Li-ion charging / balancing

[roman.isaikin]

Hello,
I'm working on a battery powered device which needs 12V @ 6A, so I'm planning to use 3 li-ion cells in series. Each cell would be approximately 3000mAh. I'm planning to use rapid charge method, since 2-3h charing is not acceptable. So my question is - how to properly charge this pack, so that no one would burn their house in the process?

On the one hand everybody says that you need to balance cells, and on the other - pretty much every charge IC that I found is supposed to charge them in series. There seems to be not very much info about it, someone suggests that maybe it is ok for cells with protection circuit inside, or for charge at a low current (0.5-0.7C).

Here is some links: MAX1737 charge controller https://www.maximintegrated.com/en/products/power/battery-management/MAX1737.html
High current Li-ion cell, with rapid charge support at 4A - Samsung INR18650-30Q https://www.nkon.nl/sk/k/30q.pdf

 

[Audioguru]

The Maxim IC does not "balance charge" so it probably assumes your battery cells each have a "protection circuit". The Samsung battery cells do not have a protection circuit.
When the voltage or current is too high on one cell then it either explodes or it catches on fire. Lithium burns violently like magnesium and titanium. See the "ium"?

Your circuit will need a "low battery voltage disconnect" that occurs when the voltage on each cell drops below 3.2V. A protection circuit in the cell will do it.

 

[treez]

If you need to supply 6A, then the 3AH batteries in series will not be able to do that.
As long as each cell has an overvoltage disconnect then its ok without balancing while charging if they are in series.

The worst problem may come in discharging, where a battery may go undervoltage when discharging, and so you would need an undervoltage disconnect aswell, or else one cell could end up getting busted.

 

[roman.isaikin]

So if I will use cells with protection and set current on MAX1737 to ~1C everything will be ok?

 

[treez]

I think charging at 1C always needs temperature protection. Maybe there is a new type of particular lithium battery that doesn’t need temperature monitoring when charged at 1C…I think bosch use it in their drills, but I cant remember the name of it, its something like lithium polymer or something.
I would monitor the temperature, and looking at page 15 of the battery datasheet, you should stop charging if the cell temperature goes above 80degC. I think a sudden sharp rise in temperature in any case, is a sign that a lithium cell being charged is fully charged.
I am pretty sure that when the cell is quite depleted, it doesn’t really heat up much at all when charged at 1C…but if you keep on pumping in 1C when a cell gets full, then it can overheat very quickly.
I suppose to be safe you have to monitor the temperature of each cell.
I know what you mean though, finding out the exact charging requirements of particular batteries is very challenging….unless you have orders in the 100,s of thouands and the battery manufacturer is then always at your side.

 

[Audioguru]

If you need to supply 6A, then the 3AH batteries in series will not be able to do that.

The 3Ah Sanyo battery cells have a maximum discharge rating of 15A on their datasheet. At 15A their capacity will be a little less than 3Ah, maybe 2.9Ah. 10A is shown to be 2.98Ah on the datasheet.
My radio controlled model airplanes use a little 7.4V Li-Po battery rated at 325mAh. Its continuous allowed output is 70 times that at 22.75A and its momentary peak output is 45.5A. If it is shorted then the resulting explosion is spectacular.

- - - Updated - - -

I think a sudden sharp rise in temperature in any case, is a sign that a lithium cell being charged is fully charged.

Not for Lithium, that is for a Ni-Cad and a Ni-MH cell.

I am pretty sure that when the cell is quite depleted, it doesn’t really heat up much at all when charged at 1C…but if you keep on pumping in 1C when a cell gets full, then it can overheat very quickly.

Again not for Lithium but instead for Ni-Cad and Ni-MH. The Lithium charger limits the voltage to 4.2V per cell and when a cell is full its charging current drops then its warming also drops. You cannot keep on pumping in 1C unless the charger is defective and feeds more than 4.2V which will probably cause an explosion. That is why using a balanced charger is important when charging cells in series because it keeps each cell from exceeding 4.20V.

 

[treez]

ok thanks but isnt the figure of "4.2V" temperature dependent... i mean, at higher temperatures, 40.5V i think is the "battery full" voltage, and keeping on charging up to 4.2V in this case will be dangerous?

 

[Audioguru]

You should let a recently used Lithium cell cool down before charging it. Also it should not be charged if its temperature is below about 40 degrees F.
www.batteryuniversity.com says 4.2V is the maximum voltage for charging a lithium cell without talking about ambient temperature. At less voltage then the cell is no where near a full charge. The cell continues to charge even when the voltage reaches 4.20V until its charging current drops to be fairly low (1/40th its mAh rating) when the charger should be disconnected.

 

[roman.isaikin]

Thank you very much for answers!
MAX1737 supports temperature monitoring, so I'll test it first.