Can LifePo4 cell capacity be determined through charging up (as against discharging from 100%)

[bigbrovar]

Hello guys,

I just got some eve lf280k from china and I am setting out to determine their capacity while also top charging them.

The plan is to discharge each cell to 2.5v then charge them (using the EBC A40L tool) to 3.65v while noting how much capacity in Amps it takes to get it to 3.65v.

This way I am about to have each charged battery to their fullest. Each fully charged battery is added to a parallel connection.

Once all the batteries go through this discharge and "100%" charge cycle, they would all then be given a final top balanced charged this time in parallel.

I am doing this to save on energy and time (as against fully charging, then testing capacity by fully discharging and then charging them back to full)

Just want a different set of eyes to look into this plan and explain what I might not be seeing and if there is any danger or wrong assumptions.

Appreciate.

 

[TorC]

You might be able to get a decent idea by how much it takes to charge from full discharge, but it's non-standard at best. New cells, however, come at an unknown state of charge, so how much it takes to get them to either charged or discharged tells you nothing useful.

 

[bigbrovar]

You might be able to get a decent idea by how much it takes to charge from full discharge, but it's non-standard at best. New cells, however, come at an unknown state of charge, so how much it takes to get them to either charged or discharged tells you nothing useful.

Thanks. The plan is to gauge how much it takes to fully charge from 2.5v so I first discharge to 2.5, than charge all the way to 3.65v and take measurement.

 

[TorC]

Remember that, due to current tapering as full charge approaches, wall time alone isn't much use. You need something that can monitor actual power in, ideally in Ah, as that's independent of wiring losses.

 

[LakeHouse]

Hello guys,

I just got some eve lf280k from china and I am setting out to determine their capacity while also top charging them.

The plan is to discharge each cell to 2.5v then charge them (using the EBC A40L tool) to 3.65v while noting how much capacity in Amps it takes to get it to 3.65v.

This way I am about to have each charged battery to their fullest. Each fully charged battery is added to a parallel connection.

Once all the batteries go through this discharge and "100%" charge cycle, they would all then be given a final top balanced charged this time in parallel.

I am doing this to save on energy and time (as against fully charging, then testing capacity by fully discharging and then charging them back to full)

Just want a different set of eyes to look into this plan and explain what I might not be seeing and if there is any danger or wrong assumptions.

Appreciate.

Yes, this should work well. The Ah to charge and the Ah to discharge should be at least very close to the same with LFP. Energy (Wh) will be different, but as discussed elsewhere on the forum, Ah is a better capacity measure anyway.
One thing to consider is at what current you’ll stop charging. If you stop as soon as you hit 3.65V at 40A, there will be some small amount of current that the battery could still absorb. If you were doing a discharge test, you might consider some absorption time at 3.65V, and I’d do the same here if you want as close a comparison as possible.

 

[bigbrovar]

Remember that, due to current tapering as full charge approaches, wall time alone isn't much use. You need something that can monitor actual power in, ideally in Ah, as that's independent of wiring losses.

Thanks. The EBC A40L charger does this. It is able to log current that goes into the battery during the charge stage and I confirm it's reading with an amp clamp reader and they check out. Attached is a sample of the log of a fully charged battery.

Here the charging started at 2.559 and stopped at 3.650 exactly when the current going into the battery dropped to 2.0A. The total current that went into the battery during the charge stage was 282.40A.

Remember that, due to current tapering as full charge approaches, wall time alone isn't much use. You need something that can monitor actual power in, ideally in Ah, as that's independent of wiring losses.

 

[bigbrovar]

Yes, this should work well. The Ah to charge and the Ah to discharge should be at least very close to the same with LFP. Energy (Wh) will be different, but as discussed elsewhere on the forum, Ah is a better capacity measure anyway.
One thing to consider is at what current you’ll stop charging. If you stop as soon as you hit 3.65V at 40A, there will be some small amount of current that the battery could still absorb. If you were doing a discharge test, you might consider some absorption time at 3.65V, and I’d do the same here if you want as close a comparison as possible.

Thank you for your reply. The charger is set to stop charging at 2A. This means once the battery voltage gets to 3.65 and current gradually tappers to 2A, the charging is stopped.

 

[justgary]

Your cells probably specify a current cutoff of 0.05C at 3.65v. For a 280 Ah cell, that is 14 Amps.