LFP battery absorption and float - report

[Almot]

Bought this 120AH LiFePO4 a while ago https://www.lynaclithium.com/wp-content/uploads/2022/03/12.8V-100Ah-True-Series-Quick-Sheet-USA.pdf. 120AH marketed as 100AH.

Don't see this now but on the older page (and most other LFP manufacturers) recommendations for LFP are - very short or zero Absorb time, no Float or set ~13V (for 14V battery). Now I see them recommending Tail Current 2A-5A, this is where you should completely stop charging, be it Bulk or Float or whatever but I doubt this would work for me (see below).

Set Victron MPPT to Float 13.2, Absorb time 10 min, controller wouldn't accept values <10 min.

Was drawing ~ 30-40 AH a day. Was harvesting slightly less, because it wasn't fully charged upon reaching Absorb 14.4, and there was zero Float current. When loads kicked in, Victron increased Float current to slightly below the load demand - ex. fridge thermostat 2A, Float 1.5A , pump 10A, Float 8A etc - so no joy here.

Victron MPPT won't start Bulk charging again until it can't sustain 13.2 Float anymore - which is usually the last few minutes before sunset - doesn't make much difference.

As a result the battery was getting lower every day, with plenty of sun.

Interestingly, Victron pre-set mode for some "Victron Smart LFP" is 2 hours Absorption time and 13.5 Float.

Set Absorb to 3 hours to see how long it takes to bring tail current to zero. After a few days when battery was finally fed well and was getting some charge during Float to compensate for Loads draw during the day, it took 1 hour 45 minutes to stay in Absorb to drop the current to roughly what loads demand. Well, at parallel 28 it's still winter, max 20A current from a modest array (8A when it became getting full charge every day).

Same story with Renogy DC-DC converter - when set to Li, it was getting lower every day with 6-7 hours driving and the only load - 4A fridge at 60% cycle. Set Renogy to Led Acid battery. Hopefully it won't try running equalization.

[pianotuna]

2oldman wrote:
My 835w would too if I wasn't too lazy to get up on the roof and tilt them. Thanks for the info.

No one ever fell off the roof by being lazy.

[Almot]

time2roll wrote:

Almot wrote:
Victron MPPT won't start Bulk charging again until it can't sustain 13.2 Float anymore - which is usually the last few minutes before sunset - doesn't make much difference.

As a result the battery was getting lower every day, with plenty of sun.

OK so I would expect the re-bulk to start about 30 minutes earlier each day if the state of charge was dropping each day.

Didn't notice, not saying yes or no. With my very low draw any sun, even minutes before sunset, results in keeping the battery above 13.2. It has to drop below Float for at least a minute for Victron to start re-Bulk.

What I did notice was progressively lower pre-dawn voltage day after day, as witnessed both by checking voltage on the app before dawn, and Victron records of "minimum voltage" for each day.

[2oldman]

3 tons wrote:
my solar 660w does entirely all the work…

My 835w would too if I wasn't too lazy to get up on the roof and tilt them. Thanks for the info.

[Cptnvideo]

Almot, I have a Victron 150/100 MPPT controller. I set bulk to 14.4 volts, absorption time to 25 minutes, float to 13.6 volts, tail current to 3 amps, and the re-bulk voltage offset to .20 volts.
When the switch from absorption to float is reached, I lose a couple amp hours during the transition. Which, of course, is no big deal since I have 1030 ah total capacity.
My SOK batteries state full charge to be 14.6 volts.

[3_tons]

2oldman says, “My LFP acts strangely when finished with Bulk. On generator charging the gen will surge up to bulk for about 30 seconds, then 'rest' on float for another 30 seconds. It will do that forever….”

I’m not totally sure what the issue might be (because I my solar 660w does entirely all the work…), but shortly after having reached 100% SOC (per Victron BMV 12), starts the cell-balancing regime (say, after a minute or two at rest) observed via the on & off again alternating of current vs volts (a temporary generator loading - I donno?)…This sometimes takes about 20 or so minutes till completed…Aside from this ‘occasional only’ meter re-sync and cell rebalancing regime, I purposely avoid a routine full charge to 100% SOC, instead opting for say 85-low 90’ish %…. I find that a full 100% SOC is otherwise unnecessary - JMO

3 tons

[time2roll]

Almot wrote:
Victron MPPT won't start Bulk charging again until it can't sustain 13.2 Float anymore - which is usually the last few minutes before sunset - doesn't make much difference.

As a result the battery was getting lower every day, with plenty of sun.

OK so I would expect the re-bulk to start about 30 minutes earlier each day if the state of charge was dropping each day.

[2oldman]

My LFP acts strangely when finished with Bulk. On generator charging the gen will surge up to bulk for about 30 seconds, then 'rest' on float for another 30 seconds. It will do that forever.

AFAIK my solar controller doesn't do that. It goes to float then 'charged.' Dunno.. maybe it's something goofy in the shore charger.

[Almot]

Don, input for Victron was 60V from 490E solar.

Also the controversial Float matter with LFP. It might not be strictly necessary but in camping there is always discharging going on, at least 1A standby current of fridge, radio, LP detector. I consider Float a continuous (in daytime) charging of battery that is being discharged continuously and thus is never really full. By setting Float to 13.5 I am getting 1 or 2 amps above the standby current in daytime only, ensuring SOC close to upper 90s by the evening.

3 tons - yes, the harvest was less than daily draw, because with zero Float current the controller stops charging shortly after the Bulk is reached. I set it up this way according to the LFP manufacturer recommendations - zero or short Absorb time, zero Float current. This is not a controller defect. Just wanted to note that contrary to the manufacturer's promise this LFP was not getting fully charged just by reaching Bulk 14.2V (my mistake, it was 14.2 not 14.4). Not even close to full. After reaching Bulk it was drawing same 8A for almost 2 hours when I set Absorb time to longer.

Again, Victron factory pre-set for their "Victron Smart LFP" is 2 hours Absorb time and 13.5 Float so I can't blame my particular LFP either. Makes me wonder about the popular mantra "no Absorb time, no Float".

[3_tons]

You might check manual section 9.3 for solar charge troubleshooting, or possibly raise the bulk voltage to a higher setting (mine is at 13.5v since float is unnecessary), yet I purposely avoid achieving a full charge as a matter of the routine since with LFP it’s unnecessary…the only time I seek a full charge is to periodically re-synchronize the SOC meter and to allow for cell re-balancing…I suspect (admittedly from a distance…) that if your SOC keeps getting lower each day it may be sky-harvest vs consumption related rather than a controller defect…JMO

3 tons

[StirCrazy]

the 40 and 20 amp dc to dc from renogy don't have an equalization phase so you don't have to worrie about that.

How big is your solar set up?

as for adsorbtion time I set mine for an hour then switch to a 13.6 float when I am camping if I hit it I hit it if not I don't usaly worry to much as its only so my BMS can do a top ballance and I have enough battery capacity to go 10 days in the fall with out the solar

[pianotuna]

Almot

Thanks for the report.

what is the input voltage for the Victron?

The DCC50S DC-DC MPPT Manual doesn't mention equalization. Which one do you have?