HELP PLEASE I think I may have some bad cells

[Moltensurf]

I think you are mistaken.
The current(amperage) should approach zero as the charge voltager and battery voltage converge typically at 3.65 volts.

You are right that is what I am speaking about the current amperage, it never dropped completely down to zero. It was jumping around all over the place

 

[John Frum]

You are right that is what I am speaking about the current amperage, it never dropped completely down to zero. It was jumping around all over the place

How were you measuring.
A bench charger that could do 30-80 amps would be very rare and expensive.
Do you perhaps mean 30-80 milli-amps?

 

[Fallingaway]

Is the BMS hooked up? Sounds like it is and its stopping the charge to protect 1 or all cells and then turning back on as the voltage drops.

 

[Moltensurf]

How were you measuring.
A bench charger that could do 30-80 amps would be very rare and expensive.
Do you perhaps mean 30-80 milli-amps?

How were you measuring.
A bench charger that could do 30-80 amps would be very rare and expensive.
Do you perhaps mean 30-80 milli-amps?

How were you measuring.
A bench charger that could do 30-80 amps would be very rare and expensive.
Do you perhaps mean 30-80 milli-amps?

Is the BMS hooked up? Sounds like it is and its stopping the charge to protect 1 or all cells and then turning back on as the voltage drops.

I never hooked up the bms. I tested the cells individually after top balancing them. The cells are way short on amp hour capacity. I opened up two disputes.

 

[Fallingaway]

What are you using to test the cells individually?

 

[Fallingaway]

What’s “way short” too?

 

[Dzl]

I never hooked up the bms. I tested the cells individually after top balancing them. The cells are way short on amp hour capacity. I opened up two disputes.

I think you need to slow down and regroup, and approach more systematically and slowly/deliberately.

Speaking for myself (and from context I think others too) its hard to follow some of what you are saying possibly because it seems there are some misunderstandings on the fundamentals (units, process, amps vs volts, series vs parallel, etc).

Mentally, it would be beneficial probably to dismiss the feeling of urgency/fear, if your cells are all currently at 3.3V you are okay there is no urgency to do anything. As others have said. Hit the pause button until you are confident you understand what you are doing and have the proper tools to do it. The cells are quite happy at their current voltage. Don't overstress.

Questions that have been asked that need answering:
1. Explain what you mean when you say current was jumping all over the place from 30 to 80. Is this milliamps? In what context and with what charger did this occur?

2. When you say "the cells were way short on amp hour capacity" how short are we talking (what are the numbers) and how did you test/verify?

3. You have a 12V charger and a 24V BMS and battery, why? Is the 12V charger just something you had laying around, is a 24V charger a planned future purchase? I would not charge in series until you have a charger that will work with your BMS.

 

[Fallingaway]

Excellent response! OP is very confusing, mixing series and parallel, V and A. From the picture of the test and assuming he tested in the 3.3v range. The fact that you got 130ish amps is encouraging that it’s not the cells that are the problem. I think once he figures out what he’s doing everything will be fine. This community is priceless! So many good people willing to help!

 

[Moltensurf]

What’s “way short” too?

I am getting 133-139 amp hours running a test at 20 amps continuous load. I am using a little mini heater.

 

[Moltensurf]

Excellent response! OP is very confusing, mixing series and parallel, V and A. From the picture of the test and assuming he tested in the 3.3v range. The fact that you got 130ish amps is encouraging that it’s not the cells that are the problem. I think once he figures out what he’s doing everything will be fine. This community is priceless! So many good people willing to help!

The cells are rated 200 amp hours. The test the manufacturer used was taking the batteries down to 2.5 V.

 

[snoobler]

The cells are rated 200 amp hours. The test the manufacturer used was taking the batteries down to 2.5 V.

At 20A/3V, you are likely getting premature cut off due to excessive voltage drop. Your connections should be rock solid, and there's resistance in the tester itself. The tester measures the loaded voltage. You care about open circuit voltage.

Run another test. After letting it run for 60 seconds, measure the voltage with a meter AT THE CELL and compare it to the tester readout.

If the difference is 0.3V, then you should run the TESTER voltage to 2.2V rather than 2.5V.

 

[Fallingaway]

From the OP’s original posts it sounds like he’s having problems actually charging the cells and running capacity tests in the 3.3v range.

 

[Just John]

The cells are rated 200 amp hours. The test the manufacturer used was taking the batteries down to 2.5 V.

First, don't assume what state the cells are in (charged, discharged, or anything else). They need to be fully charged before you can capacity test them. You do this by hooking up a power supply and charging them at exactly 3.65 volts until current going into them drops below 1 amp.

Second, don't be in a hurry, this is a time consuming process.

That little heater you are using, has some known problems. If you run it at 20 amps, you will notice it gets smoking hot, not underneath the heatsink, but right next to the MOSFET is one or two diodes. The one I measured gets over 100 degrees C when running at 20 amps, I highly recommend 10 amps (since these testers also have a habit of letting out magic smoke and creating a dead short). This means 20 hours per cell (did I mention patience?)

Unless you have a BMS, I recommend charging each cell separately, everyone ALWAYS says "I'll pay attention and stop", then we hear about "it was only a few minutes". If you have a BMS and it is set up properly, then you can charge them as a pack until the BMS stops you, that does NOT mean the cells are fully charged, it means A SINGLE CELL is fully charged. So, you will need to then take apart the pack, and charge each cell up to 3.65 volts anyway (since that is the spec for capacity testing). Charging as a pack simply makes it easier and faster to put in a bulk charge, you will have to fill each cell up individually anyway. The specification also says discharge until the cell is at 2.5 volts (that tester has of course voltage drop across the wires you are using, so when it says 2.5 volts on the tester, the cell is actually at 2.7 volts). There is so little capacity at that voltage, I wouldn't even bother to account for that to be honest.

Please explain what you have to charge these with, is it a 10 amp adjustable bench supply like most people use?

I wouldn't go to the trouble of top balancing them yet, since you seem to be concerned you might have gotten a bad cell or two. Plenty of time for that once you have verified capacity (and having the cells empty makes it faster to charge them up as a pack to get ready for top balancing). Putting 10 amps into a 12 or 24 volt pack means 120 or 240 watts, but 10 amps into a 3.65 volt cells is 36.5 watts. This means you can charge 4 or 8 cells from fully discharged in the same time as filling a single cell, but it really only works well if the cells are in the same state of charge.

It may very well be easiest to capacity test them as a pack, but first you need to get all cells at the same state of charge (and from your descriptions so far, nobody has a clue what state they are in).

 

[Moltensurf]

First, don't assume what state the cells are in (charged, discharged, or anything else). They need to be fully charged before you can capacity test them. You do this by hooking up a power supply and charging them at exactly 3.65 volts until current going into them drops below 1 amp.

Second, don't be in a hurry, this is a time consuming process.

That little heater you are using, has some known problems. If you run it at 20 amps, you will notice it gets smoking hot, not underneath the heatsink, but right next to the MOSFET is one or two diodes. The one I measured gets over 100 degrees C when running at 20 amps, I highly recommend 10 amps (since these testers also have a habit of letting out magic smoke and creating a dead short). This means 20 hours per cell (did I mention patience?)

Unless you have a BMS, I recommend charging each cell separately, everyone ALWAYS says "I'll pay attention and stop", then we hear about "it was only a few minutes". If you have a BMS and it is set up properly, then you can charge them as a pack until the BMS stops you, that does NOT mean the cells are fully charged, it means A SINGLE CELL is fully charged. So, you will need to then take apart the pack, and charge each cell up to 3.65 volts anyway (since that is the spec for capacity testing). Charging as a pack simply makes it easier and faster to put in a bulk charge, you will have to fill each cell up individually anyway. The specification also says discharge until the cell is at 2.5 volts (that tester has of course voltage drop across the wires you are using, so when it says 2.5 volts on the tester, the cell is actually at 2.7 volts). There is so little capacity at that voltage, I wouldn't even bother to account for that to be honest.

Please explain what you have to charge these with, is it a 10 amp adjustable bench supply like most people use?

I wouldn't go to the trouble of top balancing them yet, since you seem to be concerned you might have gotten a bad cell or two. Plenty of time for that once you have verified capacity (and having the cells empty makes it faster to charge them up as a pack to get ready for top balancing). Putting 10 amps into a 12 or 24 volt pack means 120 or 240 watts, but 10 amps into a 3.65 volt cells is 36.5 watts. This means you can charge 4 or 8 cells from fully discharged in the same time as filling a single cell, but it really only works well if the cells are in the same state of charge.

It may very well be easiest to capacity test them as a pack, but first you need to get all cells at the same state of charge (and from your descriptions so far, nobody has a clue what state they are in).

I do have a 10a bench charger.
i am charging a single cell at the moment its at 3.3 iam charging it at 3.65
When it gets fully charged I am going to turn the heater down to 10 A And see if I can pull more amp hours out of a cell.

 

[John Frum]

I do have a 10a bench charger.
i am charging a single cell at the moment its at 3.3 iam charging it at 3.65
When it gets fully charged I am going to turn the heater down to 10 A And see if I can pull more amp hours out of a cell.

Remember to the compute the voltage offset as @snoobler mentioned ^.

 

[Just John]

Remember to the compute the voltage offset as @snoobler mentioned ^.

Yes, in the past 15 capacity tests I've run, that will get you an extra .1 amp hour measured capacity (max). I wouldn't bother, but of course to be accurate, 3.65 and 2.5 measured at the cell are the correct voltages.

 

[Just John]

I do have a 10a bench charger.
i am charging a single cell at the moment its at 3.3 iam charging it at 3.65
When it gets fully charged I am going to turn the heater down to 10 A And see if I can pull more amp hours out of a cell.

Yes, be sure and disconnect the cell, measure the output of the power supply and set the voltage to exactly 3.65 (or as close as it will let you get). Then hook it up without changing the voltage and charge. The closer the cell gets to 3.65, the less current the power supply/charger will put out. It is very time consuming. When it drops to below one amp output, you are done. I've got a better supply that automatically shuts off when current drops below .1 amp, but I wouldn't bother with a cheap supply, you again might get an extra .1 amp hour capacity.

 

[Dzl]

I am getting 133-139 amp hours running a test at 20 amps continuous load. I am using a little mini heater.

For the sake of clarity could you explain the steps you took for your first round of testing. e.g.:
1. When I recieved the cell voltages were ___
2. I charged ___ # cells in [parallel/series/individually] until voltage was ____ and current was ____
3. Then I discharged to a voltage of ____ measured by ____
4. The capacity was ____

 

[Fallingaway]

Wouldn’t there be some big efficiency problems running that heater off of one cell through a dc-dc booster? I’d imagine you’d get a better capacity test from a cell configuration that was closer the V requirement of the heater. Not only that but you could test your BMS at the same time and have protection for your cells.

 

[John Frum]

Wouldn’t there be some big efficiency problems running that heater off of one cell through a dc-dc booster? I’d imagine you’d get a better capacity test from a cell configuration that was closer the V requirement of the heater. Not only that but you could test your BMS at the same time and have protection for your cells.

You could do this but assuming a perfect top balance then you only get the capacity of the weakest cell.
If the top balance is bjorked then all bets are off.