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Charging voltage for LFP?

zedconnor

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Feb 16, 2023
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Myanmar
I am confused about the charging. I got a old conventional 12V inverter charger which is intended for Lead-Acid batteries.
It's charging voltage is measured at 13.6V and 10Amps

I am using it with a 12V 200Ah lfp

My question is whether this inverter can fully charge my lfp or not.

LFP resting v is 3.4ish and charging is 3.65ish. But what if I only charge with 3.45V per cell until the amp trails off. Will it fully charge or not?
 
At 13.6V it can mostly charge the battery. The issue is that the cell balance function of a BMS starts at 3.4V on the cell. You will not be running the charge up enough to get cell balancing.
 
I got a old conventional 12V inverter charger which is intended for Lead-Acid batteries.
It's charging voltage is measured at 13.6V and 10Amps
Any lead acid charger will be able to completely charge and then overcharge LFP.
The question is more of how you can prevent the latter from happening.

LFP resting v is 3.4ish and charging is 3.65ish. But what if I only charge with 3.45V per cell until the amp trails off. Will it fully charge or not?
The fully charged resting voltage of LFP is 3.37V/Cell.

Any voltage above this without tail current monitoring will slowly overcharge the battery and throw balance out of whack !
You say your lead acid charger is only capable of 13.6 V. Giving 3.4V/Cell

Have you measured this with a properly calibrated multimeter or just specs?
 
Any lead acid charger will be able to completely charge and then overcharge LFP.
The question is more of how you can prevent the latter from happening.


The fully charged resting voltage of LFP is 3.37V/Cell.

Any voltage above this without tail current monitoring will slowly overcharge the battery and throw balance out of whack !
You say your lead acid charger is only capable of 13.6 V. Giving 3.4V/Cell

Have you measured this with a properly calibrated multimeter or just specs?
Sorry for the late reply. The post got burried under other notifications. Yes i have measured inverter's charging voltage with a calibrated multimeter.

The answers are conflicting under this thread.

How can one monitor the tail current and cut off charging? Via the bms? I have got further confused by the concept of charging now.
You can charge it with higher voltages like 3.5V (Or 3.65V) but you cannot charge it with 3.4V until current becomes zero?

Let's say if the lead acid charger is capable of 3.6V (14.5V) will it be worse than if it were only 13.8V?
 
LFP resting v is 3.4ish and charging is 3.65ish. But what if I only charge with 3.45V per cell until the amp trails off. Will it fully charge or not?
May not be fully charged but should be 95+% and work just fine. 13.8 would be slightly better but there is no reason to go to the max on a day to day basis. Eventually you may want to change the charging system but not immediately necessary.
 
Sorry for the late reply. The post got burried under other notifications. Yes i have measured inverter's charging voltage with a calibrated multimeter.

The answers are conflicting under this thread.
Simple! It is because they are unaware of true LFP charging behaviour.

How can one monitor the tail current and cut off charging? Via the bms? I have got further confused by the concept of charging now.
Here's the reality: When it comes to monitoring tail current and cutting off charging current, Most equipment built for lead acid batteries simply CAN'T. You are bound to get confused.
In most cases, it comes down to implementing a safe compromise with the hardware that you've got.
You can charge it with higher voltages like 3.5V (Or 3.65V) but you cannot charge it with 3.4V until current becomes zero?

Let's say if the lead acid charger is capable of 3.6V (14.5V) will it be worse than if it were only 13.8V?

Screenshot_20240102-133401.pngScreenshot_20240102-134135.png

Look at these two screenshots carefully taken roughly 7 minutes apart.
The first shows the battery at basically ~98% SOC still charging at voltages below 3.4 V/Cell. The second one shows the battery at 100% SOC shortly after cut-off (56.4 V @ 0.05 C).
I have the older JK BMS that shows 95% SOC when the battery is 100% charged. But you get the gist by looking at current going into the battery.

The takeaway is this simple:​

Any charging voltage above ~ 3.37 V will in theory will fully charge a LFP cell.
The tail current for all voltages in the range (3.37 V to 3.65 V) is given by this handy graph.
Most charging hardware for lead acid can't implement cut-off charging AKA charge termination. So, you gotta compromize in some way.
download.png
 
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