Passive Top Balancing ? How/Why does it work ?

[lupo]

just to prove that there will be current flowing if the voltage is different:

@2Big2B - maybe we can simplify it. I also performed a test you may be interested in.

If you get your nearly half-discharged cells freshly shipped out of the box, and put them together in parallel, you'll be "mid-balanced" after the small amount of current flows between each cell. But as we know, in the flat part of the curve, this means nothing. From a voltage standpoint, one cell could be at 80% capacity, and another at 30% despite the voltages appearing to be the same when you take them apart and measure again! (unless you have lab equipment)

Practically - mid balanced by paralleling means nearly nothing useful in the real world.

yup, I encountered exactly this yesterday. I found this page https://diysolarforum.com/ewr-carta/topbalancelfp/ which describes the passive/mid balancing for new cells that are matched. I trusted the page and tried this approach yesterday (I also published a video about it (see link in the signature)) with new cells that were in range of <10mV difference. I ended up with the BMS disconnecting while charging due to one cell reaching 3.7V. The others were at around 3.31, so the pack was definitely not fully charged. My guess is that the SoC was different and not matched, but I am still verifying.

 

[upnorthandpersonal]

I found this page https://diysolarforum.com/ewr-carta/topbalancelfp/ which describes the passive/mid balancing for new cells that are matched. I trusted the page and tried this approach yesterday (I also published a video about it (see link in the signature)) with new cells that were in range of <10mV difference. I ended up with the BMS disconnecting while charging due to one cell reaching 3.7V.

I don't believe that page describes passive/mid balancing - but that paragraph was confusing. I put that paragraph in strikethrough.

 

[Supervstech]

I completely understood the initial question in this post, because I I was told this "passive" method would work - but I don't think it ever actually did. When I built my 280ah lifepo4 battery from 4 EVE cells back in February, I didn't have access to a power supply that wouldnt take weeks to charge/balance the cells (which was unrealistic while traveling)... So I did the "passive" method I was told would work - and even though I had a feeling it didn't, I let that suffice and have been using the battery for the last few months.

I hope no one minds if I squeeze in this question here, but - my 4 cells are currently all very low and uneven (I've been in cloudy/rainy weather for a while), and right now I do have access to a power supply with high amperage and I have the cells connected in parallel, ready to hook them up to the power supply at 3.65V. Am I correct in understanding that doing this will both a) fully charge, and b) successfully top balance all 4 cells?

Thanks for any answers here, and for the side-bar ??

Yes

 

[RyanNicholas]

Yes .... just make absolutely sure you set the voltage while not connected to the battery .... and verify the voltage with a DVM ... Then connect it to the cells and don't touch anything on the power supply after that. If there is a current adjustment, I would set it before the final voltage adjustment.

Disconnect the power supply and Leave them in parallel for 24 hrs or so after the current has decreased to near zero.

Hey, maybe you can advise me on something here. I'm puzzled about the experience of having with this power supply. I set it at 3.65 volts. Turns out it's only a 10 amp power supply (not the higher amperage we thought it was) - but with 280 amp hour cells, it shouldn't have taken much more than a day to fully charge them, right? The amperage has been reading just over 10 the whole time, but I've had them plugged in for a couple of days and the amps still haven't dropped yet. One puzzling thing, though, is that the voltage is not properly at 3.65. It was like this at first, then I left it overnight and in the morning the other day it had reached 3.65 so I figured it was just an issue of needing to sit longer to find the right voltage. But since the current had not dropped, I let it keep running until we left the shop for the day. Then I turned it off overnight, turned it back on yesterday when we got back to the shop, and the volts were back down a little... so I left it running overnight again, thinking the volts would be correct this morning (or the amps would have dropped to zero because it finished charging), but it still reads as the picture shows here. Can you please help me understand what's wrong here? I'm feeling confused and concerned...

[edit: I just disconnected and reconnected it, and it went back up to 3.65... But still running full at 10+ amps.]

 

[Hedges]

4 cells in parallel x 280 Ah/cell = 1120 Ah
1120 Ah / 10A charge = 112 hours (if initially 0% SoC)
If 50% SoC, then 56 hours or a bit more than 2 days to charge them.

Got a volt meter? measure voltage at cells. Measure voltage drop from supply positive terminal to battery positive, supply negative to battery negative.

 

[lupo]

Hey, maybe you can advise me on something here. I'm puzzled about the experience of having with this power supply. I set it at 3.65 volts. Turns out it's only a 10 amp power supply (not the higher amperage we thought it was) - but with 280 amp hour cells, it shouldn't have taken much more than a day to fully charge them, right? The amperage has been reading just over 10 the whole time, but I've had them plugged in for a couple of days and the amps still haven't dropped yet. One puzzling thing, though, is that the voltage is not properly at 3.65. It was like this at first, then I left it overnight and in the morning the other day it had reached 3.65 so I figured it was just an issue of needing to sit longer to find the right voltage. But since the current had not dropped, I let it keep running until we left the shop for the day. Then I turned it off overnight, turned it back on yesterday when we got back to the shop, and the volts were back down a little... so I left it running overnight again, thinking the volts would be correct this morning (or the amps would have dropped to zero because it finished charging), but it still reads as the picture shows here. Can you please help me understand what's wrong here? I'm feeling confused and concerned...

[edit: I just disconnected and reconnected it, and it went back up to 3.65... But still running full at 10+ amps.]

just some wild guesses: the CC (constant current) led is on, right? usually CV (constant voltage) makes more sense. Have you double checked if the shown amp is the max or the current flow?

 

[Gazoo]

just some wild guesses: the CC (constant current) led is on, right? usually CV (constant voltage) makes more sense. Have you double checked if the shown amp is the max or the current flow?

The PS should be in CC mode until the voltage reaches or is close to 3.65 volts. Then the PS should switch to CV mode and the amps going into the cells will begin to drop.

 

[RyanNicholas]

4 cells in parallel x 280 Ah/cell = 1120 Ah
1120 Ah / 10A charge = 112 hours (if initially 0% SoC)
If 50% SoC, then 56 hours or a bit more than 2 days to charge them.

Got a volt meter? measure voltage at cells. Measure voltage drop from supply positive terminal to battery positive, supply negative to battery negative.

Aahh, OK... I forgot the amp hours add up when they're in parallel, unlike when they're in series. So that day will be more like 4 or 5 days. That explains it. Thank you!

 

[Bob B]

Hey, maybe you can advise me on something here. I'm puzzled about the experience of having with this power supply. I set it at 3.65 volts. Turns out it's only a 10 amp power supply (not the higher amperage we thought it was) - but with 280 amp hour cells, it shouldn't have taken much more than a day to fully charge them, right? The amperage has been reading just over 10 the whole time, but I've had them plugged in for a couple of days and the amps still haven't dropped yet. One puzzling thing, though, is that the voltage is not properly at 3.65. It was like this at first, then I left it overnight and in the morning the other day it had reached 3.65 so I figured it was just an issue of needing to sit longer to find the right voltage. But since the current had not dropped, I let it keep running until we left the shop for the day. Then I turned it off overnight, turned it back on yesterday when we got back to the shop, and the volts were back down a little... so I left it running overnight again, thinking the volts would be correct this morning (or the amps would have dropped to zero because it finished charging), but it still reads as the picture shows here. Can you please help me understand what's wrong here? I'm feeling confused and concerned...

[edit: I just disconnected and reconnected it, and it went back up to 3.65... But still running full at 10+ amps.]

What they said ..... It depends on where you check the voltage. The power supply looks like it is dropping to 3.46 when you connect it .... and that is normal.
Kinda surprised you can sustain that amount of current with those clip leads .... they must have really strong springs.
Looks like you are doing it right and are in the .... just have patience phase.

 

[RyanNicholas]

4 cells in parallel x 280 Ah/cell = 1120 Ah
1120 Ah / 10A charge = 112 hours (if initially 0% SoC)
If 50% SoC, then 56 hours or a bit more than 2 days to charge them.

Got a volt meter? measure voltage at cells. Measure voltage drop from supply positive terminal to battery positive, supply negative to battery negative.

OK, I'm still puzzled because this thing has been connected for well over four days and the amperage still has not dropped. And, oddly enough, the voltage has risen to 3.66 instead of 3.65 where it's been for days. That's not bad, is it? How long should I let this thing sit on this charger before I start to worry? I mean the amperage will drop naturally, right? I'm not going to somehow overload the cells, am I? Help ?

 

[Gazoo]

What is the voltage at the cell terminals? The voltage at the cell terminals could be substantially lower due to voltage drop.

 

[RCinFLA]

Just passively paralleling cells will not balance them. Even if cells are perfectly matched once they get within 20-40 mV there will be little current exchange.

All batteries, including LFP require over-potential for current to flow in or out of cell. LFP just require less over-potential for given current flow compared to other chemistries.

 

[Hedges]

3.66 vs. 3.65 is 0.3% higher, could easily be drift of parts in the power supply.
But to be safe, disconnect leads for a moment and make sure it doesn't shoot up to 4V or something (like if the knob got bumped.)

Maybe you're almost there, right at the corner of setting with both CC and CV, about to have current start decreasing.

 

[RyanNicholas]

What is the voltage at the cell terminals? The voltage at the cell terminals could be substantially lower due to voltage drop.

The terminals individually, both while connected and when disconnected from each other (I just checked), all read 3.31. The power supply says it is sending 3.66, but with everything connected I checked the terminals and they read 3.31, oscillating to 3.32.

 

[RyanNicholas]

What is the voltage at the cell terminals? The voltage at the cell terminals could be substantially lower due to voltage drop.

Hey, thanks for all the sidebar engagement in this thread, but I have another quick question. I'm feeling a little concerned because, after having to scoot the whole setup over a foot or so, several minutes later I walked by and saw the voltage was at 0.28. I immediately wondered why, then noticed that the wire had come out of the positive contact clip and was sitting over where it was actually touching the negative bus bar! So I guess that means that the 0.28 voltage was actually happening across all the negatives of the parallel set up. Do you think this did any damage to my cells?? I took everything apart and checked the individual cell voltages and they all match and they seem fine. But I don't know how else to determine if this mishap caused any damage.

 

[Gazoo]

Hey, thanks for all the sidebar engagement in this thread, but I have another quick question. I'm feeling a little concerned because, after having to scoot the whole setup over a foot or so, several minutes later I walked by and saw the voltage was at 0.28. I immediately wondered why, then noticed that the wire had come out of the positive contact clip and was sitting over where it was actually touching the negative bus bar! So I guess that means that the 0.28 voltage was actually happening across all the negatives of the parallel set up. Do you think this did any damage to my cells?? I took everything apart and checked the individual cell voltages and they all match and they seem fine. But I don't know how else to determine if this mishap caused any damage.

If you had a dead short across the cell terminals then there wouldn't be any wires left. And if the cell voltage had dropped to .28, you would not be seeing normal cell voltages. All your cells still read 3.31 volts right?

That clip might have had a bad connection. If all connections are good then the voltage of the cells should be higher with the power supply showing 3.66 volts and still putting out 10 amps. Did you set the voltage of the power supply before connecting it to the cells?

 

[Hedges]

I think that was 0.28V drop from supply, out one wire, down the busbar, back in the other. All you did was put amps through the busbar. Just like when charging. No harm.

 

[RyanNicholas]

These cells have now been connected to this 10 amp charger for over a week. 10 days, actually. In that 10-day period, there were either two or three nights during which the power supply was turned off, but the rest of the time it's been on and connected day and night. That means they've had almost double the 4.5 or so days it should have taken to fully charge them, even from a 0% SoC. I'm feeling a bit concerned about all of this. I just disconnected everything and the voltages on each cell are pretty close to the same (*3.35 to 3.37 - not exactly balanced). I really hope my battery cells are OK.

I also just started a new Thread about this:

Why are my cells still charging??

I have had my 4 280ah EVE lifepo4 cells hooked up to a 10 amp bench power supply for over a week now, and the amperage still has not dropped. They've been connected for about 10 days. On two (or possibly three) different nights, the power supply was turned off- but it has been on day and night...

diysolarforum.com

 

[upnorthandpersonal]

Disconnect them and do them one at a time. That way you know for sure if there is a bad cell or something.

 

[Gazoo]

3.5 to 3.7 - not exactly balanced

But in the other thread you posted They are all reading from 3.35 to 3.37. Is that what you meant?