Quote:
Originally Posted by s/v Jedi
You are correct, 3.4V/cell termination voltage during charging would undercharge the battery. But what if this is taken as an absorption voltage for let’s say 2-4 hours?
I’m not sure how they charge to that voltage, but I do know that you can fully charge (or 99.5% or so) at 3.4V/cell when it is held in absorption long enough.
The wish to use a CC/CV charge regime as described by cell manufacturers looks great on paper but it is designed for EV applications, where you charge the car and then it sits until driven. This is not how boats work, they use power continuously and when the battery is full we want to keep using solar power, not switch that off and use battery power instead. We only want to use battery power when the sun comes down and we lose solar power for the night.
The only way to achieve that is by configuring a float voltage that isn’t so low that power from solar is wasted. During years of living aboard with LFP and solar, I have found that 3.3V/cell is the ideal float voltage. It does discharge the battery to 92% but most of the solar power is used.
Our charge settings:
Absorption voltage: 3.45V/cell
Absorption current: I can reach 0.5C, have 1,875W solar for a 10.5kWh battery.
Absorption time: fixed 2 hours
Float voltage: 3.3V/cell
When the absorption charge finishes and the controllers switch to float, you see all power coming from the battery instead of solar. This is good because it’s better for the LFP to not be kept at full charge. When they are down to 97% SOC, solar delivers most of the power again and at low 90’s SOC solar takes 100% of power consumption.
The BMV is configured to trigger “fully charged” a couple mV below absorption voltage as described in the manual. It works perfect.
So here are the results: maximum cell deviation after battery was installed was 4mV. After two years of cycling with settings as described above, with no cell balancing of any kind, maximum cell deviation was 8mV. I packed my cell balancer because I don’t think it’s useful to do this for 4mV.
I must add that this battery is an 8s configuration with perfectly matched cells.
|
3.4v will hold the cell close to fully charged, but it won't fully charge a cell. Even LFP cells have an internal resistance, the 0.05v above the standing cell voltage is required to add more capacity into the cell, it is just so slow it's like watching
grass grow. However, 0.1v above the standing voltage will greatly increase the charge rate, if solar is the
charging source, then 13.8v float will trickle more capacity in for as long as the sun shines, but it stops when the sun is no longer on the
panels.
This is different to
shore power charging or mains charging as it's known in the land based community. That will hold the 0.05v above the standing 99% charged point and gradually push the cell that is closest to full capacity to an even higher voltage, at the expense of the lowest capacity cell. This drives an out of balance situation and this is the reason for the blanket declaration to not float charge lithium
batteries.
If you can drop the charge rate to equal or less than the active balancer capacity balancer capability, then as long as the
charger isn't still running for days, there will be no harm done to the cells.
Resistor based balancers aren't the same as active balancers, these things will just keep generating heat until something fails, they are just a problem waiting to show up at the very worth time.
Quality chargers have an auto "storage mode" they drop into once they see a percentage of charge current over a number of hrs. This can be set to what ever percentage of the battery capacity you want, if you do have resistor type cell balancing though, keep the percentage below the current flow all cells in balance mode would draw.
Those chargers with minimum current switching from absorption to float can have the float voltage set as the
storage voltage, just remember to set the "rebulk" to a higher point than the difference between
storage voltage and absorption voltage ... or the poor controller will develop a migraine and eventually refuse to do anything

Yes, I've seen more than one system set up that way, some "off grid" gurus need to go back to
school at basic maths level.
Once the voltage differential is reached to kick the
charger back into "rebulk" it generall means there is a load greater than the charger capability to hold the battery voltage above the storage/float voltage and the system will go back through the same procedure . This method of ending absorption voltage is far better than the set time method, unfortunately, not all chargers have this function.
Battery management systems like the "Electrodacus" (made by a Canadian go fund me type developer) has the smarts to bring the cell voltage up to what ever you want to program into it, then drop back to a lower cell voltage for the rest of the day. The great thing about this idea, the cell voltage is the controlling factor, not battery voltage, so much better conditions to promote long cycle life.
T1 Terry