diy solar

diy solar

2 STAGE OR 3 STAGE CHARGING?

Assume you are referring to dropping float or absorb cycle.

For LPF, higher absorb level, say 3.60v per cell, can give you a faster charge top off time, but you don't want a float level greater than 3.40-3.43v per cell. With some imbalance and inaccuracy of charger voltage it is better not to push float above 3.40v per cell.

3.43v per LFP cell is magic number of 100% state of charge. A constant float of 3.40v will have zero cell current once cell is full. Greater than 3.45v float will always push some current into cell which is bad for LFP cell as it means some overcharging.

If running charge current greater than 0.3C(A) always use separate float and absorb voltages. At less than 0.1C(A) charge rate very little absorb time is required so you could eliminate absorb voltage cycle. It will take a bit longer to charge the last few percent of capacity but if that means a lot to you, you probably don't have large enough AH battery for your needs.

Cell temp only comes into play when there is cell current, charging or discharging. There is very little deviation in cell voltage over temp at zero cell current.
 
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Assume you are referring to dropping float or absorb cycle.

For LPF, higher absorb level, say 3.60v per cell, can give you a faster charge top off time, but you don't want a float level greater than 3.40-3.43v per cell. With some imbalance and inaccuracy of charger voltage it is better not to push float above 3.40v per cell.

3.43v per LFP cell is magic number of 100% state of charge. A constant float of 3.40v will have zero cell current once cell is full. Greater than 3.45v float will always push some current into cell which is bad for LFP cell as it means some overcharging.

If running charge current greater than 0.3C(A) always use separate float and absorb voltages. At less than 0.1C(A) charge rate very little absorb time is required so you could eliminate absorb voltage cycle. It will take a bit longer to charge the last few percent of capacity but if that means a lot to you, you probably don't have large enough AH battery for your needs.

Cell temp only comes into play when there is cell current, charging or discharging. There is very little deviation in cell voltage over temp at zero cell current.
Thank you for detailed answer.

I will charge EVE280/48v array. About 0.2C-0.3C on charger and pv array The plan is to use Bulk and Absorb modes. I'm thinking to set absorb to 3.40v. I have quite a bit of extra capacity (3 days) in battery array so I do not need to run near dead or approach overcharge zone. Cells are in the corner of basement in a concrete bunker so ambient temperature is fairly constant at 20c. Charge/Discharge rates are fairly low so not too much heating going on.
 
The reason to float is to use solar during the day to power the loads instead of going into the evening with a low battery. With three days of battery capacity that might be a non issue. Although 3.25-3.30vpc would never hurt and in normal conditions provide nothing.
 
I think the most important thing is to limit the current when you are in “float” mode.

The reason i keep the voltage at 3.4 is to allow the cells to voltage balance at that level.

If you charge at 0.5C to 3.45V, then stop charging, your cells have limited time to balance.
 
Just keep in mind that BMS likely has some +/- tolerance to the 3.4v start of balance voltage. With a non-smart, non-adjustable BMS with fixed set point separate discrete supervisor I.C.'s for each cell, there is likely more tolerance variance between cell's balancing voltage trip point.

Should periodically take absorb to at least # of series cells X 3.50v to ensure you are above tolerance high side for start of balancing voltage to ensure every cell will get some balancing dump time in.
 
Assume you are referring to dropping float or absorb cycle.

For LPF, higher absorb level, say 3.60v per cell, can give you a faster charge top off time, but you don't want a float level greater than 3.40-3.43v per cell. With some imbalance and inaccuracy of charger voltage it is better not to push float above 3.40v per cell.

3.43v per LFP cell is magic number of 100% state of charge. A constant float of 3.40v will have zero cell current once cell is full. Greater than 3.45v float will always push some current into cell which is bad for LFP cell as it means some overcharging.

If running charge current greater than 0.3C(A) always use separate float and absorb voltages. At less than 0.1C(A) charge rate very little absorb time is required so you could eliminate absorb voltage cycle. It will take a bit longer to charge the last few percent of capacity but if that means a lot to you, you probably don't have large enough AH battery for your needs.

Cell temp only comes into play when there is cell current, charging or discharging. There is very little deviation in cell voltage over temp at zero cell current.
Hello,
I have 24v LFP bank that haven't use for 6 month.
I want charge them with adjustable power supply.
So what should I do...?
Just sett the voltage of power supply to 27,2v and wait until the meter show 0a...?
Sorry for my basic question and thank you for your repply
 
Hello,
I have 24v LFP bank that haven't use for 6 month.
I want charge them with adjustable power supply.
So what should I do...?
Just sett the voltage of power supply to 27,2v and wait until the meter show 0a...?
Sorry for my basic question and thank you for your repply
This looks good to me. Not sure I would wait to zero but close is fine.
 
My understanding is that most batteries do cell balancing in the float cycle. To restore a state of balance between packs 16sXp the BMS balancing wires do passive or active cell balancing. For a out of balance battery module, it seems it would be beneficial to occasional float for longer periods to maintain a state of balance.
 
Almost no BMS's do balancing in float mode. They require a cell to be greater than 3.4v to get balance bleed. To get an LFP cell above 3.4v pretty much requires charging current injection to raise cell voltage.

If you have a BMS that allows you set balancing below 3.4v and during discharging, you are getting into flat portion of cell voltage curve and discharge overpotential voltage slump from discharge current will impact cell voltage more than its state of charge voltage. You are likely to misbalance cells more balancing below 3.4v due to lack of overpotential voltage slump matching of cells for a given discharge current.
 
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Almost no BMS's do balancing in float mode. They require a cell to be greater than 3.4v to get balance bleed. To get an LFP cell above 3.4v pretty much requires charging current injection to raise cell voltage.

If you have a BMS that allows you set balancing below 3.4v and during discharging, you are getting into flat portion of cell voltage curve and discharge overpotential voltage slump from discharge current will impact cell voltage more than its state of charge voltage. You are likely to misbalance cells more balancing below 3.4v due to lack of overpotential voltage slump matching of cells for a given discharge current.
This doesn't make sense to me. Most BMS systems do cell balancing at the end of and following a charge cycle. My understanding is that these balancing operations can continue through and even after the charge cycle completes. I admittedly am not an expert in cell balancing and no only a little about active cell balancing with capacitor cycles or passive balancing through resistors. You mention requiring voltages above 3.4V per cell, but a float cycle occurs After the completion of bulk and absorb cycles. So those higher charging voltages like 28V of his 24V battery would have been achieved. If you are more knowledgeable then please elaborate. I arrived at this thread today after a bms error message noted a very low cell in one of 16s batteries. Simply charging to float voltage without hitting bulk may not trigger a balancing cycle if you are right and I am trying to figure out how to get my BMS to rebalance. I am also curious how a single one of my cells could have gotten so low when the others all seemed to be above 3V.

Here is an example from OrionBMS that discribes this. https://www.orionbms.com/manuals/utility_o2/param_balancing_description.html
 
This doesn't make sense to me. Most BMS systems do cell balancing at the end of and following a charge cycle. My understanding is that these balancing operations can continue through and even after the charge cycle completes. I admittedly am not an expert in cell balancing and no only a little about active cell balancing with capacitor cycles or passive balancing through resistors.

There is zero point in balancing LFP cells under 3.40V. It can be counterproductive.

You mention requiring voltages above 3.4V per cell, but a float cycle occurs After the completion of bulk and absorb cycles.

And "safe" float is 3.375V/cell - below 3.40V. Yes, some balancing may occur if cells are over 3.40V, but you're not getting much for it and not for long.

So those higher charging voltages like 28V of his 24V battery would have been achieved. If you are more knowledgeable then please elaborate. I arrived at this thread today after a bms error message noted a very low cell in one of 16s batteries. Simply charging to float voltage without hitting bulk may not trigger a balancing cycle if you are right and I am trying to figure out how to get my BMS to rebalance.

Hold pack at 3.45V/cell.

I am also curious how a single one of my cells could have gotten so low when the others all seemed to be above 3V.

It's the lowest capacity cell, the cells are imbalanced, or the cell is failling.


How an expensive "premium" BMS handles things is not representative of how cheap Chinese BMS handle things.
 
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