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LiFePo4 Charge drop after charging

TankFish

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Feb 2, 2021
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Hello everyone!

Sorry if this has been asked a thousand times before. I've been searching online and the forums and couldn't find my question or missed it.

I have 16 brand new 280ah cells. When they arrived they were all 3.29v to 3.3v. I charged two sets of 8 in parallel that I will be making 24v packs out of to 3.65v. After days of charging, my packs were finally at 3.65v and I let them sit in parallel overnight.

Now for my question. Do cells normally drop voltage with no load or anything connected? In the parallel, they dropped to 3.61v over night. I charged them back to 3.65v and then disconnected them from each other. They were all 3.65v and I let them sit separated over night. This morning, each cell is around 3.58v. Is it normal for them to drop this way?

My main concern is the Overkill Solar BMS's I ordered say specifically to charge each cell and top balance them at 3.65v before putting them in a series to 24v and setting up the BMS. They drop that 3.65 fairly quickly it seems. Do I need to keep charging them and letting them sit overnight until they hold the 3.65v or should I just go ahead and hook up the BMS? I want to take good care of these and have them last. So, I want to do it as right as possible. Any help is always appreciated. Thanks!
 
One of the Best Explanations you will will find here

About mid way down the page in the "Top Balancing" Section

The cells should hold above 3.50V for at least 30 minutes (after charge is disconnected). If not, bring them up again and hold them for a little longer until they do. At 3.60V, you may need to insist a little more than if using 3.65 or 3.70 volts; that’s all.
 
Yes. The normal resting voltage is about 3.35 volts per cell. No capacity is lost when they settle to that voltage although it might take a day or two for them to settle. Enjoy your new pack.
Good info to keep in mind thank you!
One of the Best Explanations you will will find here

About mid way down the page in the "Top Balancing" Section
I'll make my way over there now and have a read. Thank you!
 
Had the same question and here's the answer! Now I won't get yelled at for not searching before asking. Charged 16P x 150Ah cells to 3.6 and they've dropped to 3.53 12 hours later
 
Another question....while I wait for my BMS, I've disconnected the cells and 15/16 stays around the same voltage (3.45) and one is down to 3.4 - should this be of concern?
 
Another question....while I wait for my BMS, I've disconnected the cells and 15/16 stays around the same voltage (3.45) and one is down to 3.4 - should this be of concern?

Not necessarily, keep track of that cell during use. I would leave cells connected to parallel until final assembly
 
It is normal for cells to drop from 3.65v topping charge to 3.45v to 3.55v no load equilibrium voltage.

There are a couple reasons for it. First is transitioning from over-potential kinetics voltage that is required to drive the charging current flow dropping to no current equilibrium state terminal voltage. This levels out in a few minutes depending on absorb current level when charge was terminated. Second is surface charge built up on internal layers, primarily negative anode. It is like a supercap but for LFP cells the total charge capacitance is fairly low, amounting to about 0.01% of cell capacity rating (28 mAH for 280 AH cell). It can be discharged in about 30 seconds with a 1 ohm power resistor load. If not discharged it can give up to a few tenths of a volt cell adder that can take several days to leak off if cell left unloaded.

Beyond that there is normal self leakage rate. 1.5% to 5% capacity per month depending on age of cell and storage temperature. Rate is initially higher due to higher post charge cell voltage.

Leakage vs temp.png
 
Hello everyone,

Bumping this thread, I have a situation here...

I have hooked 3 Renogy batteries in parallel 12v. I have charged all of them separatley to 14.4 absord 1 hour, and float 13.6. Left them to rest 1-2 days before paralleling them, and they were pretty within 0.2v, 13.25,13.25,13.49. Not sure why the 2 batteries droped to 13.25, isn't this a little low?

Now I have charged them all together to 14.4 absorb again for 1 hour, float at 13.6 for an hour and left them for 3-4 days to rest, 0w load, disconnected the switch, the only thing I can relate to be connected is the victron smart shunt that has a blue LED blinking, I have connected that before the on/off switch.

When I measured with the multimeter, the voltage was 13.25...seems a bit low for a fully charged lifepo4 that has had 0 load and sat for 3-4 days.

Should I try to charge for a day at 14.4v absorb to wake up and sync the cells or not a good idea for a long time to charge at 14.4v...

Thank you guys!
 
Not sure why the 2 batteries droped to 13.25, isn't this a little low?
I know you bumped this thread but did you read it? As mentioned many times it is normal for Lithium to settle. Perhaps it is a little low at 3.31 per cell versus 3.32 or 3.33 that some people report. What do the Renogy specs say about the normal resting voltage?
It is not likely that you have lost capacity but to put your mind at ease, do a capacity test.
 
I know you bumped this thread but did you read it? As mentioned many times it is normal for Lithium to settle. Perhaps it is a little low at 3.31 per cell versus 3.32 or 3.33 that some people report. What do the Renogy specs say about the normal resting voltage?
It is not likely that you have lost capacity but to put your mind at ease, do a capacity test.
Hey there,

Yes I read it twice and sorry if my question is useless, but I'm just wondering 13.25 is a bit low, following this image as a general rule:

1674579649552.png

In my mind is that reaching float at 13.6, the battery should stay at 13.6 when resting, which is 100% SOC from this image, 13.25 is somwhere between 70%-90%.

Renogy does a very poor job in putting out battery specs, so resting voltage I couldn't find in the manual.

How should I perform a capacity test, just our of curiosity?

Also found this online - A 12v lithium LiFePO4 battery fully charged to 100% will hold voltage around 13.3-13.4v, if that's true than my 13.25 (0.05 belo 13.3 fully charged) is really the LED from the shunt? That statment there is very different from the image above though...
 
If LFP cell is fully charged they should not drop below an equalization (rested, no load) voltage of 3.43 vdc. That is 13.72vc for four cells.

You did not state the charging current but likely you did not fully charge batteries.

The 13.72v for battery assumes all cells are fully charged and the four cells are balanced. If cells are not balanced you might get a BMS cell overvoltage shutdown. You have to allow BMS to bleed high SoC cell and reset itself to resume charging.

Common issue with these self contained 12v LPF batteries is the user does not do enough balancing charging which requires about 14.2v absorb charging. Time to hold 14.2v depends on how much balancing is required to level the SoC of the four cells. It can require an hour or two normally, or up to a week or two if cells are severely out of balance due to not being allowed to balance for months.

BMS does not balance a cell until it gets above 3.4v. Less than 1% difference is SoC can get you BMS cell overvoltage shut downs. BMS bleed current is likely between 50 mA and 150 mA. 1% cell SoC misbalance on 100AH battery with 100 mA balance bleed takes 10 hours to rebalance.
 
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If LFP cell is fully charged they should not drop below an equalization (rested, no load) voltage of 3.43 vdc. That is 13.72vc for four cells.

You did not state the charging current but likely you did not fully charge batteries.

The 13.72v for battery assumes all cells are fully charged and the four cells are balanced. If cells are not balanced you might get a BMS cell overvoltage shutdown. You have to allow BMS to bleed high SoC cell and reset itself to resume charging.

Common issue with these self contained 12v LPF batteries is the user does not do enough balancing charging which requires about 14.2v absorb charging. Time to hold 14.2v depends on how much balancing is required to level the SoC of the four cells. It can require an hour or two normally, or up to a week or two if cells are severely out of balance due to not being allowed to balance for months.

BMS does not balance a cell until it gets above 3.4v. Less than 1% difference is SoC can get you BMS cell overvoltage shut downs. BMS bleed current is likely between 50 mA and 150 mA. 1% cell SoC misbalance on 100AH battery with 100 mA balance bleed takes 10 hours to rebalance.
Hey, I'm charging with a multiplus 2 3000va, it has a 120a chargin current. I have limited it to 40a for the 300ah battery bank. Bulk/absorb at 14.4 and float at 13.6. I'm under the impression I have fully charged them, once the inv/charger goes into float mode.

I'm not using self contained batteries. The Renogy batteries use pouch cells as far as I know, and I cannot balance each ot them, I hope they come balanced by the manufacturer. What I did is to ballance each battery before connecting them parallel, but that really does not matter as if you are really close the batteries will auto balance. These batteries have RJ45 ports, which are used to connect each battery for auto-balancing function when connected in parallel, which I don't know how good that works, but it's there.

Will leaving the 3 batteries in parallel chargin at 14.2/14.4 for longer periods of time help with anything? I can increase the absorbtion phace on the multiplus.

Thanks!
 
In my mind is that reaching float at 13.6, the battery should stay at 13.6 when resting
What does "reaching float" mean to you? If you mean float just started, the batteries are not full. If you mean that charge current is very close to zero and the float voltage is 13.6v, they are full. I would not leave them charging above 13.6v for an extended period of time. You can charge them at 13.5v more or less indefinitely, but keep in mind that they don't really want to be left full for long periods of time.

The easy button for you might be to set your charger to 13.6v (open circuit), connect the batteries, and then let the batteries have at it until the current is 0.005C or lower. For 100Ah batteries, that would be 0.5 Amps or less (or 1.5 Amp for three in parallel). Once you get below that charge current, disconnect them, let them rest an hour, and test the voltage.

Reference: https://nordkyndesign.com/charging-marine-lithium-battery-banks/
 
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