Help with low charge current on new eve 280 cells

[radioknow]

Hi,
I've recently received 16x LF280K (Grade B). Individual cell voltages and polarities were all 3.27 - 3.28 volts with a multimeter. Yesterday I connected them in parallel for top balancing.

I bought a 30V 10A Bench power supply (https://www.amazon.co.uk/gp/product/B083BVGRLN/)

Set the voltage to 3.65V but only saw a very low charge current of a few amps.

I was expecting the PSU to max out at 10A (or limit it to 8A), so I took apart the pack to test the cells individually.

I've connected each cell to PSU individually and they are only pulling 3A or less at a charge voltage of 3.65V

Could they be bad cells? or more likely what am I doing wrong?

Thanks for any advice!

 

[time2roll]

As a brief test turn the voltage up and verify current increases. Just a quick test that the settings are not limiting the amps. Disconnect and reset voltage before continuing to charge.

Then make custom leads with #10 wire and ring/spade terminals. Put the skinny wire and alligator clips in a storage drawer.

 

[radioknow]

Increasing the voltage 0.1v increases the current to 3.6A - I could be wrong but was expecting a much higher current, maybe the cells are at a higher soc than I thought.

I'll make some new leads up tomorrow.

 

[brum]

Measure the voltage at the cell terminals. I bet it will be ~3.3V, not 3.75V.

First - your wires are tiny. And second - the "crocodile clip is not making sufficient contact for that current. Go for 10AWG (6mm2) wires with crimped terminal and you'll see significant improvement.

Missing such an obvious issue raises concerns if you have sufficient knowledge to build a LiFePo4 battery. The improper build will lead to poor performance. There is a serious risk of putting things on fire. 16 280Ah cells are 15kWh of power - this is not a toy. Read more, ask in advance and then proceed with the experiments.

 

[Substrate]

I've recently received 16x LF280K (Grade B). Individual cell voltages and polarities were all 3.27 - 3.28 volts with a multimeter. Yesterday I connected them in parallel for top balancing.

Set the voltage to 3.65V but only saw a very low charge current of a few amps.

STOP. You are being pulled over by the battery-police.

HOW did you set this voltage? Did you rely solely on the charger's voltage display, or did you set the voltage as measured on the clamps with your *multimeter* AND with batteries disconnected? (the latter is how it is done)

I've connected each cell to PSU individually and they are only pulling 3A or less at a charge voltage of 3.65V

That is normal. What will NOT be normal, is if you have changed your voltage after you have set it with your multimeter, to try and enable more current to flow from the charger.

This temptation is common - what happens is that at the very very end of charge, while you are not looking, that voltage from the supply shoots up much further than what you set it at had you left it alone, and tapped foot at the lower charge current. And now you have a puffy bloat, or an accordion on the bench if you've strapped them together.

So just a warning from the battery police to make sure you aren't doing something shady (but common) and doing it right instead.

 

[Northernchateau]

I will suggest, you connect them in series, with a BMS and charge them with something with more then 10 amps.
When you get them close to 100% charged, stop and connect them parallel and top balance to 3.65 volts.
The way you have them now can take many moons. Because the curve is so flat with LifePO4 batteries right now you really do not know the state of charge.

 

[time2roll]

I suggest put them in series with the BMS and put them in service. Limit the charging voltage to avoid over volt at the cell level. If the cells are way out get an active balancer to speed up the process. In the mean time all is up and running even if at reduced capacity until the balance completes.

 

[radioknow]

All are working fine with thicker leads! For future readers, I was just going off the PSU display voltage at 3.65V. As the replies above said there was enough resistance / voltage drop on the thin leads to reduce the voltage at the cell terminals to around 3.3v, so therefore it wasn't taking much current.

Thanks for the advice all.

 

[Just John]

Hi,
I've recently received 16x LF280K (Grade B). Individual cell voltages and polarities were all 3.27 - 3.28 volts with a multimeter. Yesterday I connected them in parallel for top balancing.

I bought a 30V 10A Bench power supply (https://www.amazon.co.uk/gp/product/B083BVGRLN/)

Set the voltage to 3.65V but only saw a very low charge current of a few amps.

I was expecting the PSU to max out at 10A (or limit it to 8A), so I took apart the pack to test the cells individually.

I've connected each cell to PSU individually and they are only pulling 3A or less at a charge voltage of 3.65V

Could they be bad cells? or more likely what am I doing wrong?

Thanks for any advice!

Like everyone else with the same problem, you need better cables. 10 gauge wire with ring terminals. FYI, never try to run current through the studs.

 

[Just John]

All are working fine with thicker leads! For future readers, I was just going off the PSU display voltage at 3.65V. As the replies above said there was enough resistance / voltage drop on the thin leads to reduce the voltage at the cell terminals to around 3.3v, so therefore it wasn't taking much current.

Thanks for the advice all.

FYI, the resources section here has excellent directions. It also includes how to get the cells almost full 4 times faster using a BMS (section 3).

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...

diysolarforum.com

 

[brum]

FYI, never try to run current through the studs.

Why?

I was planning to put longer studs and have the following layers: cell terminal, busbar, nut, balancing lead, nut. But if there is a reason not to run current through the stud I will use a different approach.

 

[time2roll]

The typical stainless does add some resistance. Same for the nut and any washer. I drilled and tapped my bars for the balance lead to keep the stack to a minimum.

 

[brum]

If this is the only reason then it is not a problem for balancing leads. All leads will see the same resistance increase. This would be comparable to let's say 5cm longer leads.

 

[Martijn]

Why?

I was planning to put longer studs and have the following layers: cell terminal, busbar, nut, balancing lead, nut. But if there is a reason not to run current through the stud I will use a different approach.

This is frowned upon. Even though the balance lead will only have a small current, putting it on top of a nut instead of on the busbar, will add resistance. If you're worried about the nut on the busbar loosening up ( I was ), then use a nylon locking nut.

Cell terminal > busbar > balance lead > ring washer > locking nut

 

[Just John]

Why?

I was planning to put longer studs and have the following layers: cell terminal, busbar, nut, balancing lead, nut. But if there is a reason not to run current through the stud I will use a different approach.

Always have the highest current directly in contact with the cell terminal (about 100% of the time, this is the busbar). Then next highest, etc.

The stainless steel stud is not the terminal, don't use it for charge or discharge (like clipping alligator leads to for top balancing). You are just adding unnecessary resistance into the mix. Best connect a busbar, torque it down properly, then use the alligator clip on the busbar.

Better yet, build real leads from 10 gauge wire (cheap and easy to find) with crimp-on ring terminals (again, cheap, easy to find, and WAY superior connection to both power supply and cell). Not doing this is a VERY common problem when trying to top-balance cells.

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