General > General Technical Chat
LiFePO4 balancing.
(1/9) > >>
paulca:
It's been a while since I looked into lithium cell balancing.  I already see a few "new" techs thrown at it.  Like "active flying capacitor balancers", which looks like a set of capacitors, a PWM circuit and some mosfets which connect the capacitors to each cell in turn.  Such that higher cells charge the caps and lower cells discharge them.  Efficiency seems to be the issue, certainly when testing extremes which they probably would never be used over.

It made me start thinking of active balancing again as I had similar or better ideas for top balancing.  Passive resistors are just pants.

I figured I could do better than a PWM circuit.

An other example I seen "out there" which is more my style was using a completely separate MCU (STM32G030) per cell, running from the cell and connecting back to the mother MCU via opti-couplers, probably just UART.  I like it, but it does seem a bit overkill.  Also it was designed to clip into one of those 18650 grid arrays, not LiFe bulk cells.  And... it doesn't actually do anything special in terms of balancing... just a passive power resistor to cap the top cells.  Making it a bit snake oil for Lithium shedphiles.

So.  My thoughts are to take the two ideas and merge them.

A single MCU connected to (initially) cell 1.  It sees 2.5-4.2V.  Via a 50Ohm resistor it charges a supercap.  The MCU spin monitors that cap and waits for it to stabilise.  Notes the voltage.  Disconnects entirely from cell 1, pauses a few miliseconds and reconnects to cell 2.  Again via the 50Ohm resistor the supercap equalises to that cell voltage and it notes the voltage and moves on cell, 3, 4 etc.

After a pass of all cells it can now make intelligent decisions about which cells need the most attention and focus there.  It can also determine when the cells are "close enough" and put itself into a shutdown state, disconnected from all cells.  Waking up periodically to scan the cells, balance if absolutely necessary and return to sleep.  Data can be reported back to base via Wifi from an ESP-AT module.

I figure as long as the capacitance and surges are managed during disconnects/reconnects and the device is always 100% floating, even if powered from a wall wort it should be doable.
paulca:
One challenge which might need addressing separately are the "fail safes".  Low/High voltage cut outs for protection. Overcurrent protection (asides a basic fuse!).  Soft fusing, cell fusing etc.  I doubt a "floating" device can help much here except to monitor and inform any "intelligent" control system something needs done.  For me, that is not enough, so a completely separate set of interlocks in the form of cell or pack cut outs ...  I can probably source those pre-made, it's just hard to find ones that DONT also try and balance or charge monitor these days.
tszaboo:
LiFePO4 has very stable voltage across it SOC, so small measurement errors (and balancing then) leads to imbalance in the pack. So they are usually balanced when the SOC is low, maybe below 10% or when it is almost fully charged. If you do it when it's charged, you can get away with cheaper parts, with some wasted energy, that's why it's usually done that way.
Otherwise yeah, why not put a motor on each cell, and a gearbox selects which cell to charge and discharge. I'm sure we can come up with many bad ways of balancing cells.
paulca:
Well.  For now I am top balancing them manually to 3.65V with a PSU and 5A.  They were about 100mV across the pack out of balance at 14.40V pack charge, which is a bit too much.  I'll go round them tonight bring each to 3.65V individually until they take less than an amp, then repeat.  Then I'll leave them with only the solar charge controller load on them till morning and remember to check them before the sun comes out.

I have ordered a simple little passive balancer/monitor.  The plan being, if I balance them first, then it shouldn't have much to do.  It only has like 100R resistors or something, so shouldn't waste too much until I decide if I need a better solution and if then what that is.

I believe I am correct they "can" technically go to 4.1/4.2V, but there is little point past about 3.7V.
lastguy:
Where you get the power source for each cell? it might be easier for 12-16V battery array, how about high volt?
if you use isolated power source for each cell, system would have high cost I think?
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod