Lithium Phosphate

Off the Grid

New Member
Am struggling to get my head around our new lithium phosphate batteries.After 30 years of lead acid banks we replaced with 4 x SimpliPhi OES3 LFP 3.5kWh 24V batteries.If they dip below 24.8volts the generator kicks in to recharge.I have been told this is normal as they are "rated to 80% discharge".The lead acid`s were bulky and needed constant maintenance but i could run them down,occasionally to 21volts and this new system seems so fragile compared to the old bank.Have read the science but struggle with the concept of a 24 volt battery being technically "flat" at 24.7v.Anyone else out there??
 
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toms

Solar Addict
Yep, my 48V system is “flat” at 51V, so that sounds about right.

Mine is a 400ah system, and i often take close to 300ah out of it without dropping to 51V.

How much capacity had you used out of your lead acid batteries when they got to 21V ?

Don’t stress about voltages, use the 80% capacity that you have available.
 

Off the Grid

New Member
thanks toms,you would think that a replacement for lead acid would be at least as robust as before but it appears that,s not the case.
we converted from 32 x 12volt 225ah which is heaps,down to 65% from memory,but required constant maintenance.
about to add another 3.5 so total storage will be17.5kwh which sounds ridiculous to an old timer.....
 

rmaddy

Full-time Solar-powered Trailer Life
24.7V for Lithium Iron Phosphate (LiFePO4) is roughly 12% SOC. That's getting pretty low. 28.8V is basically 99.9%. You have to adjust your mindset to the different working voltage range compared to lead acid.

I'm not sure what makes you refer to the LiFePO4 as fragile. Just keep in mind that the voltage reading on LiFePO4 isn't quite as useful since 99% down to 20% is less than 0.75V difference. A shunt that lets you actually see the true state of charge (SOC) of the battery is almost a must-have with LiFePO4 batteries so you really know the current status of the battery.
 

Off the Grid

New Member
thanks rmaddy
24.7V for Lithium Iron Phosphate (LiFePO4) is roughly 12% SOC. That's getting pretty low. 28.8V is basically 99.9%. You have to adjust your mindset to the different working voltage range compared to lead acid.

I'm not sure what makes you refer to the LiFePO4 as fragile. Just keep in mind that the voltage reading on LiFePO4 isn't quite as useful since 99% down to 20% is less than 0.75V difference. A shunt that lets you actually see the true state of charge (SOC) of the battery is almost a must-have with LiFePO4 batteries so you really know the current status of the battery.
i meant fragile as compared to the previous lead acid bank-no offense to Simpliphi-as i said,just getting my head around the different parameters...
will check the shunt on the SPMC241 :cheers
 

Bossrox

Solar tinkerer
What you're confusing is thinking that lith's low voltage cutoff is similar to lead/acid & that far from the case. A lith's 21.7v as another mentioned is about 12% remaining & is about equal to a lead acids 20.5v for the same drain. If you were draining lead acids down that far, that torture on them & lucky if you got any kind of decent lifespan from them. Lith's can handle that kinda discharge depth way better. Here's a typical volts by drain % chart that will graphicly show how they differ.
 

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Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
Unlike Lead, LFP has a very flat voltage curve. The working voltage range is 3.000-3.400Vpc (24.0V-27.2V) and the simplihi's A+ cells can do better but there are BMS limits. The gross voltage range is 2.500-3.650Vpc (20.0-29.2). Many people cutoff at 22.0 (2.750Vpc).

Your Auto Generator Start settings will require adjustment for the voltage thresholds for LFP.
Also, it is extremely essential that the system is Voltage Corrected. Due to the flat curve, a 0.5V difference in readings will throw things off. All Lithium Based Batteries & their BMS' operate at millivolt & milliohm rates, they are not "brute force" like lead. This is in fact a common error that occurs and is often overlooked, especially for those converting from Lead to Lithium base.

Solar Charge Controller while charging will commonly sense/show a Lower Voltage while Charging, when no charging is occurring they tend to show a higher voltage, They should be voltage matched for the Charging differential between the battery bank batt post terminals & the SCC.

Inverter/Chargers act similarly as well. when charging they sense lower voltage, while or discharging they sense higher. In this instance, it should be voltage matched to the Discharge differential.
* There is a link regarding calibration in my signature as well as other material that may be helpful to you.

Below are Thumbnails, click to expand, you may wish to save these for reference.

1637800881435.png 1637800813950.png

Hope it Helps, Good Luck.
 

toms

Solar Addict
about to add another 3.5 so total storage will be17.5kwh which sounds ridiculous to an old timer.....

Not sure why that sounds ridiculous.

I usually recommend people have a battery that is 150% of their maximum overnight useage.

20kwh is the average size of LiFePO4 systems around here.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
I hafta chuckle. Went from 956AH (428 useable) Rolls Surette S-550 to 30kWh of LFP and now the Lead is retired to a secondary backup bank.
No such thing as too much storage LMAO ! Given that I use 3.5-4.0kWh per 24 Hours on average, I got reserve but I'm also farther north than most LOL.
 

JimJr

Solar Enthusiast
I hafta chuckle. Went from 956AH (428 useable) Rolls Surette S-550 to 30kWh of LFP and now the Lead is retired to a secondary backup bank.
No such thing as too much storage LMAO ! Given that I use 3.5-4.0kWh per 24 Hours on average, I got reserve but I'm also farther north than most LOL.
I have relatives in that area . "3.5-4.0kWh per 24 Hours" is because the freezer compartment of your refrigerator is on the outside and the inside
is to keep the milk from freezing over night :)
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
My home runs on that, not just a fridge. BUT I have very efficient Energy Star-5 Electric appliances, on-demand hot water (dual system, 1 Radiant, 1 Hot Water) and stuff. I do not have the Fridge inherited from Granny or any wasteful devices at all... When I started this, it was from Scratch, I even had to buy my tools and equipment as everything had been destroyed.
 
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