Lithium Capacity Question

[sailjumanji]

Previously had two Mastervolt 270 Ah AGM's, that we have replaced with a Mastervolt MLI Ultra 5000. (Also put in two MacPlus DC-to-DC chargers, safety relay, everything on Masterbus system, changed charge parameters, etc.) The MLI is LiFePO4 chemistry battery with stated capacity of 360 Ah. So from a usable Ah metric, we have about the same capacity (288 vs 270).

For the AGM batteries, I know the capacity is based on the discharge rate (current). Specs for the 12/270 AGM show discharge at 13.25 amps takes 25 hrs to deplete to 10.8 volt cutoff, equals 265 Ah. At 45 amps it takes 5 hours, equals 225 Ah capacity.

So here is my simple question. Does the LiFePO4 battery have 360 Ah capacity, regardless of discharge rate? Within reasonable limits, for instance 10-25 amps.

Second question. Whether on solar charge or shore power charge, the MLI gets to 99% SOC in the bulk phase. Charger switches to absorption, and that may go on 1-3 more hours, with the amps just doing instantaneous cycling of 0 to 30-50 amp peaks. Its like its turning on and off, but when it comes on, it is full current. Then it drops to float eventually, and settles in at 13.5 volts. I don't see any reason to have this absorption phase, although it is part of the Mastervolt MLI charge setting. It is adjustable using the MasterAdjust program, so am considering making the max absorption period 30 minutes. Am I missing something that says I shouldn't do this? That it needs that absorption top off? (I don't think so but thought I would confirm via the braintrust!)

[smac999]

Generally absorb is set to 0 for lithium

[sailjumanji]

Originally Posted by smac999

Generally absorb is set to 0 for lithium

Yeah, I thought so too. But my solar controller is a Mastervolt Solar Chargemaster. It has a specific MLI setting - 14.25 volts bulk and absorption, 13.5 volts float. The stock "maximum absorption period" is set for 240 minutes. Strange to me as it's a Mastervolt controller with a Mastervolt lithium setting specific to the MLI.

That said, I tend to agree with you. Maybe I will set it to something really short like 10 minutes. A little concerned that it might have a problem if set just to go from bulk to float.

[lmxr]

Originally Posted by sailjumanji

[...]
So here is my simple question. Does the LiFePO4 battery have 360 Ah capacity, regardless of discharge rate? Within reasonable limits, for instance 10-25 amps.

You can expect 360 Ah or more. The internal resistance of an LFP, one in good shape of course, is so low that there is little to no capacity reduction effect at the currents you mention.

Originally Posted by sailjumanji

[...]
Second question. Whether on solar charge or shore power charge, the MLI gets to 99% SOC in the bulk phase. Charger switches to absorption, and that may go on 1-3 more hours, with the amps just doing instantaneous cycling of 0 to 30-50 amp peaks. [...]

I don't think LFP likes a 3 hour absorption charge... nor that it needs it. But I'm not Mastervolt .

[sailjumanji]

Is there any reason at all to have absorption phase, if bul gets to 98-99%? Sounds like not.

[CatNewBee]

Originally Posted by sailjumanji

Is there any reason at all to have absorption phase, if bul gets to 98-99%? Sounds like not.

To give your balancers time to do their job at low charge current.

[sailjumanji]

Originally Posted by CatNewBee

To give your balancers time to do their job at low charge current.

This is probably a pretty hostile environment to do balancing. Note the current just spikes up and down. Have done two tests of Solar Chargemaster, and both look similar. (Although previous test had absorption period of less than an hour.)

[lmxr]

Originally Posted by sailjumanji

This is probably a pretty hostile environment to do balancing. Note the current just spikes up and down. Have done two tests of Solar Chargemaster, and both look similar. (Although previous test had absorption period of less than an hour.)

Thank you for the interesting graph. It illustrates the issue very well. To be honest, the voltage regulation of the solar charge controller appears quite poor... I expected better from a Mastervolt product.

I would limit absorption time to 10 minutes or less. 10 minutes daily should be sufficient for balancing because, as far as I know, Mastervolt uses high-quality Winston cells together with rather powerful active balancers. Some people run these cells for years without any balancing, which shows that these cells don't require much balancing. However, maybe Mastervolt put some secret sauce in their MLI batteries that requires a 3 h absorption duration... you should ask Mastervolt about that, I'm afraid.

I hope limiting the absorption time solves most of your issues. Maybe there are other, possibly better, 'solutions'? How does your solar charger currently decide when to switch to float? Does it only use the absorption timer? Does it take battery current into account? Does it allow specifying a taper current? If so, what taper current is currently set?

[more]

Originally Posted by lmxr

Mastervolt uses high-quality Winston cells together with rather powerful active balancers. Some people run these cells for years without any balancing, which shows that these cells don't require much balancing.

i use Winston cell 2x 200ah 3 year*now be 4 year without BMS
Victron MPPT
and also 1 pack to another pack is connected with 8 meters 95 mm2 cables*limited space
active balancer 5 A ,

[sailjumanji]

Originally Posted by lmxr

However, maybe Mastervolt put some secret sauce in their MLI batteries that requires a 3 h absorption duration... you should ask Mastervolt about that, I'm afraid.

I hope limiting the absorption time solves most of your issues. Maybe there are other, possibly better, 'solutions'? How does your solar charger currently decide when to switch to float? Does it only use the absorption timer? Does it take battery current into account? Does it allow specifying a taper current? If so, what taper current is currently set?

It actually limits the absorption period to no more than 4 hours. It's never gone that long. This is a graph of the previous test we logged. Absorption lasted 55 minutes, but had the same spiking amps and voltage as the other test.

I need to look into what triggers shift to float. Unfortunately when Seawind programmed the MLI, they also locked the configuration changes on my Solar ChargeMaster. (So I can't see all of the parameters.) I've emailed Seawind, who tends to work pretty closely with Mastervolt.

[psahd]

With the SOC getting to 99%, I would rather go with zero absorption and probably gain an extra 5-10 years of life from the bank. There is no need to screw the last ounce of capacity out of the cells.

[sailjumanji]

Originally Posted by psahd

With the SOC getting to 99%, I would rather go with zero absorption and probably gain an extra 5-10 years of life from the bank. There is no need to screw the last ounce of capacity out of the cells.

Agreed. I was actually thinking about 10 minutes max time, in case for some reason the float phase needs to see an absorption first.

[T1 Terry]

As far as battery capacity, if they are in fact Winston LYP cells and the advertised capacity is the same as the advertised capacity on the Winston cells, then the whole 100% of the capacity can be used as long as you have a BMS that will stop the discharging if a cell drops to 2.8v under load.
Winston test for capacity at 0.05CA or C2 rate, discharged from 100% to 0% SOC in 2 hrs .... that's a continuous load of 180 amps for 2 hrs.

As far as absorption cycle charging, the bulk/absorption voltage is the key. If the absorption voltage can not be set lower than the bulk voltage, then set the bulk voltage to 13.9v and absorption to 30 mins and look to see what the current flow is at the end of the 2 hrs.

The on/off/on you are seeing is the batteries BMS disconnecting due to over voltage, that is the reason I'm recommending a lower absorption voltage to allow the balancer to do its job without risking cell over voltage.

What battery monitoring do you use, if it has Ah counting set the Peukert factor to 1.00 and the charge efficiency to 100%. The real figures are 99.?% but generally the accuracy of these devices isn't close enough to get within 1% either side so by using these settings you effectively disable the monitors calculation for losses so you get a far better accuracy reading.

T1 Terry

[Pete7]

Continuing nonsense by a member has been deleted, back to LFP capacity questions.

Pete

[T1 Terry]

Just to clear up the misconception about the 80% depth of discharge being the limit for LFP and LYP cells. This all came about by someone not understanding what they were reading and fuelled a belief in something that isn't actually factual by quoting their definition of what the believed the manufacturers graphs said. This has been repeated so many times that it is now become an accepted fact, even though it is completely wrong.

The 80% DoD (Depth of Discharge) and 5,000 cycles actually means that after 5,000 cycles to 100% DoD the cells will still have a minimum of 80% of their original advertised capacity.
This can be confirmed by digging deeper into the test conditions used by the likes of Winston Thundersky.
These are extreme cycling conditions they are using to prove the viability to the military, something we would never do with them as house batteries.
By using the advertised capacity as the bench mark of 100% DoD the cells are not stressed to the extent the factory stresses them, yet the factory shows graphs like this

The graph shows each progressive stage of capacity loss, not the number of cycles you can expect if the DoD is limited to 80% to get 5,000 cycles and limited to 70% DoD to get 8,000 cycles.

T1 Terry