diy solar

diy solar

3.65v

grAMPS

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Dec 14, 2021
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How long can I safely let my cells sit at 3.65v after top balancing?
 
The instant there is no room in the anode for further intercalation of lithium ions, and charging is still occurring- metallic lithium will begin to form.

The lower the current, the more localised the plating (ie: dendrite formation)

The question you should be asking yourself is why take a LiFePO4 cell close to saturation?

My advice is to stay as far away from the top of the upper voltage knee as you can.
 
Once you have reached 3.650Vpc and even if you allowed full saturation down to less than 1A being taken. The moment you start charging they start settling. Within an hour it would not be unusual to see them at 3.55Vpc and even 3.50 after 2 hours. THIS IS NORMAL BTW for LFP ! The working Voltage Range is 3.000-3.400 with Nominal Voltage being 3.200.

Gross Allowable Voltage is 2.500-3.650
General Working Voltage is 3.000-3.400. With A+ 2.900-3.450. The Actual Working Voltage Curve.
The AH between 2.500-2.950 and 3.450-3.650 only represent about 5-7% of Gross Capacity. (see gross voltage)
A Grade A Cell will deliver its AH from the working voltage range.
A 280AH A+ Cell will typically test out at 288-290AH GROSS
A 280AH B Cell will typically test between 277-282AH GROSS
A 280AH B / Bulk Cell will always test below Rated AH

Below from: Luyuan Tech Basic Lifepo4 Assembly Guide

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1641031141770.png
 
The instant there is no room in the anode for further intercalation of lithium ions, and charging is still occurring- metallic lithium will begin to form.

The lower the current, the more localised the plating (ie: dendrite formation)

The question you should be asking yourself is why take a LiFePO4 cell close to saturation?

My advice is to stay as far away from the top of the upper voltage knee as you can.
Very interesting. So are you suggesting not top balancing? They are brand new grade A EVE cells. All at 3.28v. Can’t I just set them up in my 8S2P configuration, hook up the bms and let them balance?
 
The original question..
They are not going to sit at 365 after top balancing. As soon as you remove the charger, they will come off of 365.
However, I didn't see any responses that said do not top balance. You should top balance.
 
Ideally, you would top balance them one cell at a time (each cell to 3.6V or even 3.55V or so, doesn't have to be 3.65V). The main reason for doing this is so that a full cell isn't kept at voltage waiting for other cells to catch up and you can overcharge a cell this way. Another way to do it is assemble them in a battery with BMS, charge the battery until you reach high voltage disconnect on one of the cells, and then top up the rest individually.
You can also just throw them together and have the BMS balance them over time. This works, but you might have to be patient before you can use the full battery capacity, and you need a BMS with a decent balance current preferably.
 
The original question..
They are not going to sit at 365 after top balancing. As soon as you remove the charger, they will come off of 365.
However, I didn't see any responses that said do not top balance. You should top balance.
Thanks for the response. I’m not trying to get out of top balancing. I’ve got everything set up and ready to go. I plan to do 8 cells at a time with bench power. So do I need to manually discharge them afterwards, or will they come off to a safe idle voltage?
 
So how long is ok at their settling voltage after top balancing.
 
Ideally, you would top balance them one cell at a time (each cell to 3.6V or even 3.55V or so, doesn't have to be 3.65V). The main reason for doing this is so that a full cell isn't kept at voltage waiting for other cells to catch up and you can overcharge a cell this way. Another way to do it is assemble them in a battery with BMS, charge the battery until you reach high voltage disconnect on one of the cells, and then top up the rest individually.
You can also just throw them together and have the BMS balance them over time. This works, but you might have to be patient before you can use the full battery capacity, and you need a BMS with a decent balance current preferably
I use your second option with my 48v 100Ah battery. Whilst assembled I charge each batch of 4 cells with a Victron 12v
charger at 20 - 30 amps until the BMS indicates an OV disconnect at 3,55v. Then I top up individual cells with a bench PSU. Takes a bit of time but beats stripping the battery and connecting all the cells in parallel for a day or two - what a PITA.
 
Very interesting. So are you suggesting not top balancing? They are brand new grade A EVE cells. All at 3.28v. Can’t I just set them up in my 8S2P configuration, hook up the bms and let them balance?

I definitely would not parallel top balance, that is a youtube circle jerk gone mad.

If you are using mismatched charge sources and BMS you are asking for trouble long term in either case.

If your BMS/charge controller can be set up to lower the charge rate when a single cell hits its high voltage, then you will most likely get your desired performance without any intervention.

If this is a battery for your own use - think in terms of decades rather than moments with what you are trying to achieve
 
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