault area - and then, at least for electric cars, to dump the car in a container full of water and keep it there until dismantling, so that the water acts as a thermal buffer to prevent reignition.

I suspect to have the right answer you will need to read the relevant standards/cert and understand how it interacts with your design.

But the general theory as I understand it generally is not that the plastic may not burn, only that they are self extinguishing. V-0 like that filament means it will extinguish itself within a maximum of 10 seconds after the ignition source is removed.

Regular filament acts more like an accelerant, put a flame near it and it burns quite vigorously, and for a long time. Meaning anything near it that can burn will likely catch fire too.

I've become a large user of PET-G for some of the reasons you outline. I've never had a problem with strength the way that I use it in my designs, PET-G is plenty durable for a small device which spends 99% of its life inside a larger device. Also the heat resistance makes it tolerate the summer heat of cars which is also important.

Also PET-G has the added benefit of not giving me cancer like ABS. It definitely has a learning curve and I wouldn't fault anyone for going other routes rather than taking the time to get a feel for it.

Yep, same reasons.

My printer prints PETG pretty cleanly with no fuss. I imagine if someone used the same exact printer (Prusa i3 MK3), the same exact filament (eSun), my same exact settings (https://github.com/dheera/3d/blob/master/settings/slic3r-con...), and my same exact textured bed sheet, same exact procedure (Windex, then Magigoo, then print), they should have zero problems as well. It's not even tricky. It prints just like PLA with this "recipe".

The only problem seems to be that there is a lot of conflicting information about what works for PETG but people just need to think in terms of configuration sets like the above. If you use the Prusa PETG settings with eSun filament it fails miserably -- the prints come off the bed mid-print.

> I'm really not sure how to get the word out there that PLA is outdated and should be deprecated for just about every 3D print use case.

It works fine for 90% of use cases and is easy to use. Aside from that, you can buy virgin PLA filament which is pretty darn nontoxic. Most other plastics have a variety of negative effects on humans, from physical interaction with the finished product but also from the printing process.

I'd like to do this for my old Thinkpads since I don't trust the random brand replacements to either last a decent amount of time or not blow up. Is there a source of loose matched cells and the protection circuit? Ideally with a guide like the one in this post.
Those batteries are generally 18650 cells welded together, but you do need a spot welder for that, and to figure out their voltage so you know how many to put in series. You may be able to just take the protection circuit out of the pack you have now.

Sony and Panasonic are good 18650 vendors.

I love to (or used to before we switched to WFH) check in on the battery recycling bins in the office and look for discarded laptop battery packs. Usually they are full of good 18650 cells (except for 1-2 bad ones that bricked the pack).
Oh yeah, that's usually like $40-$50 worth of cells just tossed.
All the laptop salvage cells I have test with high internal resistance and low capacity, though the packs I salvaged them from were not dead, just low in capacity.

They may be worth $5/cell new, but not so much in their current state.

It's not recommended, but I have soldered in replacement cells with a regular 30W iron. It can be as easy as open the pack, identify and replace bad cells, manually balance charge the cells (one at a time), tape the pack back together. It's a bit janky, because the plastic cases are sonic welded and you have to butcher them a bit to get them open and then glue or tape them back together, but it can be difficult to locate batteries for old laptops and I don't want to spend money.
The problem with soldering them with an iron is that you get the cells way too hot by the time solder has managed to melt, even if you're quick about it.
Not if you work quickly and use 63/37 0.6mm rosin core tin-lead solder with a 30W iron. I never need to apply the iron for more than half a second. I've done this more than a few times, and I realize that it goes against standard safety recommendations, but I'm going to keep doing it.

And if you're recycling cells from other packs, they are likely to have remnants of old tabs still connected, so you can solder to the tabs instead of directly on to the cells, which gives a little more thermal wiggle room.

You can also stick the cells in the freezer before soldering. It adds a fraction of a second to coming up to temperature, and gives the cell proper a lot more thermal wiggle-room. Nothing bad will happen if you keep them above about -20° C.

Definitely file this under "bad advice you probably shouldn't follow, and don't tell them I sent you", but 'allegedly' this works...

That's extremely, extremely risky to do if you don't know the cell's composition, especially the liquid electrolyte, and its performance in cold temperatures. Last thing you want to do is manage to get it frozen and physically damage the layers.
Yes, you should look it up, and pretty much any 18650 will have -20° C or lower as the lower end of its range.

It would be extremely, extremely risky to sell a product which could be destroyed by leaving it in the car in the winter.

I think this advice only earns one "extremely" or maybe a mere "very"...

I didn't think that the absolute series voltage would matter much - the right number of cells will be close enough for the voltage regulation in the laptop. My big concern with that approach is matching the cells. It's not practical for me to do that myself since I would need to test a lot of cells. Or maybe I am overestimating the negative consequences of using unmatched cells. But I don't think so.
How do you mean matching them? Just get 8 NCRs or whatever and that's that.

The absolute series voltage matters in that you don't want to over- or undervolt it by 3.7V usually.

Matched in terms of voltage and discharge characteristics. Many of the battery guides mention the possibility of the weakest cell in an unmatched pack being improperly charged. And that that would result in shorter life for the pack or worse consequences.
Eh, sure it matters somewhat. But if you think you're getting matched cells when you buy anything with a battery pack from China you're kidding yourself. So a home made pack isn't going to be any worse off that the cheapest kit you already have. If you go to the trouble to assemble your own pack with matched cells you'll be ahead of the game, if that floats your boat.
Hmm, I was assuming the charger would have a balancer, if it doesn't you can get in trouble, yes.
Maybe you are right that it is a balancing charger. That would simplify things a lot. Although some of the packs have some of the cells in parallel so would have to be carefully matched.
FWIW, old thinkpad batteries make for a great source of 18650 cells to use in flashlights.

Usually only one of them in a dead pack is bad and the rest are great.

Using them in flashlights that take only one 18650 - fine. Using multiple ones - I am not taking the chances (mismatched li-ion is asking for trouble).
Assuming you charge them separately, I don't think there is an issue.

(though usually I use flashlights that take one 18650).

It can be. The really bad outcome is reverse-charging, where one cell is completely drained, then charged to a negative voltage. The probability of an explosion is significant.

If the flashlight has a low-voltage shutoff for the series at 2.5V per cell, it would take a series of four before that's possible, and even then it's pretty improbable. You'd have to mix a discharged cell with three full cells; a moderate difference in capacity or internal resistance wouldn't be enough by itself.

It is easily possible to over-discharge a cell under those conditions, after which charging and using it again is risky, but if you're salvaging laptop cells you probably already know that.

Thanks! I hadn't even considered the possibility of reverse charging really imbalanced cells. Not an issue for anything I use those recovered cells for, but certainly something I'll watch out for.

[this also reaffirms my decision to store lion batteries and their charger in a metal cabinet in a detached garage... because clearly I didn't know all the ways I could turn them into bombs.]

It's my opinion, having been using and reviewing Li-ion flashlights for several years now that single Li-ion cells are very easy to handle safely provided they and the devices you use them in are of reasonable quality. It's mostly a matter of avoiding short circuits and mechanical damage.

Multi-cell devices require a little more knowledge and care, but the main thing is just to make sure the cells match. Being the same model and charged to full before installation will do for new cells, but salvaged ones need to be tested and binned.

Not that easy. They have a battery management controller which contains cell charge data and a fuse which bricks the pack if it’s not handled properly.

Have a good look around on the internet.

I've looked plenty and I think I can handle the issues with the controller though it would be better if I could just replace it. I guess I could also get a quality battery pack (Turnigy?) and take apart the cells, but I'd prefer to buy matched loose cells from a known-good brand (Sony, Panasonic). I haven't seen anyone selling matched loose cells though.
For a while I was scrapping dead laptop batteries to build an 18650 pack for my esk8 build. I ended up building a battery validation rig with a Turnigy battery charger that charges the battery to 4.2V, drains it to 3.8V, and charges it back up to 4.2V to figure out its true storage rating. Then I sorted them by manufacturer, model, and storage capacity, with a recycling bin for anything below 80% of their rated capacity, open cells, and shorted cells.

It's a lot of work, but the validation was all automated via the charger. The pack I ended up with had healthy cells all within 10% of each other in terms of capacity.

By "matched", do you mean binned by the vendor for capacity and internal resistance? I don't know of anybody doing that, but dealers like Illumn might be willing to sell you multiples with the same lot number. I'd be surprised if most OEMs do more than that when building packs.

You can get a cheap analyzing slot charger ($30-40) and do it yourself. That won't be quite as accurate as a big expensive industrial battery tester, but it will be accurate enough to make safe battery packs for consumer electronics.

I have the same problem except it is the battery pack for my original Sony Aibo (1st gen robot dog.) I had the cells replaced once using an eBay service but they didn't last very long.
I really wish a significant fraction of consumer electronics ran on removable rechargeable cells of standardized form factors. There are only bad reasons most Bluetooth speakers (to give an arbitrary example) don't run on a removable 18650 cell.
Safety is a top reason.
When companies do use that approach, they usually include a warning insert about not shorting, mechanically abusing, or eating the battery. Li-ion cells aren't very difficult to handle safely, especially in single-cell applications. This has gone well for flashlights, but has yet to be widely adopted in other product categories.

I understand that major consumer-product companies are reluctant to offer a product with any risk of attracting personal injury lawsuits, but I'm still classing it as a bad reason.

How so? It seems to me like standardizing would help focus efforts on building safer batteries.

I don't really know what I'm talking about though.

Standardized Li-On battery could be safe. IMO it's now available as "Portable Power Banks", but its form is not standardized.

Simple 18650 cell can't be safe. Normal consumer may just put/charge raw 18650 cell, then burn.

It seems that no one intend to make standard Li-On battery. Some manufacturers just use other brand's battery. (e.g. Blackmagic products uses Canon's battery)

Slightly off topic, but I wish some company with capability in volume production of battery packs would develop a modular system of safe, reliable, light and economical battery packs. There's so much potential for various products that would benefit from having multi-kWh storage capacity available, and this potential will only increase as the cost per kWh drops further.

As a case in point, this Slovenian company makes sustainer systems for sailplanes, which is awesome, but their system is limited to two battery packs. I imagine that pack design, form factor and availability is a big limiting factor in projects such as these.


YES! I have been toying around with this concept and there are no good solutions right now.

There are few providers doing something similar (Vruzend off the top of my head), but they all have huge downsides.

What I would love is: 1.) Integrated high quality BMS. 2.) Modular design so that I can add capacity or voltage simply without welding. 3.) High amp discharge capability. 4.) SAFETY! Can be used in electric mountain bikes, eFoils, electric paramotors, whatever. Vibration, drops, etc. do not disturb the cells or the pack.

If you could accomplish this then you would unlock so many cool features. 1.) Construct your own packs regardless of voltage or size requirements. 2.) Travel on airplanes with your batteries! Deconstruct the pack simply and store in <100Wh battery blocks or as 18650 cells and you can take theoretically an unlimited amount of lithium across oceans (likely limited somehow). 3.) No vendor lock in. 4.) When your BMS tells you that a certain subpack has failed just pop the cells out easily and replace the one that is busted. 5.) Because of #4 far better for the environment and cheaper.

Does anybody have any thoughts about how to bring this to fruition?

The Vruzend thing works but is clunky, has low amperage capacity because of the bus bars, and the BMS is not integrated.

If anybody has thoughts I would VERY much appreciate it.

Seems like you're describing this product exactly:


By the way, the company that makes this, Grin Technologies, is really great. I've toured their facility and met the owner. (I don't have any sort of stake in them, though).

Only 36V, each pack has a BMS. It's basically spot welded 96 wh packs @ 36V with individual BMS. Works OK I guess but lots of wasted space and weight.
I think most people interested in this already string together their own packs with 18650 cells.

Apart from that, I wouldn’t feel comfortable creating packs that could be fire hazards if the users do things like mix cell types or quality or capacity.

You don't want to string together a homemade pack of 18650 cells and put them in the wing of an aircraft, or anywhere else a fire will be particularly dangerous.

My point is, I'm sure there's a market for modular battery packs that have actually made these considerations, along with charging/discharging management, power electronics, insulation/cooling/heating and so on.

I love this approach: clean, accessible, shareable.

I have an old Nikon D1h SLR which I loved, but it used horrible Ni-MH battery packs that didn't last long--both in terms of shots per charge, and total lifetime. They're all dead and I can't bring myself to order another one, now that I'm used to li-ion packs in modern cameras. There's one German company that makes a compatible li-ion pack but they don't sell to the U.S. for some reason.

There's a bunch of blog posts about how to take one of these old battery packs and jerry-rig it to accept a couple of rechargeable li-ion cells. But it looks like a pain--cutting open the old pack with a razor blade, gluing in battery tabs, etc. Maybe 3D printing would offer a better way to do the same thing...

>but they don't sell to the U.S. for some reason.

Shipping lithium batteries by themselves (as opposed to installed in/packed with equipment) is a huge pain in the ass to do internationally.

MiniDisc players were beautiful gadgets. I had one of the lower-end models but it was still a delight to use.
As a younger person who missed out, I recently enjoyed this mini-documentary— especially the surprisingly lengthy history of the many devices for a supposedly "failed" format: https://www.youtube.com/watch?v=kU3BceoMuaA
MiniDiscs were technologically interesting but everything about them was a pain in the ass. In the US they were also stupid expensive in just about every aspect.

Console MD player/recorders were ridiculously expensive in the US. Portable players were expensive but not nearly as bad as the mains powered console ones. This meant most people just went for CD players.

Because they didn't sell well here music labels didn't release many (any?) MD albums in the US. That meant you were buying new albums from the "import" section of the store (if you had a record store that sold imports) for full integer multiples of CD album prices.

Unless you were stupid rich then you bought blank MDs and recorded your own music on them. That didn't mean dropping tracks onto a playlist and pressing the "Burn" button. You had to plug your MD deck into an audio source, hit record, and then play the track/disc back in real-time. If you just did a whole CD at once it would record as one long track but you could go back and mark start and end points to break it into actual song tracks.

So you either spent tons of money buying commercial MD albums or bought blank discs to make your own. Making your own discs meant every album you listened to required the cost of the blank disc and at least an hour of your time to record it.

Or you could just save yourself tons of money and effort and listen to CDs.

Source: a friend of mine was obsessed with MiniDiscs and I thought they were cool until I spent an afternoon recording two discs. I realized my DiscMan was a way better deal.

Per the Wikipedia article [1], it seems like in the 2001 timeframe there were "NetMD" devices which could receive digital audio directly from a computer at much faster than realtime; I imagine this was a direct response to the iPod and other MP3 players, and was probably too little, too late. Particularly when it sounds like the process was bogged down by making you use proprietary, copyright-aware software.

[1]: https://en.wikipedia.org/wiki/MiniDisc

The NetMD players were way too little and way too late. While a NetMD player could load music via USB onto a disc the software on the PC was terrible and the process wasn't much better than recording from the line-in.

If you were loading music from MP3 or WMA the software was transcoding those formats to ATRAC, NetMD players didn't natively support either format. So you went from one lossy compression to another. If your MP3s were already at the minimum threshold of quality because they came out of a shitty encoder they were just going to get worse in ATRAC.

If you were loading WAVs into SonicStage your quality would be way better but you first had to rip your CD to disk and then load it in to SonicStage. So you had no time savings over ripping to MP3. ATRAC also had a bunch of DRM so it limited how many discs your could load a track on. Not an automatic problem but putting a song on more than a couple discs wouldn't work.

Keep in mind that by the time NetMD players were out CD-R drive's were cheap and pretty common. A lot of CD players had also started supporting native MP3 playback and dedicated MP3 players were readily available.

The extremely inconvenient NetMD experience was up against cheap CD-Rs, much more convenient MP3 players (including the iPod), and in general a better MP3 experience. The whole MiniDisc ecosystem was just inconvenient unless you had spent a lot of money to live in some sort of end-to-end MiniDisc world.

I had a NetMD from Sony, and I absolutely loved the thing. I still have a couple disks I made, with no way to play them back.

My apartment was burgled in 2003, and the insurance payout covered an iPod to replace the stolen minidisk player.

Night and day, no comparison, that iPod was the coolest thing I owned at the time, and stayed that way for many years. It's in storage right now, but the last time I plugged it in, a couple years ago, it still worked. The battery is toast, though.

Pretty much every generation of iPod has upgrade options available for both the storage and the battery. Definitely dig out your unit and see what you can do for it.
iPods are still cool, and becoming quite collectible. I'm starting to collect them myself, especially older ones that are repairable, and even upgradable with higher capacity batteries and CF to SD card converts - so you have a higher capacity device, with a power-sipping SD card, that lasts longer with a higher capacity battery. Far better experience that streaming on a flaky connection on a device that is dead in hours.
Coincidentally I found my old minidisc player just last week while sorting my box of old gadgets. I brought it to the car since I have a bunch of mix discs that turned out to have aged well. Mine runs off of a single AA battery, and after at least 10-15 years with no use it still had a charge. The Sony UI certainly was no Ipod...
I still have my MD recorder. A few weeks after I discovered that I could make perfect digital recordings, I sold my mint condition Studer-ReVox B77 MkII reel-to-reel tape recorder to a friend, and never looked back. The B77 was a lovely thing, but a pain in the ass to haul around, and the tapes were exorbitant.

I now have a little Tascam pocket digital recorder that I love.

Awesome youtube channel. Here is the one he did on battery packs for hand tools.

DIY super capacity BATTERY PACK


Good stuff to see how you can just take those packs apart and recombine them, but aren't you supposed to take care to match cells and make sure your BMS can balance the charging or risk letting the explosive magic smoke out? Seems like he just took any random cells and put them all together.

He didn't really talk about any of that, so this approach feels like quite the fire hazard.

The battery he's ended up with looks like it does have charge protection after all (6-pin chip under the kapton tape), which is probably a good thing.
Is there a market for replacing battery packs of PHEV cars? The latest Passat GTE apparently has a 33% bigger capacity in the same volume. I would love to have more electric range in the same car. I am very hesitant though because automotive parts have to be resistant to higher stresses that consumer electronics like laptops.