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.
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.
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.
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.
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.
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.
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.
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).
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...
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?
The actual power required does not change..
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#
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.
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.
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.
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.
A review of the original pack teardown reveals it has a small pcb with what looks like some voltage cutoff or thermal fuse.
At least with lithium-ion technologies, for larger applications, this ignores a lot of the degradation phenomena and electrochemistry of the battery.
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!
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.
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...
very small drones for inside the house, they are super fun and you can fly them whenever you want