Can a 12V LiFePo4 battery be fully charged with only 13.8volts?

I want to know if a mains charger in power supply mode (13.8v) can fully charge a LPF battery, and if so will it take alot longer than at the recommended 14.4v?

If not what % of charge would 13.8v achieve?

Lithium iron phosphate batteries are typically charged in a two-stage algorithm, first charging with a constant current and then switching to a constant voltage charge to get the battery all the way to 100 percent.

You can charge one with a constant voltage only, and a lead-acid battery constant voltage charger will work as long as the voltage is about 14.4 volts (as noted in the OP). You don’t want to go much higher than that in voltage or you’ll damage the battery. A lower voltage, like the 13.8 volts that lead-acid chargers often use for float charging, will not fully charge a lithium iron phosphate battery. I’m not sure exactly where it will stop charging, but my guess is somewhere around 80 to 90 percent of charge.

Many lead-acid battery chargers will drop to float-charging when they think that the battery is charged almost to capacity. This type of charger will not fully charge a lithium iron phosphate battery.

It depends on the charge current. Also the cells have to be balanced.

If you charge four-in-series well-balanced LiFePo4 with a high current (say, 0.5C) and cut off as soon as it hits 13.8V (= 3.45V per cell) then the cells might be charged to about 85%.

If you keep them at 13.8V for a while and let the current drop (e.g. to less than 0.05C), then the charge level would get quite close to 100%.

That’s what I deduce from the charge curves about halfway on this page: https://forum.solar-electric.com/discussion/351996/charging-lfp-lifepo4-batteries-to-90-soc-or-not

That also assumes the cells are very well balanced. If the cells get out of balance, you would very easily be overcharging one of them.

As for how long it takes:

A normal CC/CV charge at 0.5C to 14.4V from zero to 98% takes a bit over two hours.

Charging to 13.8V at low current (0.05C) all the way from 0% might get you to 98% in 25 hours or so.

Charging to 13.8V at high current (0.5C) initially gets you to that 85% in some two hours, then keeping it there and letting the current taper to 0.05C might get you to that 98% state in about four more hours.

Thanks Frankenstein Monster! Very helpful to me.

Just one other thing Im a bit confused by, you say "
Charging to 13.8V at low current (0.05C)"

At CV I thought the current was the varying figure, starting off high (only limited by what charger is rated for) when battery is flat and reducing. I dont see anyway to charge at .05C unless the charger is very small in comparison to the battery.

With CC/CV you have the Constant Current phase followed by the Constant Voltage phase.

Constant Current means you limit the current, e.g. to 0.05C. Charging an empty battery at 0.05C means the voltage is low initially, then you let it creep up.

Once you hit the specified voltage (13.8V), you keep it there and thus you will let the current drop.

Or to try to be more clear:

The CC phase is characterized by the current. The CV phase is characterized by the voltage. So by “Charging to 13.8V at low current (0.05C)” I meant CC/CV charging with the current during the CC phase at 0.05C and with the voltage during the CV phase at 13.8V.

Not sure what you mean. You should never exceed the rated charging current of the BATTERY during charging. You can in fact do this by using a charger whose maximum output current is lower than the battery’s charging current, which is a common trick for charging batteries with relatively simple power supplies. But the battery dictates the allowed current.

My understanding is in CC mode the amount of current is only limited by the charger’s capacity or what the rate is set to.
When switching to CV the current will drop down alot & begin to taper down further more.
I don’t see how a charger can be so low at 0.05C (25hours) in CV unless the rate & size of charger is tiny.

In CV if the charger is big & the battery is low the current going into the battery will be big for a while. Like an alternator & a depleted car battery.

Well thats my understanding. Cheers.

(My bolding) That’s what I meant, it could indeed be a tiny charger, or it could be a big charger with the ability to limit the current.

E.g. this charger (just a random Amazon search) can charge at 6 Amps but you can limit it to 0.1A to charge a 2Ah LFP at 0.05C.