LiFePO4 batteries...Rule of thumb changer?

DustyDusty Posts: 197Solar Expert ✭✭✭
Since LiFePO4 batteries are capable of much higher drain and recharge current, does this battery chemistry alter the rule of thumb of 100 Ah battery capacity per 1KW of inverter output?
XW6048, 3.4KW PV, Grid-Tied, always tweaking.

Comments

  • mcgivormcgivor Posts: 2,044Solar Expert ✭✭✭✭
    Rule of thumb? 100Ah at 12V is vastly different from 100Ah at 48V, 1Kw for how long, time is critical to the equation, there are too many variables to blanket with a simple rule of thumb, I would think, but considering depth of discharge ability, charging efficiency LiPo4 definitely have an advantage, theoretically.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • BB.BB. Posts: 27,682Super Moderators admin
    The rule of thumb also gives you (hopefully) enough stored energy that you can run the inverter with loads for a reasonable amount of time without sun (as an example: C/20 discharge rate--5 hours for 2 night, 50% maximum discharge for longer lead acid battery life).

    If you can support a C/1 discharge rate--That is great if you have loads that run for short timeframes with heavy power needs (say a deep well pump). But 30 minutes a day for a couple days is not a typical off grid home usage pattern.

    If you cycle that Li Ion battery from 20% to 80% state of charge (for longer Li Ion battery life)--That is 60% capacity cycling compared to the 50% cycling we suggest for lead acid... Not that much difference for basic capacity planning.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DustyDusty Posts: 197Solar Expert ✭✭✭
    mcgivor said:
    Rule of thumb? 100Ah at 12V is vastly different from 100Ah at 48V, 1Kw for how long, time is critical to the equation, there are too many variables to blanket with a simple rule of thumb, I would think, but considering depth of discharge ability, charging efficiency LiPo4 definitely have an advantage, theoretically.
    Thanks, I understand your point.  But that 100Ah/1KW inverter output has been posted so many times on this forum that I nicknamed it a "rule of thumb." Didn't mean to trivialize it, but since the LiPO4 batteries are so superior, I wondered if they significantly changed the equation in computing battery bank capacity requirements.
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
  • DustyDusty Posts: 197Solar Expert ✭✭✭
    BB. said:
    The rule of thumb also gives you (hopefully) enough stored energy that you can run the inverter with loads for a reasonable amount of time without sun (as an example: C/20 discharge rate--5 hours for 2 night, 50% maximum discharge for longer lead acid battery life).

    If you can support a C/1 discharge rate--That is great if you have loads that run for short timeframes with heavy power needs (say a deep well pump). But 30 minutes a day for a couple days is not a typical off grid home usage pattern.

    If you cycle that Li Ion battery from 20% to 80% state of charge (for longer Li Ion battery life)--That is 60% capacity cycling compared to the 50% cycling we suggest for lead acid... Not that much difference for basic capacity planning.

    -Bill
    I read that with LiFePO4 batteries, it's a good idea for extended life not to charge them past 95% SOC, but 80%?  I was looking at the Discover batteries because they incorporate xanbus.  Is 80% SOC what their BMS tells the SE MPPT-60/150 controller to float them at? 

    I thought that being able to discharge them down to 20% and up to 95% would dramatically increase net capacity, so I'd still get my 3 days autonomy with a much smaller battery bank.  IMO, that was one of the biggest selling points with shifting to Lithium.  Thanks for the insight.
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
  • BB.BB. Posts: 27,682Super Moderators admin
    100 AH per 1 kW of inverter capacity is @ 48 volts. At 24 volts that would be 500 Watts of inverter per 100 AH, and at 12 volts would be 250 Watts per 100 AH.... (this is also suggested as the nominal maximum solar array size for lead acid batteries).
    • 1,000 watts * 1/0.85 inverter eff * 1/48 volt nominal battery voltage = 20.8 amps nominal current
    • 20.8 amps / 100 AmpHour battery capacity = ~ 1/5 hour discharge rate
    • Surge power capacity of inverter is ~2x rated capacity, then 2kW surge ~ 1/2.5 hour discharge rate
    Yes, Li Ion and other battery designs (such as AGM--Some are rated to C4 or 15 minute discharge) can support higher discharge rates -- But that still leaves us with the question of what are your loads and energy needs... Do you need an AC inverter+loads for only 15 minutes and then you need a full charging cycle for the battery bank?

    And there are batteries which support high discharge currents (approaching 15 minute discharge rate) but C/20 hour recharge rate such as a typical US GEL (gel lead acid battery). These high discharge rate systems are typically use for UPS (uninterruptible power supplies) and are intended to keep the power up just long enough to start the backup genset. Many times, these systems are design for the batteries to be replaced every ~2 years (if used or not), and many will only survive a couple such cycles before they fail and need replacement anyway.

    Obviously a Li Ion battery bank would be very interesting for these types of installations (UPS+Genset)--But high drain ability of Li Ion does not seem to offer a huge advantage over Lead Acid (more expensive price wise). Although cycling and storage life of LiFePO4 batteries do seem to be very interesting (longer cycle/storage life and less maintenance than Lead Acid--And integrated BMS battery management systems seem to getting there).

    We (I) use the rules of thumbs to quickly estimate sizing of an off grid solar power system for a "typical" installation. If your usage is not typical, then we have to go through your needs and figure out what works best for you...

    Remember that we are designing for (nominally) worst case loads and weather conditions that should give you a reliable and relatively cost effective system without having to redesign the wheel every time.

    If your needs are different, we can talk about those specifically.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DustyDusty Posts: 197Solar Expert ✭✭✭
    I was encouraged that I could install LiFePO4 batteries in the same location as my current AGM set and at least double my current usable capacity.  But even then, I would still be slightly below the 100Ah per 1Kw of inverter (48v) recommendation.  But I was wondering if the ripple induced in an undersized battery bank was as much of a concern with a lithium bank that can handle so much more current than an AGM bank.
    I live in an area prone to hurricanes, but our grid is pretty stable.  After a major storm, however, it could be several days before power is restored.  My critical loads total about 2Kwh/day, and for three days of autonomy, I was looking at a Discover AES 48v 6.6Kw LiFePO4 battery to replace my two strings of AGM batteries when they start to show signs of age.  The only surge my XW6048 gets is my refrigerator and chest freezer compressors, so it is oversized for my current needs.  When both refrigerator and freezer are running, I'm using about 250W. 
    I also like the fact that the Discover AES has xanbus, and I was wondering if anyone on this forum had personal experience with this battery system and how it integrates with the XW/XW+ line of Schneider inverter/chargers. 
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
  • karrakkarrak Posts: 304Solar Expert ✭✭✭✭
    edited August 8 #8
    I think the main reason that this rule of thumb has come about  is because of Lead Acid battery's poor energy efficiency and reduced lifespan at high charge/discharge rates (peukert effect).

    Poor efficiency equates to more heat being generated in the battery which severely affects the lifespan and can cause damage to the battery.

    With all Lithium batteries the Peukert effect is not an issue. It is still important to limit the charge/discharge current so as to not heat the battery up as it will decrease the lifespan and if taken too far could become a safety issue.

    From all the data I have seen I think a good rule of thumb for LFP (Lithium Iron Phosphate) batteries is to limit the charge rate to 0.5C (2500W per 100Ah @48V) and discharge current to 1.0C (5000W per 100Ah @48V).

    These high discharge rates make it possible to run power hungry equipment like ovens, induction cooktops, microwaves, and welders.

    The high charge rates, and no need for long "absorb" times make it possible to use large solar arrays which reduces the size of the battery needed to tide one through cloudy weather.

    Simon
    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 4p8s (24V), 4kW Latronics Inverter, 1160W of Solar Panels, homemade MPPT controller
    Homemade BMS https://github.com/simat/BatteryMonitor
     

  • karrakkarrak Posts: 304Solar Expert ✭✭✭✭
    Dusty said:
    I read that with LiFePO4 batteries, it's a good idea for extended life not to charge them past 95% SOC, but 80%?  I was looking at the Discover batteries because they incorporate xanbus.  Is 80% SOC what their BMS tells the SE MPPT-60/150 controller to float them at? 

    I thought that being able to discharge them down to 20% and up to 95% would dramatically increase net capacity, so I'd still get my 3 days autonomy with a much smaller battery bank.  IMO, that was one of the biggest selling points with shifting to Lithium.  Thanks for the insight.
    From the research I have done and my experience of using LFP batteries for over five years I would say that it is the time the batteries are at high SOC rather than what SOC you charge them too that is the issue. Sure, keeping the batteries in the SOC range 70%-30% will increase their lifespan, but I don't think the increase in lifespan can justify the loss of usable capacity and more importantly the utility of being able to have a battery full at the start of cloudy weather.

    The two batteries that I installed over five years ago which are charged to >95%SOC when there is sufficient sunshine show little reduction in capacity and more importantly show no charge in internal impedance/resistance which is the thing that leads to the death of LA batteries.

    Simon

    Off-Grid with LFP (LiFePO4) battery, battery Installed April 2013
    32x90Ah Winston cells 4p8s (24V), 4kW Latronics Inverter, 1160W of Solar Panels, homemade MPPT controller
    Homemade BMS https://github.com/simat/BatteryMonitor
     

  • DustyDusty Posts: 197Solar Expert ✭✭✭
    For my use, the high cycle rate of lithium is not as much a priority as the energy density is. I have very limited space next to the inverter in my attached garage, and lithium would allow me to expand my capacity in the same footprint.  And these batteries would not be cycled unless the grid goes down, which is typically only a couple times a year.  But when the grid does go down, it's usually due to a hurricane pushing trees/branches onto power lines--and the utilities don't start making repairs until the storm moves out of the area.  Although I have a manual start generator, I don't roll it out and start it up during the storm.  So having three days of autonomy to give the storm time to pass through is what I'm planning for.  So in that case, I'd be sure to have the batteries topped off before the storm hits.  But in normal day-to-day operation, I'm okay with keeping the batteries less than 100% charged. 
    So my question is, does the Discover AES batteries with xanbus tell the MPPT 60/150 the ideal float voltage to keep the batteries at the ideal level of charge for maximum life, and if so, what float voltage value does it set? 
    Is anyone on this forum using the Discover AES batteries with a Schneider XW/XW+ grid-tied inverter?
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
  • Dave AngeliniDave Angelini Posts: 3,882Solar Expert ✭✭✭✭
    I do but I am in business ;)
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • DustyDusty Posts: 197Solar Expert ✭✭✭
    edited August 8 #12
    Unfortunately, NAWS does not sell them yet.

    @Dave Angelini , can you shed some light on what parameters the AES batteries control with the Schneider MPPT and inverter/charger through xanbus, and what the charge settings are? I'm really curious what state of charge the batteries tell the SCC to maintain them at.  Thanks!
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
  • Dave AngeliniDave Angelini Posts: 3,882Solar Expert ✭✭✭✭
    I have a different supplier.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • mcgivormcgivor Posts: 2,044Solar Expert ✭✭✭✭
    Dusty said:
    For my use, the high cycle rate of lithium is not as much a priority as the energy density is. I have very limited space next to the inverter in my attached garage, and lithium would allow me to expand my capacity in the same footprint.  And these batteries would not be cycled unless the grid goes down, which is typically only a couple times a year.  But when the grid does go down, it's usually due to a hurricane pushing trees/branches onto power lines--and the utilities don't start making repairs until the storm moves out of the area.  Although I have a manual start generator, I don't roll it out and start it up during the storm.  So having three days of autonomy to give the storm time to pass through is what I'm planning for.  So in that case, I'd be sure to have the batteries topped off before the storm hits.  But in normal day-to-day operation, I'm okay with keeping the batteries less than 100% charged. 
    So my question is, does the Discover AES batteries with xanbus tell the MPPT 60/150 the ideal float voltage to keep the batteries at the ideal level of charge for maximum life, and if so, what float voltage value does it set? 
    Is anyone on this forum using the Discover AES batteries with a Schneider XW/XW+ grid-tied inverter?
    If one takes the time to research Schneider solar , using keywords, such as Discover AES and bridge, the answers are available, not that that's what I'm looking for, but they are there, I've taken a peek. 
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • DustyDusty Posts: 197Solar Expert ✭✭✭
    Ok, I'll dig deeper on the Schneider website.  I've already been talking with their engineers, but they haven't responded about Discover AES batteries.  I did get a .pdf from them on how their MPPT controllers and inverter/chargers can be set up for lithium, but it didn't talk specifically about xanbus parameters for batteries that support it.  I was just trying to cover all my bases--didn't mean to appear lazy--just trying to learn from the experience of those in this forum. 
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
  • mcgivormcgivor Posts: 2,044Solar Expert ✭✭✭✭
    No implementation of laziness on your behalf, just pointing out how to use keywords to get results, it's a new technology with Schneider so personal experience may be limited.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • DustyDusty Posts: 197Solar Expert ✭✭✭
    edited August 8 #17
    Thanks @mcgivor, I'll give the keyword search a try! 

    The only "Discover AES" hit I got on their search function was a boilerplate press release dated 27 June announcing their collaboration, but it had no specifics regarding parameters; what SOC the battery requests from the SCC, for instance.


    I'll keep searching.
    XW6048, 3.4KW PV, Grid-Tied, always tweaking.
  • BB.BB. Posts: 27,682Super Moderators admin
    If folks are worried about links to non-NAWS sites... No problem as long as links are answering questions asked.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.