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.
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.
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.
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.
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.
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.
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.
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.