Only getting ~30ah out of "280ah" lifepo4 batteries

[NomNomCameron]

I finally received 8 3.2v prismatic lifepo4 that I ordered from alibaba. They were advertised as brand new 280ah cells.

They came in good shape, terminals look clean, there's a barcode on each of them. They were also all matched at 3.3v

I have them all wired in series and have been running a few capacity tests at really low c rates (~10 amps) and the shunt that I'm using continues to show only 30ah once the inverter turns off??

I have the inverter low voltage shutoff set to 10.5v. When the inverter shuts off the shunt shows the batteries recover to about 22v, which seems within a reasonable range for voltage sag?

I charge the batteries (either with solar or the inverter/charger) to 28-29 volts before I start the capacity test.

I'm currently waiting for my BMS to come in the mail so there is no BMS in use at the moment. I've just been monitoring all the cells manually to make sure that they all stay the same voltage (which they have to about .1v)

This is the first lifepo4 pack I've ever made so I'm really hoping there's just something silly going on that I'm totally missing because even if these were really used cells I would anticipate AT LEAST 150-200ah out of 280ah advertised cells.

Any help on this would be awesome, thank you so much!

 

[upnorthandpersonal]

I have the inverter low voltage shutoff set to 10.5v.

Assuming you don't want each cell to go below 2.5V I would make this at least 20V to begin with.

I charge the batteries (either with solar or the inverter/charger) to 28-29 volts before I start the capacity test.

How much current is going into your battery at that time? How much power are you getting into your pack? Because of the flat curve of LiFePO4 you might not be done charging. You can't rely on just the voltage.

 

[NomNomCameron]

Assuming you don't want each cell to go below 2.5V I would make this at least 20V to begin with.

10.5v would be the actual shutoff if the inverter were 12 volts, since it's a 24 volt inverter the shutoff is twice whatever the setting is so 10.5v shutoff means it shuts off at 21v

How much current is going into your battery at that time? How much power are you getting into your pack? Because of the flat curve of LiFePO4 you might not be done charging.

I only have 2 strings of solar panels setup right now so when the solar is charging it the controller shows it gets to about 16 amps at solar noon when charging the batteries.

I'm not sure if this is correct but when the shunt is connected to the batteries/inverter and the charger on the inverter is on the shunt shows about 14 amps when it's in "fast charge" mode.

I've just been going off of the voltage of the battery pack to see what the capacity is, but from my limited understanding of lifepo4 I can understand how that may be an unreliable way to do that since the voltage is the same for most of the charge cycle. Is there a better way to determine the actual SOC of the batteries?

 

[upnorthandpersonal]

Let's assume you put in 14A at 24V nominal battery voltage gives you around 330W. Your pack has a capacity of 7kWh, meaning that at that charge rate you need 21 hours to fully charge that. Based on what you wrote I assume your batteries are just depleted and don't charge them fully at all.

As for state of charge on LiFePO4, a Coulomb counter would be much more accurate.

 

[Pierre]

Is your inverter a 12 / 24 v input type ( from what I understand from your post ) and make ?

 

[NomNomCameron]

That makes sense!

I honestly cannot tell you how much I appreciate the fast reply!

I'll take another stab at trying to get the batteries fully charged and try to run another capacity test, I'll post here again if I run into trouble

 

[NomNomCameron]

Is your inverter a 12 / 24 v input type ( from what I understand from your post ) and make ?

This is the inverter/charger I'm using:

Amazon.com

 

[Pierre]

This is the inverter/charger I'm using:

Amazon.com

It is a beast ! Will check it out a bit later.

 

[upnorthandpersonal]

When you want to make sure if your battery is fully charged or not, you need to look at the current going into the battery at a charge voltage of, say, 3.55V per cell (max 3.65). When the current drops to zero, your battery is charged.

 

[NomNomCameron]

When you want to make sure if your battery is fully charged or not, you need to look at the current going into the battery at a charge voltage of, say, 3.55V per cell (max 3.65). When the current drops to zero, your battery is charged.

This makes sense. So I should be able to read this off the shunt that's connected between the inverter/charger and the batteries then. When the charge voltage is at ~29v and the shunt is showing 0 amps (without a load on the inverter) the batteries can't take anymore amps, which means the batteries are full.

Does that sound about right?

 

[upnorthandpersonal]

Yes, sounds right. FIY, have a look at the curve below:

Check how the voltage at low charge rates stays low, your battery is still absorbing energy and getting full. You're at the 0.05C rate right now based on the numbers you mentioned.

 

[NomNomCameron]

Thank you so much for your help upnorthandpersonal! I'll post back here with my results!

 

[upnorthandpersonal]

One more thing: did you balance the cells before making your pack? It's rather important that all of your cells are at the same state of charge before putting them in series. You can do this by top-balancing and charging all of them in parallel to 3.6V. Of course, you need a dedicated charger of CC/CV power supply for this.