LiFePo4 Float charging voltage

I saw few posts with similar subjects, but just to ask the question "my way". Example is for 24x7 off-grid solar system with LiFePo4 batteries and a variable load. Charge Controller is in User mode, Max charging voltage is set to 14.4V and Float is set to 13.6V. Let say battery reaches 14.4V, but the load current increases to a level that exceeds the max. controller current and starts draining the battery. Let say battery voltage reaches 13.7V, even if the load current drops the battery might stay at that voltage by sunset, which is not a fully charged battery. In that case isn't it better to set the Float at 14.2V or even at 14.4V to ensure timely re-charge to max. capacity? Thanks!

Getting 100% is overrated. Besides if the battery gets to 14.4 it is full. If the voltage stays above 13.5 the battery is still full.

If you need 80% or more of the battery to make it from sundown to sunrise I would consider the battery too small. I would expect to have two or three days power with no solar such that if solar gets the battery to 14.2+ anytime during the day there is plenty of power to make it through the rest of the day, all night and well into the next day.

Running the LFP at full 100% is a hangover from the days of using FLA.

T2R, thanks for your quick comment! Yes, the battery bank I have can make it till the next morning, voltage is ~13.2V which according to the reference data I have is about 70% of the battery SOC. BTW, the batteries are "deep cycle" type which I assume refers to a "full charge - full discharge" cycle (let say 14.4V to 10.6V). Is going from 14.4V to 13.2V considered a partial cycle? If yes, can we say a 3,000 cycle battery spec translates to let say 5,000 partial cycles (or whatever the right number is, but more than the spec deep cycles)?

afaik, discharging 50% is half a cycle. There are other factors that will also shorten the life including heat and holding the charge up in the top voltage area for extended periods. Let the cells cycle as this alone will have them lasting close to 20 years with any luck.

Thanks much!

If the cells are good quality, well balanced, and your charge current is around 0.2C, then the cells will be at almost full at 14.0 volts charge voltage. Charging to high voltages with long absorbtion time ( boost duration), and having high float volts, seem to contribute to accelerated ageing.

Mike

Thanks, Mike, appreciate it!

Below are the settings I've come up with for my solar installation (PV power - 1,300W: 4x200W/12V; 2x150W/12V, 2x100W/12V, connected in serial pairs for 24V and then in parallel (4 pairs total). Battery bank is 560Ah - 6xEco-Worthy LiFePo4 batteries (2x150Ah/12V, 2x100Ah/12V, 2x30Ah/12V). 150Ah batteries are new (~2mo old), the rest are ~2yr old. Probably need to do some measurements and see if the old ones are as good as the new ones. I have some ideas, but certainly value the experience of this forum's gurus, so any suggestions are welcomed!

Let your battery tell you what the proper voltage is! Which may be more important with aged / grade b cells.

1) Charge your battery to full.
2) WAIT at least 12 hours for it to settle.
3) MEASURE the settled voltage at the terminal with a high-accuracy voltmeter. I use a Fluke 87V
4) Do not exceed this value in your float voltage settings. Perhaps change it to 0.1v or more lower.

Logic: Battery is already full. Exceeding the "settled" voltage puts enough of a voltage differential for it to *try* and push it beyond full into overcharge. It may be ridiculously small, but if seemingly no current is flowing, secondary reactions may be getting aggravated.

Generally, this is why with all the batts I've owned, I never exceed 13.5v. I'll see references to 13.6v being ok, but I think that even with that minute .1v difference, it is still and unnecessary voltage potential difference.

Perhaps with grade-b cells, your measured value might be no more than 13.4v. Let the battery tell you.

13.6V float is good if you're supporting loads. It's a little high for just parking it there, so 13.4 is recommended.
13.8 is a bit high, that's actually full charge for lifepo4. And 13.2 might be a shade low if you do have active loads - the battery will dip down before full charging kicks back in.

Each system is a little different, so fine tune your parameters so you get what you need for your particular system.

I agree, but given what my battery tells me (I've never had one settle after 12 hours to 13.6, but lower), and charger manufacturer tolerance (like +/- 0.2v), I think it best to err on the conservative side in case one has a unit that is high in manufacturer's tolerance.

Hence I think if one is going to publish a float recommendation I'd never go higher than 13.5v. Just to avoid that long-term knife-edge and manufacturer slop.