Best way to burn some juice of a couple runners?

[hammick]

I did top balance my 16 Lishen cells using Will's approach but I have a couple cells that are running up to 3.65v and tripping my BMS when charging at anything over 55.1 volts. The BMS balancing seems to have improved the lowest of the two runners but I don't have time to wait for the worst offender to balance out.

Can I safely burn off some juice with a 50 watt ceramic resistor or is that a bad idea. Thanks.

 

[curiouscarbon]

High watt rated resistor yes good. Take it easy and watch real-time feedback

 

[hammick]

High watt rated resistor yes good. Take it easy and watch real-time feedback

Do you consider a 50w resistor appropriate for a 3.2v cell? What do you mean by take it easy and watch real-time feedback? I need to burn off roughly .25 volts. Not sure how long I will need to keep the resistor on. If it starts glowing I will have to do it in stages.

 

[curiouscarbon]

50W seems plenty.

gently hold opposite resistor leads against cell positive and negative for short periods of time, removing to check voltage

each time you remove it, look at the voltage readout for the cell. if it’s still too high, touch the leads again for a few seconds.

sit there for a few minutes and repeat this loop

above 3.400V the voltage should drop pretty quickly as most capacity is below that threshold

 

[hammick]

50W seems plenty.

gently hold opposite resistor leads against cell positive and negative for short periods of time, removing to check voltage

each time you remove it, look at the voltage readout for the cell. if it’s still too high, touch the leads again for a few seconds.

sit there for a few minutes and repeat this loop

above 3.400V the voltage should drop pretty quickly as most capacity is below that threshold

Can I do this while the pack is charging or do I need to isolate the cell? At fully charged resting voltages the runners are the same voltage as the other cells.

 

[curiouscarbon]

the key is to do short bits at a time so the delay of heat traveling to the outside of the resistor doesn’t catch you by surprise

 

[curiouscarbon]

personally i’d disable charger and give attention to high cells then resume charge observe and repeat if they still run high

 

[hammick]

personally i’d disable charger and give attention to high cells then resume charge observe and repeat if they still run high

Thanks. Do I need to remove the buss bars from the two high cells for the discharge?

 

[curiouscarbon]

by the way.... ohms on 50W resistor?

 

[curiouscarbon]

Thanks. Do I need to remove the buss bars from the two high cells for the discharge?

it should not be necessary if you are strictly only touching that one cell’s positive and negative terminals with only the opposite sides of the resistor

 

[AussieSim]

I didn't have a resistor lying around, so I used a 12v PC fan, with stiff/malleable breadboard wires between the fan connector and my grub screws.

 

[BarkingSpider]

Has anyone tried using an old style incandescent 100w light bulb to draw down the voltage?

It could avoid the issues with excessive heat you get with a resistor.

I wouldn't expect any light, but it should draw some power.

I'll try it out myself later today ...

 

[Luthj]

Those cells don't seem to be well balanced. Remember the Orion Jr only has 65mA of balance current.

If that resistor is too low a resistance it will overheat quickly. If its too high you won't get much current. 0.5-1 ohm should be fine for a 50W resistor.

 

[Ampster]

current. 0.5-1 ohm should be fine for a 50W resistor.

Yes that is what i have had luck with. Also before doing that check the internal resistance readings and see if they are cosistent. You may have a loose connection that is contributing to that cell reading high.

 

[hammick]

by the way.... ohms on 50W resistor?

Sorry I have two 10w 68 ohm resistors and two 5w 47 ohm resistors if I'm reading them correctly. Which do you suggest? In parallel and how many seconds per drain? Thanks.

 

[JoeHam]

Has anyone tried using an old style incandescent 100w light bulb to draw down the voltage?

It could avoid the issues with excessive heat you get with a resistor.

Yes to an incandescent bulb.

I use these for precharge or burning off power:

Amazon.com

If it gets too hot to touch give it a break but I doubt that will be a problem with the aluminum case.

 

[hammick]

Yes that is what i have had luck with. Also before doing that check the internal resistance readings and see if they are cosistent. You may have a loose connection that is contributing to that cell reading high.

Hey Ampster can I rely on the internal resistance readings from the Orion BMS utility or do I need to disconnect and put a meter on my two offending cells. I sanded all terminals, buss bars and washers and then cleaned with alcohol.

 

[Ampster]

According to Ohms Law a 25 Watt resistor will burn off 1/2 a Watt. The time might be measured in hours.
54.8 pack voltage is better than I am getting without seeing some runners. That could be 90% SOC or more. My Constant Voltage setting is 54.4 volts.

 

[Ampster]

can I rely on the internal resistance readings from the Orion BMS utility

I only use them to see inconsistencies. They should all be close. I actually don't understand the physics, but things got better when I tightened ones with inconsistent readings. I was also inconsistent on cleaning.

 

[Luthj]

If you have major variance in the IR readings on the Orion BMS, that indicates connection issues. Debris, loose connections, unflat bus bars, burrs, etc. A torque wrench and studs/nuts are the best approach for consistent connections. 20% is fine for lower C rates, and 50% or greater will cause voltage spikes during charging.

Only use very fine (600+ grit) sandpaper on the terminals. You are wanting to polish, not scratch.