LiFe cell internal resistance

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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #32

    Hi mcgivor, I came up with something.  The schematic shows 50W power resistors at .5 ohms, two toggle switches and heatsink.  I think the branch circuit (3 resistors) is about 19 amps and .17 ohms.  One switch will reduce current to 6 amps when necessary and second switch will cut power allowing cells to readjust in-between cycles.  I guess heat dissipation will be similar to a 60W incandescent bulb.  And 25W resistors are available instead of 50s.  

    If you, Bill or someone can look at this and give me your opinion, suggestions…that’d be great.  Cheers

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #33
    Where you have the "V" meter... That would be "A" (Ampere Meter) for current to flow through the meter (if that is what you want to measure).

    To measure voltage, a "V" Meter would be connected "in parallel" with one of the resistors (or sort of near the resistors). Or on the "red" side of the battery (positive) before the left switch and on the black side (negative) for voltmeter (depends if you want to measure voltage with or without loads.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #34
    Yeah I’ll use a multimeter to monitor cells as they near 2.5V and string an 8S voltage checker too.  
    It sounds like this thing should work...Thnx.  


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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited May 2021 #35
    That would work however the connection should be diagonally opposite on the parrallel cells, the volt meter as pointed out should be in parallel. Just be aware that when bottom balancing  the cell voltages will begin to  vary during charging  once they approach ~ 3.400 VPC, in other words they won't be able to be fully charged, but assuming you will be operating in the discharged state more frequently this shouldn't be of too much concern.

    Limit the charging to 27.2V maximum since there is no safeguards to prevent a runaway cell, the cell voltage devices may provide some sort of indication assuming someone is watching the bank 24/7, other than that they provide little more than a meaningless  random number that offers no protection.

    Not using a BMS is kind of like buying a new car but opting out of the seat belts and airbags to save money, assuming the odds of having an accident are lower than the possibility of not having one.




    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #36
    mcgivor said:
    That would work however the connection should be diagonally opposite on the parrallel cells, the volt meter as pointed out should be in parallel....

    Ok.  Revised schematic, let me know if incorrect.  Thnx

    Yes, I’ll be operating in a discharged state more frequently.  And the SOC is minimum 10% = 24V (3.00 vpc) and maximum 90% = 26.8V (3.35 vpc).  But I’ll condition and monitor the battery at first, SOC 20/80. Btw I got these numbers from a chart and assume correct.

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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #37
    Parts ordered and expect delivery in about a week.  After battery discharge, I understand slow charge is preferred and have a 10 amp DC power supply.  Any suggestions for amp setting? 
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #38
    I also read apply max dc power supply amps (10) to charge the battery after discharge.  And when the cells are (near) charged, what is the CC reading, 0 or something else?  I’ve not used a power supply and instructions are scanty.  Thoughts Bill?  Thnx
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #39
    What output voltage is the power supply capable of? The bottom ballanced cells once connected in series is now the finished battery, the charging current will be what the manufacturer recommends, it's probably best not to use the maximum.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #40
    mcgivor said:
    What output voltage is the power supply capable of? The bottom ballanced cells once connected in series is now the finished battery, the charging current will be what the manufacturer recommends, it's probably best not to use the maximum.
    PS output voltage is 0-30V and current 0-10/5A.  The LiFe manufacturer said 1C max charge current for 280AH cell.  If I understand correctly, charge at 10 amps to 26.7V (approx. 85% SOC).  And as you suggested, dial down the charge current to condition the cells.  Any info appreciated....
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #41
    As I understand, you either Top Balance the pack. or you Bottom Balance the pack.  Top balance being less risky.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited May 2021 #42
    At 10 amps it would take 28 hours to fully charge, or 85% of that for reduced voltage limit, from a completely discharged state, you might want to consider something a little higher like 70 amps  0.25C. Since the choice was to bottom ballance, which is now complete, the battery is ready for normal service.

    The individual cell voltage  will remain at extremely close until 3.400V  or 27.2V, after which the imbalances at the top will begin to increase, the choice of 26.7V is a safe conservative value for a bottom ballanced system.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #43
    mcgivor said:
    ...The individual cell voltage  will remain at extremely close until 3.400V....
    Excellent, good to know 👌.  Eventually charge to 3.35 vpc (90%).
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited May 2021 #44
    Invest in a multi cell voltage monitor at very least, that way is possible to see what's happening, particularly in the upper limits where things may go strange with a bottom ballanced bank, once an individual cell voltage exceeds 100mV, or 0.100V, of another it's time to pay attention, as things will esculate rapidly. 

    Additionally it will provide information in the lower regon by ensuring the cells are in fact ballanced, nothing is 100%, be aware of this, have an accurate low battery cut out to prevent individual cell damage, this is why a BMS is so very important, it can think in miliseconds as to act, whereas take, a human is significantly slower, even with their finger on the pulse.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #45
    mcgivor said:
    Invest in a multi cell voltage monitor....

    Yes, a multi cell voltage monitor w/alarm.  

    Here’s my theory to protect cells-  Passive balance in a parallel configuration (for weeks).  Next bottom balance because I read this method is preferred without a BMS.  The SOC is 10-90%, a very stable range for LiFePO4 chemistry (recommended by the manufacturer).  Once cells are balanced and charged, an 8S active balancer will maintain the status quo, that’s key.  Also the system is monitored by Bluetooth, shunt-relay disconnect and inverter, charge controller sensors. 

    Anyway I’m not suggesting others run without a BMS, it’s just not for me.  I have concerns about potential issues when boondocking and would rather have control of the circuit.  I’m not into battery management but battery monitoring (once a month).  That said, I thank you for your suggestions and knowledge! 

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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #46
    I removed the 700 watt microwave after learning of low efficiency (40-60%) and replaced it with an instant pot. What’s cool is extra cabinet space with minimum modification, just make doors. The pot fits in there and an existing receptacle.
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #47

    I chose to bottom balance LiFePO4 cells and will build a DC discharger (power resistors, switches and heatsink) to slowly achieve 2.5+ volts per cell.


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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #48
    ...more solar system pics, my schedule is crazy at the moment  😬
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #49
    Lithium Batteries for RV solar (with a 300-600+ Watt array) is certainly making a dent in the propane and generator based RV 120 VAC market...

    My wife is really looking to do a bunch of RV travel in the next couple of years--So Lithium+Solar for a Class B (i.e., Sprinter type van) is interesting...

    Avoiding propane. Also, wondering about the Diesel cook top (sort of like glass/ceramic surface electric stove top, but burns diesel instead of electric element).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #50

    Also consider an Instant Pot, versatile and electric (solar powered).  I’ve heard it’s wholesome cooking.

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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #51
    A diesel cooktop sounds interesting, not familiar with it. 
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #52
    If you have sine wave power, a Induction hot plate is close to 90% efficient a putting the energy into the metal cooking pan.  Way more than microwave or any stove top burner
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #53
    Diesel Cooktop:

    https://www.webasto.com/fileadmin/webasto_files/documents/international/rv/data-sheet/rv-diesel-cooker.pdf
    https://www.stealthsprinter.com/van-build/diesel-cooktop/
    https://www.dieselheat.com.au/product-category/application/caravans-and-rv/cooking-caravans-and-rv/ (Australia price list. AU$?)

    Don't know anything else about the stove--Just looks interesting, especially for less sunny locations where solar is less available.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    edited June 2021 #54
    mike95490 said:
    If you have sine wave power, a Induction hot plate is close to 90% efficient a putting the energy into the metal cooking pan.  Way more than microwave or any stove top burner
    Yes, my house has an electric range w/induction cooktop.  It’s great, easy to clean too!

    @Bill.  Not sure of ROI after factoring in unit and conversion costs but I can see it in a diesel pusher or van as a factory option.  As for a (solar powered) instant pot, I’ll know more about efficiency when in use. 
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    edited July 2021 #55
    She’s ready for solar, I’ll post pics of the install.  Again thnx Bill and mcgivor for tech support...hope this thing flys.  lol

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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    edited July 2021 #56
    Installed solar hardware; inverter, converter, charger controller, breakers and cutoff switch.  ...it’s getting there! 


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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #57
    made copper busbars...coming along.
     

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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    #58
    Solar hardware installed in a small cubicle - battery, wire next.  And I’m not sure if generated heat will be an issue (the system is oversized and fan on hand if necessary).  I exchanged an 8 year old AC wall unit for something more efficient, solar friendly! 
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    edited July 2021 #59
    Beware of U.S. market induction cooktops.  Because of lack of residential power factor regulations (with possible exceptions of CA) they often only use simple rectifier-filter cap for AC to HV DC to run their inverter.  This gives a poor power factor which increases peak current causing greater battery powered inverter losses and cabling losses.

    Same goes for variable compressor speed inverter refrig's.  My expensive Samsung 28 cu-ft has terrible power factor of 0.6 and consumes 2 kWh's per day.  My previous 27 cu ft standard GE refrig averaged 1.2 kWh per day. The Samsung has an energy-saver switch that does nothing more then shuts down the stainless steel door heater that is there to keep door outside surface from condensation formation.  Energy Star ratings are a joke.  Samsung also seem not to be able to make an ice maker that does not continuously jam up with ice bulid up.  Regret purchasing it.

    Since mini-split air-conditioner market in U.S. is small.  Most mini-split A/C units do have PF correction to meet EU regulations.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #60
    Another issue with Samsung (and some others)--The evaporator in rear of the refrigerator has a drain that ices over after a few years of operation. There are some hacks and fixes that bring heat from the defrost calrod down to the drain--But iffy results (I have not fixed the one in a rental unit--Have to wipe up the pond under the vegetable trays ever couple of weeks). Also the ice maker is darn near useless (over 5 years old now).

    And had an LG that would ice up the freezer drain.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Suntrekker
    Suntrekker Registered Users Posts: 146 ✭✭
    edited August 2021 #61

    Solar conduit through AC vent

     


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