Because of slight cell capacity differences, that will likely result in two of the cells (of a 3-cell battery) not being fully charged.i measure every cell voltage and stop charging when it gets near 4.20V for any of the cells.
No, they don't. E.g, when Q7 is on, the gate of Q4 is close to 0V, so Q4 (which is an N-MOSFET) is off.I think when Q6/Q7/Q8 have Vbe 5V they turn on Q3/Q4/Q5
Cell 0 is GNDYou should show the polarity of the battery cells also on the schematic.
I didnt explain this part... I would NOT charge and "balance" at the same time. Charing will be in series with current around 2A. When cells get disbalanced, charging will stop and they will get balanced through 1/4W (or maybe 1W) shunt resistors.To shunt 500ma of current (for example) at a voltage of 4.150 volts would require a precise resistor value of 8.300 ohms.
That won't change the switching function. An N-FET and NPN both turn on with a positive control signal.Maybe i should use 2N7002 N-CH mosfet insted of 2N2222 NPN transistor?
How do you propose to introduce the bypassing that Kubeek mentions in post #23?
Cell 0 is GND
Cell 1 is 4.2V when full
Cell 2 is 8.4V when full
Cell 3 is 12.6V when full
I didnt explain this part... I would NOT charge and "balance" at the same time. Charing will be in series with current around 2A. When cells get disbalanced, charging will stop and they will get balanced through 1/4W (or maybe 1W) shunt resistors.
Once they are balanced, and cells are below 4.15 od 4.20V, charging will continue
You mean that you will balance them first, and then start charging? The problem is that if one of the cells has lower capacity than the others its voltage will grow faster than the others and it will be fully charged first.Why should i be able to pass ALL the charging current? I think its not neccesary because i would charge my battery pack ONLY when all cells are at the same voltage. When they are not, passive balancing will be active
If you simply limit the voltage on each cell separately with a parallel transistor and make sure that it can dissipate the full charging current
If you charge 4 cells in series and cell #2 charges up to 4.15v and the others are at 4.00v, then why would you load the 4.15v cell?
Then, if the cell is really at 4.15v, as soon as you start to charge again (for the other three at 4.00v) the one at 4.15v would trip again right away, so you'd be back to balancing. Therefore the three lower voltage cells would never charge.
Unless you have a different way of explaining this, this wont work.
In rough terms yes. The transistor itself can act as a resistor and ensure that the voltage on that cell is never above the limit, so when the cell reaches 4.20V the transistors open just enough to keep that voltage and thus bypasses the charging current around the cell.What do you mean with paralallel trasistor? You mean i should charge them in series and when any cell charges to 4.20V, its mosfet turns on and that cell is shorted (bypassed) with resistor that can pass 2A of charging current?
Is that what you were thinking?
In rough terms yes. The transistor itself can act as a resistor and ensure that the voltage on that cell is never above the limit, so when the cell reaches 4.20V the transistors open just enough to keep that voltage and thus bypasses the charging current around the cell.
What do you mean with paralallel trasistor? You mean i should charge them in series and when any cell charges to 4.20V, its mosfet turns on and that cell is shorted (bypassed) with resistor that can pass 2A of charging current?
Is that what you were thinking?
I meant to charge my battery pack without balancing till any cell gets to 4.15.
Lets say that voltages at that moment are 4.12V, 4.13V, 4.15V.
At the next step i would dissipate cells 2 and 3 energy till their voltages are all 4.12V. Then the charging resumes and when any cell is greater for 0.02V, charging stops and balancig begins.
So after few minutes voltages would be 4.15V, 4.16V, 4.16V. Everything is fine and charging is still ON. Than the first cell charges to 4.17V and first cell is stil 4.15V. Charging is stopped, and balancing is ON till all cells gets to 4.15V.
Something like that... Do you think bypassing is a better solution?
That is true. But i think the biggest difference in cell voltages in my battery pack would be 0.05V after full discharge. I think that if cells of my battery where always balanced (and they were), they cant disblanace very much. Thats my opinion.What if only one cell is 4.00 while the other three are 4.15 ? We really want to have to discharge three cells for the benefit of one cell? I dont think so
This is not a problem because i have calibrated my voltage measurement and it is really good now. For example Cell 2 in my schematic has 7.83V, and with 2 10K resistors i get correction factor = 1.944 (2 would be with 2 perfect 10.0000K resistors). So the measurement is not the problem.As you're using a resistor dividers to scale down he voltages at each node between the batteries, these will need to be at the very least 1%
This is true, but i just wanted to make a very simple balance charger that has dimensions no bigger than 4x8cm. Current i want is not so big, just 2A, even though my battery is 8Ah. I thought maybe i could skip constant voltage charging state so i removed ACS712 for current measurement (cheap, small and simple charger).What is rather worrying is the fact that you don't provide a constant current source
Could you please draw and post me a schematics of this part? Also, can i use a for exampe a 0.001oohm resistor as a current sensing? I have never tried that, but it would be very cheap and simpleI would include a fuse, then a P-channel MOSFET, followed by a constant current source, that has its maximum current output set in hardware (analogue circuitry) to a limit.
Something like this?Using a current monitor/sensing IC (with external resistor perhaps) does not 'control' the current, merely measures it - and so you would need to use this output to limit the current via a control element, a MOSFET or transistor.
I dont mind, you are very helpfulI really can't short short posts can I? :/
To be honest i dont want to do this in pure digital way, i would rather do it in analog way, because it should be as simple as possible, but i dont know how to do it with curren or voltage limiters, so you all may be very helpful to me if you could help me with thatI have a feeling he wants to do this in a pure digital way
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