Best way to discharge a cell prior to top balancing?

TrailAgent

New Member
I have 8 new cells I am trying to top balance. Long story short I have one cell at 3.50v and the rest are 3.34. What is the best way to discharge the one cell down to 3.34 so I can start to top balance them safely? Would connecting an incandescent lightbulb to use as a resistor on the cell work, or are there resistors that I can use to discharge quicker? I have all 8 cells connected to an overkill BMS at the moment and it is balancing, but I think that will take forever.
 
D

Deleted member 23531

Guest
P = V*I = V*V / R, so for a 60W incandescent light bulb, R = 120V * 120V / 60W = 240 ohms. So connected across 3.5V it would bleed I = V / R = 3.5 / 240 = 14.6mA, so it would take you 8 days to discharge a 280Ah battery 1% of capacity. Not a practical option.

You could buy a lower-resistance "power resistor" that can take the heat. The big ones are mounted in heatsinks as all the power has to get wasted as heat and convected into the air.

Hopefully the above math is correct (there may be some factors of 2 floating around for peak vs RMS voltage), but you get the picture...
 

Hedges

I See Electromagnetic Fields!
I might connect all cells in series with the single 3.5V cell at the end, connect BMS with its disconnect relay/FET at the end away from the 3.5V cell, and connect a power supply to series charge 7 cells until they reach 3.5V.
Then reassemble in parallel to finish top balancing.
 

FilterGuy

What, me worry?
I might connect all cells in series with the single 3.5V cell at the end, connect BMS with its disconnect relay/FET at the end away from the 3.5V cell, and connect a power supply to series charge 7 cells until they reach 3.5V.
Then reassemble in parallel to finish top balancing.
You beat me to it!
 
D

Deleted member 23531

Guest
Couldn’t you just connect it in parallel and let the cells equalize?
Yep, and the initial current would be defined by the internal cell resistance plus the resistance of your bus-bar. I think a typical cell resistance is 0.1-0.25 ohms so the current would be about 1.6A, which is fine. EDIT: 0.1-0.25*mOhm* actually, so actually don't do this given the size of your voltage difference...
 

Hedges

I See Electromagnetic Fields!
Yep, and the initial current would be defined by the internal cell resistance plus the resistance of your bus-bar. I think a typical cell resistance is 0.1-0.25 ohms so the current would be about 1.6A, which is fine.

To be safe, connect with a lightbulb or resistor, and short it when voltage drops so about a millivolt.

Typical cell resistance is about 0.00017 ohm. Still fine?
 
D

Deleted member 23531

Guest
Typical cell resistance is about 0.00017 ohm. Still fine?
Lol, sorry dropped a factor of 1000! So that would be 160mV / 0.2mOhm = 800 Amps, too much for OP's voltage difference.
 

TrailAgent

New Member
I might connect all cells in series with the single 3.5V cell at the end, connect BMS with its disconnect relay/FET at the end away from the 3.5V cell, and connect a power supply to series charge 7 cells until they reach 3.5V.
Then reassemble in parallel to finish top balancing.
Thanks. Interesting solution. I am new at this. Which wire is tied to the the disconnect relay?
 

TrailAgent

New Member
If your cells are 3.34V at rest, they may be fairly full already, ready to top balance.
If so, you could charge them in parallel to 3.5V, then parallel the high cell and finish top balancing.
I like this idea. Seems the easiest with the set up I have at the moment.
 

Hedges

I See Electromagnetic Fields!
Another proposal which has been floated, especially considering already built into a pack, is use a bench supply to individually charge cells to a target voltage.
 

smoothJoey

Ooga Booga!
I have 8 new cells I am trying to top balance. Long story short I have one cell at 3.50v and the rest are 3.34. What is the best way to discharge the one cell down to 3.34 so I can start to top balance them safely? Would connecting an incandescent lightbulb to use as a resistor on the cell work, or are there resistors that I can use to discharge quicker? I have all 8 cells connected to an overkill BMS at the moment and it is balancing, but I think that will take forever.
I used a capacity tester to great effect.
Didn't even have to break the pack.
 

Hedges

I See Electromagnetic Fields!
Thanks. Interesting solution. I am new at this. Which wire is tied to the the disconnect relay?

You would just have to RTFM. I've never used one of them.
If battery current passes through the BMS it'll be obvious where it goes.
Some have internal FET. Some control external contactors.
You obviously want to make sure BMS can disconnect to prevent over/under voltage of cells when doing anything other than single cell charging with a regulated power supply.
 

BarkingSpider

Carbon Lifeform
I covered exactly what you want to do in a previous thread. You can lower the voltage of 1 cell very quickly and safely.

 
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