Self balancing is the biggest pile of crap I have ever heard of. It is a stupid and baseless theory.
Here is why:
If you charge 2 cells independently of each other, stop charging one at 4.0v (~85%) and the second at 3.6v (~50%) and stick them in a drawer, will they be the same voltage at the end of the month? (You must be thinking what kind of stupid question is this?) The answer is obviously no. They will not be at the same voltage at the end of the month.
Now if you were to solder a wire from cell 1 positive terminal to cell 2 negative terminal, and stick them back in the drawer, will they have the same voltage at the end of the month? The answer is still a big fat no.
There is no cell in existence that self balances with other cells in its vicinity without any intervention.
Where and why did this come about? Cells self discharge over time.
What affects self discharge? Voltage, temperature, age, internal resistance, chemical composition, etc.
A lot of these we can hold constant, like age and chemical composition. So lets assume any variance in self discharge due to this is negligible.
What would make a cell self discharge and degrade faster than other cells from the same batch? Higher voltage, temperatures outside operational range.
How does this degradation manifest itself? Higher internal resistance, lower capacity.
What happens when internal resistance is higher? Cells run hotter as the electrical power loss to heat is equal to the current passing through the cell times the internal resistance. If internal resistance is high then self discharge is high too.
What does this all have to do with self balancing?
Batteries are shipped from manufacturers at a specific voltage. This varies from manufacturer to manufacturer, but it is usually around 3.5v per cell. This means that if you buy 4 cells, they will all come “balanced” at 3.5v. Lets say we charge them without balancing in series. (No two cells are identical, they will always have a slight variance in discharge profiles. One will hold slightly more capacity than the others while one might hold less. Why would this happen? Different environmental conditions during manufacturing and shipping, etc.). We stop the charge cycle as soon as a single cell reaches 4.2v. This cell would be the weakest in the pack, maybe not by much but it is still the weakest cell. This cell holds the least capacity, has the highest internal resistance and is the hottest of them all after the charge is done. This is where self discharging comes into play. This cell would self discharge slightly faster than all the other cells and after a couple of hours might even settle to the similar voltage as the other cells. To the untrained eye, this is good, the cells “self balanced”. This is not good. It has been established that this cell has a higher internal resistance than the other cells and therefore lower capacity as the cell will lose more energy to heat when discharging and charging. If the cells are never balanced and cycled hundreds of times by discharging until one cell reaches 2.5v and then charged until one cell reaches 4.2v. Then you will be running the one cell to 100% of its capacity while maybe running the others to 90% of their capacity. Over time, the constant higher temps and internal resistance and being used to a higher standard than the other cells will lead the one cell to go out of balance and wear out increasingly faster than the others.
So that’s the long winded version of that…
Correct. That is a huge can of worms that I don’t understand. A lot of chemistry and stuff involved and I’m an aero guy that just happens to work with batteries…
Definitely! The scenario above is exaggerated in order to get my point across but even the best manufacturers don’t make identical cells every time so care must still be taken.
Quality cells do tend to be really well matched which means that you will get very little drift as long as you do not abuse the cells or have any outside influences that may cause one cell to degenerate faster than the others. Some examples would be having your hot escs sitting on top of half of your battery creating a temperature gradient across your battery or poorly welded tabs that may increase the resistance and heat generated in spots of the pack.
I LOOOOVE VTC6s. They are by far the most resilient battery ever. I once ran over 500A through a 6P pack (by accident, had a short across an ESC power stage) melting an 8awg wire and desoldering it from the load in about 10 seconds. Battery health status? Perfect… lol
When testing head to head, brand new 30Q vs VTC6, at 30A continuous with no airflow. Both batteries managed to melt my battery holder. Both batteries failed within 10-20 cycles. Neither battery vented, they just stopped holding any charge. If you try to charge both batteries reach 4.2v almost immediately when charging at 1A and get hot too. My charger recognizes there is an issue and tells me that the cells are bad. I think that regular brick chargers would get confused by this and eventually make the cells go into thermal runaway. The 30Q failed before the VTC6 by like 4-5 cycles. The 30Q is also harder to work with since the positive terminal is at the same height as the rest of the can while the VTC6s positive terminal is slightly raised.
This last point is why I prefer Sony/Sanyo/Panasonic batteries over Samsung/LG. I have never shorted a cell from one of the brands where the positive is slightly raised while I always have to be careful not to hold scratch the heatshrink on Samsung/LG cells.
Cycle life wise: I do not know which is better. Our aircraft are not made to land… It’s a one flight and done kind of deal… So I do not care about longevity.
As far as my personal stuff goes, I don’t buy VTC6s because I’m cheap lol.
Man that took long! Hope this is useful and sorry if not all of it makes sense, I’m hungry and not proof reading!