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Another top balance question for EVE 304

Darce

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Dec 4, 2020
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I'm working on getting a 16s 48v battery into service. I charged off the SCC until two of the cells hit the BMS limit and stopped the charging. From there I am attempting to charge with a 10amp dc power supply and 10g leads. I set the voltage at 3.65 and amps all the way up to 10. As the cell fills up the amps drop, makes sense I guess. However once the cell hits 3.65 volts there is still 2 amps showing on the charger and when I disconnect it promptly drops back to 3.5x in a short while. I've been pulling the charger off the cells because the voltage creeps above 3.65. I don't want to damage anything. I've read here elsewhere in the forums to let the charger run until the amps drops to near zero. Am I pulling the charger too early? Is it normal for the voltage of the cell to exceed 3.65 while it tops off and then it settles back to 3.6x? Thank you for any suggestions on helping me get this pack balanced out...

Darce
 
LFP cells will settle. That's normal.

When you say "3.65V" where are you getting that number? Is it from a voltmeter on the cell in question, or is it the power supply? if it's the power supply, what is the voltage AT the cell?

Most 10A/30V supplies have garbage leads, and you need to fab your own. This is discussed in detail in the top balance guide in resources.
 
Is your dc power supply disconnected from the battery when adjusting the voltage on the dc power supply? You have to have it disconnect from battery and adjust with a meter then hook up to battery.
 
That is normal. You need to let the current drop to a low level before terminating charge to ensure cell is fully charged,
That is the short answer. Make sure the cell voltage does not exceed 3.65v.

Here is some additional info, if you are interested.

When you charge to 3.65v and charge current drops to low level, there will be surface capacitance charge that will build up within the cell layers that is not related to cell capacity., The cell is actually fully charged when rested no-load open circuit cell voltage is about 3.45v. 'Rested' means no charge or discharge current for about 5 minutes for LFP cell.

When you fully charge cell to 3.65v, as cell approaches full charge and charge current drops off, the excess voltage above about 3.45v goes into charging the surface charge capacitance. The capacitance charge voltage creating this extra 0.2v (3.65v-3.45v) has only about 0.01% of the cells rated capacity. It can be quickly discharged within 60 seconds with a temporary 1-3 ohm resistor load. After the surface charge is discharged the cell open circuit rested voltage will be about 3.45v for the fully charged cell.

If you do not manually discharge this surface charge it will bleed off on its own but may take several days to do so if cell is left open circuit with no load. There are many folks that think something is wrong when they see this happen on their fully charged cell after removing from charger.

The reason for going above 3.45v for charging is to speed up the charging process. You can fully charge a LFP cell with limiting cell recharge voltage to 3.45v but it will take a long time to reach full charge. The reason not to go above 3.65v is it can start damaging the electrolyte in the cell.
 
The cell is fully charged when there is no more room in the anode lattice for lithium ions. If you continue to charge after this point you will be causing irreversible lithium plating of the anode (in the form of dendrites).
One of the many reasons parallel top balancing is a bad idea, and the major reason i have experienced in short lifespan of a LiFePO4 cell.
 
Yes I am disconnecting the charger to set the voltage to 3.65. I did fab my own leads from 10g wire and ring terminals after reading other posts. So 3.45 is fully charged. Thats works with what I am seeing in my cells after they settle. I am not balancing in parallel, but individual cells.

The meat of my question lies in the process. Here's exactly what I'm doing...
1. disconnect cell from charger and set charger to 3.65v and 10 amps
2. connect charger and hang out while the current slowly drop as the cell charges
3. monitor cell with voltmeter and disconnect charger at 3.65v irregardless of amps (usually 2+/-) showing on the charger

One thing I will change is to adjust the charger with the meter and not rely on the chargers readout. Thank you for that.

My confusion is that I had read that you disconnect the charger when the amps reads near zero, not necessarily at 3.65 volts. The post just didn't expand on the voltage reading to expect when the current reads zero. My voltage creeps up past 3.7 below 2 amps of current which is what has kicked off this post. If the cells are settling down to around 3.45 and thats normal and charged then I am all set. Lol I'm not trying to make this complicated for myself, I just want to understand the results I'm seeing from the process I'm using and verify its correct. Very thankful for the forum members for helping me not destroy my batteries.
 
My voltage creeps up past 3.7 below 2 amps of current which is what has kicked off this post. If the cells are settling down to around 3.45 and thats normal and charged then I am all set.

BMS's have a tolerance in their protection cutout voltages. Each cell may have its own separate protection I.C., and that may even cause a variance in protection trip voltage between cells' position in stack. They may have cutoff voltage of 3.60v to 3.75v, depending on particular protection I.C. They usually have to drop about 0.1v before BMS reconnects, again with individual I.C tolerance.

When a BMS cuts out for overvoltage during charging, most BMS's continue to bleed current from cells over about 3.4v until they drop to about 3.4v. So it is good not to reapply charging current too soon after BMS resets to active again so you allow some time for the BMS balancing bleed current to do its job. Beginning charging again right after BMS reconnects will likely result in a short time before it overvoltage cuts out again giving little time for low state of charge cells to get further charging.

If doing top balancing with BMS it is good to reduce charging current after BMS initially cuts out for overvoltage the first time. This reduces the ratio between balancing current, which may only be 50-200 mA, to charging current, extending the time before BMS cutouts out again for a cell overvoltage. Having a BMS with high balancing current really helps for this.
 
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My voltage creeps up past 3.7 below 2 amps of current which is what has kicked off this post.
It should not be possible to charge a cell to 3.7 volts with a charge voltage of 3.65 volts.
The 2 possibilities I see are...
1. Your DVOM and benchtop PSU don't agree on voltage.
2. Your benchtop PSU is malfunctioning.
 
It should not be possible to charge a cell to 3.7 volts with a charge voltage of 3.65 volts.
The 2 possibilities I see are...
1. Your DVOM and benchtop PSU don't agree on voltage.
2. Your benchtop PSU is malfunctioning.
What happens is I set the voltage to 3.65 and the max current. Connect the battery and the digital voltage and current readouts on the charger fluctuates. The voltage hovers near the 3.65 but the current drops as it charges. Would you say this is not normal? Either way, up to this point I was not verifying the charger voltage with a DVOM before connecting. From now on I will.
 
Thank you for the responses. I just had to grow a pair and charge till the current dropped to near zero. Some of the cells settled higher than others so I have a Heltec balancer on the way that should pair well with my Overkill BMS. Should be here this Friday.
 
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