Balancing question - how long?

[stevelex]

Drain them back to 40% if you are going to leave them for a while! They will be much more comfortable and won't degrade - that's why they ship them that way.

Ok, that's interesting. I will do that. I have built the 24v battery and installed the overkill BMS. Show's 100% charge. I may put them into service or just attach a load while they are in the house and push them back down a bit. It's below freezing outside right now so I've avoided putting them into the camper. Going to post another thread with questions on that.

 

[SV_Stray_Cats]

Ok, that's interesting. I will do that. I have built the 24v battery and installed the overkill BMS. Show's 100% charge. I may put them into service or just attach a load while they are in the house and push them back down a bit. It's below freezing outside right now so I've avoided putting them into the camper. Going to post another thread with questions on that.

Here's some documentation for my statement that I should have included in my original comment, but was too lazy. This is a screenshot from the specs/manual for my 202 Ah Lishen cells:


Earlier in my research, I saw graphs in an academic paper that showed storing at even lower SOC resulted in lower degredation. In one case, storing at 0% SOC resulted in zero capacity loss! Maybe that's not recommended for other reasons. But this does underscore how we have to flip our battery care ideas upside down compared to lead acid batteries.

 

[snoobler]

FWIW, this issue is likely less important than it's portrayed on TV...

I fully charged 80-ish 40Ah CALB cells about a year ago. I've recently tested about 10 of them, and they only lost 3% of their charge, and they retained ALL of their capacity. After the year, they were all sitting at 3.31V. They were stored indoors in 70-80°F environment.

Independent testing shows that lower temperatures also slow the degradation, and that same testing showed lower SoC is desirable at higher temperatures. Even 80% vs. 100% made a notable difference.

Lastly, I tested 21 Lithium NMC (maybe NCA) cells for retained capacity. I stored them fully charged for about a year in a shipping container. The interior of the shipping container definitely hit 40°C+. Capacity loss was about 3%. They also experienced temps down into the -10°C range as well. Most of the time was in the middle.

YMMV, but I wouldn't hesitate to ignore the SoC of my LFP cells for 3+ months PROVIDED they were in comfortable to cold temps.

 

[Bluewatersailor]

Just an update: On Saturday sometime the charger showed no current and reached the desired voltage. So that was 5 days. Of course, now it's 23 deg F outside and I'm not going to be working on the camper until it warms up, so the cells are sitting here, charged and balanced, in my kitchen, until then.

We keep reading not to keep cells 100% charged . No one seen to say how long not to keep them at 100% , so are you planning to discharge them some ?

 

[Jim Burrow]

Kind of interesting that everyone says not to keep your battery at a full charge if you are not going to use them right away. Then why do stores have their batteries fully charged sitting on their shelves? you go buy a car battery and it is fully charged - just a curious question, not disputing anyone.

 

[snoobler]

Apples and oranges.

Car batteries are lead-acid. They must be stored at full charge or they deteriorate.

We're talking in the context of LFP chemistry.

 

[Maast]

I've got 8 200ah lifepo4 cells. They are in parallel for top balancing.

I've got a power supply that is pushing amps. It says it is pushing 10 amps @ 3.44 volts (I set a limit of 3.65 volts). That's 34 watts approx.

Actually, its the voltage differential between 3.44 and 3.65v thats moving amperage, effectively you're moving amperage at .21v at 10A, thats only 2.1 watts, yeah, it'll take forever.

 

[Jim Burrow]

Apples and oranges.

Car batteries are lead-acid. They must be stored at full charge or they deteriorate.

We're talking in the context of LFP chemistry.

ok, thanks snoobler. I didn't catch in the questions and answers that anyone was talking specificlly about LFP. Also, I could have had Lead-acid stuck on my mind as I'm using Lead-acid at the moment until I can save up enough money to buy LFP. Being retired and on a fixed income, just can't go plunk down $1600 or more for LFP, and looks like I may need two of those batteries.

 

[snoobler]

ok, thanks snoobler. I didn't catch in the questions and answers that anyone was talking specificlly about LFP. Also, I could have had Lead-acid stuck on my mind as I'm using Lead-acid at the moment until I can save up enough money to buy LFP. Being retired and on a fixed income, just can't go plunk down $1600 or more for LFP, and looks like I may need two of those batteries.

Given that you're on a budget, I assume you're considering DIY?

 

[Bluewatersailor]

So do anyone have an answer , how long you can leave LEP at 100% safety?

 

[snoobler]

So do anyone have an answer , how long you can leave LEP at 100% safety?

There isn't an absolute answer.

You follow the manufacturer's data sheet. I've provided anecdotal evidence that it doesn't matter for my cells in my environment, but brands may vary.

 

[Gazoo]

So do anyone have an answer , how long you can leave LEP at 100% safety?

I am going to attach a graph that's been floated around the forum. Make of it what you will.

 

[stevelex]

We keep reading not to keep cells 100% charged . No one seen to say how long not to keep them at 100% , so are you planning to discharge them some ?

Ok, we had a bunch of warmer weather in early January and I got the 24v battery built and the overkill BMS installed. I hooked up all the electric and the battery is now "in use" - charging from solar and the alternator when I can get them warmed up. They've gone up to 100% and then down into the 80s a few times, but now I think they are now at 53% because my panels are covered in snow and I've been using lots of power tools in the camper. So, they didn't stay at 100% for long. I may hook up the DC power supply at 26 volts to charge them a bit (need to figure out the correct voltage...) or I guess I could push 14v through the B2V charger...not sure on that...probably post a question here on that.

 

[powerB]

I've got a simple 4S 100Ah battery sitting on my table, waiting to be top balanced for the first time. I am a novice when it comes to this stuff, but I've done a lot of reading and researching and have downloaded the Tutorial about balancing LiFePO4 with a 10A power supply. I think I have a good grasp of the important concepts but I am not sure if I'm doing everything right, though...??

The individual cells each measured in at 3.30v and I've hooked them up in parallel. I've decided not to do the pre-charging but instead go straight to the balancing. So I've taken the following steps:
- set the max voltage of the PSU to 3,65v (nothing connected)
- attached the positive (+) and the negative (-) clamps together to then dial in the amps (set to 10A)
- only after that is done clamp to and connect the battery terminals

...the PROBLEM (if it is one): the PSU display shows that the PSU is in the CV (constant voltage) mode, the display shows a permanent 3,65v and the amperage has dropped to around 4,0-4,2A and seems to slowly slowly decrease over time.

Now I'm really wondering whether this is correct?? I thought LiFePO4's needed to be charged up to 3,65v with a constant current?? Whats happening therefore seems to be the exact opposite. Or am I misunderstanding?

Here are three pictures to show whats going on: Step 1: set the voltage to 3,65

Step 2: clamp clamps to each other, as per instructions, and set the max current to 10A

Step3: connect the cell. The voltage displayed remains at 3,65v, the current is ony at 4,377... and the CV light indicates that I'm charging with a constant voltage while I thought I should be charging at a constant current

 

[stevelex]

I've got a simple 4S 100Ah battery sitting on my table, waiting to be top balanced for the first time. I am a novice when it comes to this stuff, but I've done a lot of reading and researching and have downloaded the Tutorial about balancing LiFePO4 with a 10A power supply. I think I have a good grasp of the important concepts but I am not sure if I'm doing everything right, though...??

The individual cells each measured in at 3.30v and I've hooked them up in parallel. I've decided not to do the pre-charging but instead go straight to the balancing. So I've taken the following steps:
- set the max voltage of the PSU to 3,65v (nothing connected)
- attached the positive (+) and the negative (-) clamps together to then dial in the amps (set to 10A)
- only after that is done clamp to and connect the battery terminals

...the PROBLEM (if it is one): the PSU display shows that the PSU is in the CV (constant voltage) mode, the display shows a permanent 3,65v and the amperage has dropped to around 4,0-4,2A and seems to slowly slowly decrease over time.

Now I'm really wondering whether this is correct?? I thought LiFePO4's needed to be charged up to 3,65v with a constant current?? Whats happening therefore seems to be the exact opposite. Or am I misunderstanding?

Here are three pictures to show whats going on:View attachment 37417 Step 1: set the voltage to 3,65

View attachment 37418 Step 2: clamp clamps to each other, as per instructions, and set the max current to 10A

View attachment 37419 Step3: connect the cell. The voltage displayed remains at 3,65v, the current is ony at 4,377... and the CV light indicates that I'm charging with a constant voltage while I thought I should be charging at a constant current

Not an expert, but when I changed my wires to 4awg and lugs that bolted to the battery, the display changed to show voltage below 3.65 and then it rose over time. I really don't know the details on this, and maybe I'm conflating different aspects of the top balancing, but I would recommend making yourself some thick cables with a good connection to your batteries.

 

[Hedges]

Probably series resistance.
You've got 3.65V at some point in the power supply. (less at the supply terminals? check with a meter.)
Skinny leads and alligator clips. Less at he battery terminal. check with a meter.
Try measuring just voltage drop between an alligator clip and the aluminum terminal of the cell. taking 3.xx volts out of what's being measured, you may get millivolt or lower resolution reading of the voltage drop.

 

[Gazoo]

Now I'm really wondering whether this is correct?? I thought LiFePO4's needed to be charged up to 3,65v with a constant current?? Whats happening therefore seems to be the exact opposite. Or am I misunderstanding?

It's most likely the cheap cables and it is a common problem because of the low voltage. You can search for voltage drop on google and you will see what I am getting at. I suggest using 12 AWG cable. Don't use alligator clips and banana plugs. Use ring terminals to connect to the battery and fork terminals to connect to the power supply if you can.

Also if the bolts are loose on your cells then you don't have a good connection. But the bottom of the bolt should never touch the bottom of the cell terminal when it's being torqued because it could puncture through the bottom of the cell terminal. So be careful with that.

 

[DerpsyDoodler]

Step3: connect the cell. The voltage displayed remains at 3,65v, the current is ony at 4,377... and the CV light indicates that I'm charging with a constant voltage while I thought I should be charging at a constant current

the second you “dialed down the amps” you put the supply in cv mode (Or something like that). This is what you want, anyway, since there is no BMS to protect your cells. Listen to what others are saying about your wires and connectors.

 

[powerB]

Listen to what others are saying about your wires and connectors.

It's most likely the cheap cables and it is a common problem because of the low voltage. You can search for voltage drop on google and you will see what I am getting at. I suggest using 12 AWG cable. Don't use alligator clips and banana plugs. Use ring terminals to connect to the battery and fork terminals to connect to the power supply if you can.

Also if the bolts are loose on your cells then you don't have a good connection. But the bottom of the bolt should never touch the bottom of the cell terminal when it's being torqued because it could puncture through the bottom of the cell terminal. So be careful with that.

guys, you were right! I crimped up some thicker cables with ring terminals and immediately the PSU jumped into the C mode! But: it also started singing! It makes a really high pitched sound. Reducing the amps reduces the sound so I am guessing pushing 10A is pushing the PSU to its limits!

Thanks for the advice though! Much appreciated!

 

[Hedges]

Inductors sing, also capacitors and other parts.
I boosted the switching frequency of my VFD from about 4 kHz to 15 kHz which quieted it down.

Glad it's working better.

One more improvement, although pretty minor at 10A with fat busbars, is to connect supply cable to far end of one busbar (opposite corners of the paralleled set of batteries.)
That matters more with parallel strings in the final application, running at high current.

 

[John Frum]

I've got a simple 4S 100Ah battery sitting on my table, waiting to be top balanced for the first time. I am a novice when it comes to this stuff, but I've done a lot of reading and researching and have downloaded the Tutorial about balancing LiFePO4 with a 10A power supply. I think I have a good grasp of the important concepts but I am not sure if I'm doing everything right, though...??

The individual cells each measured in at 3.30v and I've hooked them up in parallel. I've decided not to do the pre-charging but instead go straight to the balancing. So I've taken the following steps:
- set the max voltage of the PSU to 3,65v (nothing connected)
- attached the positive (+) and the negative (-) clamps together to then dial in the amps (set to 10A)
- only after that is done clamp to and connect the battery terminals

...the PROBLEM (if it is one): the PSU display shows that the PSU is in the CV (constant voltage) mode, the display shows a permanent 3,65v and the amperage has dropped to around 4,0-4,2A and seems to slowly slowly decrease over time.

Now I'm really wondering whether this is correct?? I thought LiFePO4's needed to be charged up to 3,65v with a constant current?? Whats happening therefore seems to be the exact opposite. Or am I misunderstanding?

 

[John Frum]

@powerB if the power supply is rated for 10 amps try running it at 8 or even 6.6 amps.
Cheap power supplies may not last long at their rated current.

 

[powerB]

Inductors sing, also capacitors and other parts.
I boosted the switching frequency of my VFD from about 4 kHz to 15 kHz which quieted it down.

Glad it's working better.

One more improvement, although pretty minor at 10A with fat busbars, is to connect supply cable to far end of one busbar (opposite corners of the paralleled set of batteries.)
That matters more with parallel strings in the final application, running at high current.

I've gotto admit that I don't know what "switching the frequency" of a VFD from 4kHz to 15 kHz means... but I have a feeling that would mean opening the divice to make that sort of a change. If the cheap Amazon device I have would even allow for that.

But good point on connecting the leads to the opposite ends of the battery pack! Thanks

 

[Bluewatersailor]

Can I ask why join the leads togather and setting the voltage?
I did mind not so long ago with a similar Power supply ,
I couldn't get the reason why to connect the leads ,
I just connected a meter to the leads , turn the current up a little and set the voltage to read 3.65 at the meter , then connected the leads to the batteries and turned the current up as far as it would go , in my case 5.2A
So why do people connect the leads togather? What different does it make?

 

[powerB]

Can I ask why join the leads togather and setting the voltage?
I did mind not so long ago with a similar Power supply ,
I couldn't get the reason why to connect the leads ,
I just connected a meter to the leads , turn the current up a little and set the voltage to read 3.65 at the meter , then connected the leads to the batteries and turned the current up as far as it would go , in my case 5.2A
So why do people connect the leads togather? What different does it make?

Many roads lead to Rome...
The manual of my power supply unit say to set the values according to those steps. It lets you dial in the voltage and that is reflected on the LCD screen but if you just turn the amperage knob the LCD screen shows no change. Only when you "short" the leads do you see the amperage value as you turn the know. You connecting your multi meter essentially does the same/similar thing