Hm? You can wire 1000 cells in parallel for the biggest honkin’ porta-power you ever saw!
Just make sure the cells are equalised before doing so. If you wire 1 spent cell in parallel with those 999 topped-off cells, they’ll dump as much current into the spent one to make it go boom (well, whatever happens if you don’t limit the charging current).
And you can mix’n’match 18650s, 26650s, 10440s, whatever you like, as long as they’re the same chemistry.
Whatever your charging current, they’ll all charge proportionately. And discharge proportionately. So when the brand spankin’ new 26650 is down to 3.3V, so will be the one-foot-in-the grave 10440.
There’s no need for balancing resistors or any active circuitry needed to keep them all at the same SOC. Just a wire will do it.
In parallel? It absolutely will work. Voltage == SOC, regardless of individual cell capacity, age, anything. As long as they’re all the same chemistry, that is.
Last Saturday an Airbus A320 China to Japan had to divert for a safe landing because a home-did powerbank charging a phone in overhead locker caught fire. Dangerous bloody things.
I’d think if you used battery box cellholders rather than soldering the cells in you could remove and replace them at will for use in your flashlights.
Well, maybe not the entire powerwall would be built that way for cost reasons, but enough for your daily use.
Only if each and every one of those removable cell slots is individually protected and controlled to prevent parallelled cells at mismatched voltages from equalising with each other at dangerously high currents.
This is practical for small USB powerbanks operating at relatively low power levels, where one good-quality cell can take the full load if need be, but not for large units at the scale of a powerwall.
A complete, stand-alone BMS at each cell slot is what really needs to be done if you want maximum flexibility of cell size, chemistry, age, internal resistance, state of charge, etc. With a full BMS at each cell, the cells can charge each other safely to reach equilibrium when a new cell is added, and each cell slot will “shut off” if/when its BMS detects that it reaches low voltage. You wouldn’t want the cells to always charge/discharge equally across the pack, because that means you have to force it to the lowest rate that any one cell can handle. If you have a 90mAH 10180 in there for instance, that’s going to be a huge limitation. The individual BMS will make sure each cell will charge/discharge at its optimal rate.
You may need some circuitry to adjust for the potential differences in voltage of different cells across the pack. Maybe a boost circuit in the output of each cell slot, so the pack sees a constant voltage, regardless of individual cell voltages. The pack should also have its own control circuit and a charging circuit that completely disconnects from the pack upon “full charge” and only re-engages if/when the pack reaches some point of self discharge (or low charge from swapping cells in/out) rather than operating in constant float charge, which is bad for the cells. Actually, for the best cell maintenance, the charger shouldn’t ever reach up to 100% cell charge. You can pull a cell out of the pack and put it into a standard charger if you need it to be charged up to full capacity for use in a flashlight.
You are wrong with the small capacity battery limitating the packs charge speed.
If you connect a big battery bank to a charger each cell takes the current it internal resistance lets it pull
So if you got for example a pack of one Sony VTC6 in parallel with one 10180 cell and charge it with 1A the VTC6 will draw >950mA and the 10180 <50mA
During charge all cells have the same voltage, so if the whole pack has 3400mA and I charge with 1A it takes for the 10180 4 hours from emty to full
So the charge current on you 10180 is the same as it would be in an individual charge taking the same time
you dont need to agree its simple physics that are the base of electricity
lets say we have a 1000kW tuned tractor driving 15mph max
and a car with 100kW that can drive 150mph
both with electro engine without transmission
pulling a heavy load the 100kW car cant drive faster than the tractor, no matter what you do
the same counts for if both get pushed and have to break
the tractor got 10 times bigger breaks than the car
so the tractor will do 90% of the break force and the car 10%
same counts for 18650 and 10180 charging and discharging
I’m no EI but wiring a bunch of mismatched cells in parallel hoping they will self organize for the best does not seem like a sound design, especially if you want to allow random cells replacements.
Could an appropriate BMS be as small and inexpensive as a typical protection board we have on protected cells?
