Bulk & Float Solar settings for LiFePO4 batteries

[john61ct]

Yes, remember that voltage chart is based on the vendor definition of Full, using higher voltages than is good for longevity.

In normal House bank usage, you never want to even get near the "shoulders", the points where you see the curve's slope increase.

Keeping voltages low when charging, and Stopping charge inputs when amps get low ensures that.

Charging to 13.9 and Stopping is likely very much the same as keeping going until endAmps at 13.8.

With a reliable accurate volt meter, check resting voltage when at your definition of full: no charging, no loads for five hours, best to just open you bank isolation switch.

As long as that's around or below 3.4Vpc (13.6) your charging regime's voltage is not harming longevity.

[asimba2]

Originally Posted by 44'cruisingcat

Sitting at 13.8 for a few hours, you'd be getting very close to100%. If it's only at 13.8 for a moment, then back to 13.4, that won't happen.

This has me a bit confused...if bulk, asborb and float are all set to 13.8V then I assume the solar charge controller would stop the flow of current to keep the voltage from raising above 13.8V. 14.2-14.4V is 100%.

[44'cruisingcat]

I'm no expert, but my reading on the subject suggests that at 13.6 v, for long enough, batteries will get to 100%. Time, as well as voltage, is a factor.

So IMO, a moment at 13.8, (or less if you prefer) then back to a 13.4 float, is a more dependable way of avoiding 100% SOC.

[asimba2]

Originally Posted by john61ct

Charging to 13.9 and Stopping is likely very much the same as keeping going until endAmps at 13.8.

With a reliable accurate volt meter, check resting voltage when at your definition of full: no charging, no loads for five hours, best to just open you bank isolation switch.

As long as that's around or below 3.4Vpc (13.6) your charging regime's voltage is not harming longevity.

Thanks John. The end amps method is a problem since my Magnum inverter/charger has that capability but the minimum absorb time is 1 hour, which is a problem. Same problem with the solar charge controller I am planning to buy, so I think using end amps is out. I will be charging by solar 99% of the time.

[asimba2]

Originally Posted by 44'cruisingcat

I'm no expert, but my reading on the subject suggests that at 13.6 v, for long enough, batteries will get to 100%. Time, as well as voltage, is a factor.

So IMO, a moment at 13.8, (or less if you prefer) then back to a 13.4 float, is a more dependable way of avoiding 100% SOC.

Interesting. I suppose because solar will be my method of charging 99% of the time there may not be enough sun hours even in California for that to be a huge problem, but it seems the lower float is a good way to avoid that. However I should probably set my Magnum inverter/charger to a lower voltage for those rare times I am on shore power.

Because the resting voltage of the cells is around 3.3vpc (13.2v for 4S), my last concern would be whether or not holding a float above resting voltage would cause a problem.

[44'cruisingcat]

Originally Posted by asimba2

This has me a bit confused...if bulk, asborb and float are all set to 13.8V then I assume the solar charge controller would stop the flow of current to keep the voltage from raising above 13.8V. 14.2-14.4V is 100%.

The current wouldn't stop when it reaches 13.8.

It would reduce slightly, but still keep putting charge into the batteries, holding the volts at 13.8. Over time the current would keep reducing as SOC gs closer to100%.

[john61ct]

Make sure you are accurately measuring charging amps. Set to 13.9V, let the Magnum keep going until you see the flow drop to .02C.

Let the bank rest isolated, completely disconnected.

After that is voltage really only 13.2?

And yes, you want float at or below resting volts.

Better to just stop: not "harmful", but some unknown, maybe minor loss of potential lifetime cycles.

[T1 Terry]

You might have come across this before, but I'll repeat it here. 3.6v in any cell is the max you want to go to if you want them to live a long life. 3.45v or higher rested absolutely saturation full, no need to do that unless you are capacity testing or resetting the SOC to an accurate 100%.
Under charge the cells on a constant basis will invoke a memory wall and produce symptoms that look like capacity loss, in fact any regular voltage set point will produce this effect but the 3.45v at least takes longer to produce a noticeable effect.

Understanding what the term "float charge" actually means and understand the voltage used as the "float" voltage where these tests showed cell damage is the key to understanding the subject.
Float voltage was a lead acid charging term used for batteries in a stand-by use format. The idea was to have the battery fully charge and be ready for use. This is a very different battery use compared to cyclic use, forget about using Li batteries for stand by use, they are not suited to this application and will fail very early in their cycle life.
Even lead acid battery manufacturers give different charging voltages and charging regimes for stand by use and cyclic use, Li batteries are no different. If you are solar charging a battery in cyclic use 13.8v is a fine float voltage, 14v bulk and 13.9v absorption if you can limit that to 1hr max, if not then set absorption the same as float voltage or get a better controller. This is because solar charging is not constant, it stops when the sun goes down and all charging needs to follow the same rules, no 24/7 float charging, 4 or 5 hrs max. If you don't use the batteries over night then drop to 13.6v float, if you aren't going to use the battery for 3 mths or more, discharge to 50% SOC or 3.2v per cell and disconnect the battery.

T1 Terry