LiFe cell internal resistance

Hi All. Newbie Greetings from AZ!
I’ve searched high and low for an answer to this question but no sun : ). I need help understanding principles of the resistor.
Question: I’d like to calculate internal resistance of a 3.2V LiFePO4 280 amp cell by using a resistor to cause voltage drop and apply Ohms and Kirchhoff laws. Can you suggest a resistor in a closed circuit (no other load), is this doable?
Basically I’m after a test similar to the YR1035 battery internal resistance meter. Any suggestions, comments are appreciated, Thnx!
Basically I’m after a test similar to the YR1035 battery internal resistance meter. Any suggestions, comments are appreciated, Thnx!
Comments
- V=I*R
- R=V/I
- R = "delta V" / "delta I" <=corrected to "I" current
For example., Say your battery shows 3.60 volts at 10 amps and 3.55 volts @ 20 amps:- (3.60 - 3.55) V / (20-10) A = 0.005 Ohms = 5 mOhms (battery should be roughly in the 0.005 to 0.015 Ohm range???)
You don't have to use resistors--You can use your actual loads--For example AC inverter with portable heater and set low vs high output.If you want to find some power resistors... Say you want 5% vs 10% load on cell (just round numbers--You probably want to test around your planned/actual loads):
- 280 AH * 0.10 load * 3.7 volts (full?) = 103.6 Watt rated power resistor
- 280 AH * 0.10 load = 28 Amps
- R=V/I= 3.70 volts / 28 Amps = 0.132 Ohms for 10% rate of discharge
You could, if you have some 10 AWG copper cable around make a load. For example 10 AWG cable with coils spread out (for cooling), is around 1 ohm oper 1,000 Feet:http://www.interfacebus.com/AWG-table-of-different-wire-gauge-resistance.html
- 1 Ohm per 1,000 feet * 0.132 Ohms needed = 0.132 of 1,000 feet = 132 feet of 10 AWG cable
Copper cable increases resistance (somewhat) as the cable gets hot--So actual voltage and current measurements (such as Current Clamp DMM) suggested for accurate calculations.Or you can get an electronic load:
https://www.amazon.com/s?k=electronic+load+DC+battery&ref=nb_sb_noss
You have to go through the specifications and reviews to find what you want... This one is close (down to 4.0 volts, but review said tested single Li Ion cell)...
https://www.amazon.com/Electronic-Adjustable-Constant-Current-Capacity/dp/B083V42CTY
Lots of choices out there...
-Bill
By the way, notice I had a typo in the 3rd equation. Was supposed to be "delta I", not delta "R"... Equation fixed.
By the way, "delta" is the change in something--In this case, the change in voltage over the change in current.
-Bill
Also, need to finish connecting the negative bus of the Li Ion battery bank wiring schematic.
And from your other question in the RV Grounding discussion, you want the AC Neutral bus to be NOT be connected to RV chassis ground (AC inverter has internal transfer switch to shore power which manages N+G ground bonding).
-Bill
The solar panel frames--Grounding them with a "Green" or bare copper wire to frame/chassis/battery negative ground is done to prevent energizing the frames if there is a a short from solar to frame/mounting. You do not need grounding for operation--Just for safety.
Do you rely on your mounting hardware (panel frame to racking, racking to RV roof, etc.)? If the RV is a steel skinned vehicle (like a delivery van)--You have better connections. If if this is an aluminum skinned RV (trailer, etc.)--Those have poor "frame grounds". If you have a plastic skinned RV--Then obviously no grounding... Running a green wire from panel/racking to vehicle ground--The best way to go.
If you have "tilting" frame of some sort (more RV folks are starting to do the tilting frames--Works for better farther north/winter time harvest). You want wire grounding to prevent current flow through "bearings"... Current flow (short circuit, other leakage current) can quickly ruin pivot bearings).
-Bill
@Horsefly. Yes, I have the PowerMon.
If this was for protecting against lightning strike--I would be suggesting 6 AWG minimum in theory.
-Bill
Being middle ballanced would likely results in a single cells voltage drifting as it approaches both top and bottom, reducing overall capacity due to the BMS cutting the load when a single cell is out of parameter with the others. Logic may suggest having them both top as well as bottom ballanced would be optimal, however this can't be done, it's on or the other.
To achieve top ballance either a programmable power supply or a single cell charger can be used, the charger is a simple procedure of connecting and waiting for a week or so, depending on current output, until it turns itself off, connection of the charge leads being placed diagonally opposite rather than at one end.
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.
Yes, I’m leaning towards bottom balance because of a 10-85% SOC range. Of course feedback is important.
And the system is without a BMS (though controversial) but the circuitry has protection. I just don’t trust an inexpensive BMS but that’s me.
In the meantime I’ll passive balance the cells (in parallel) and study bottom (top) balancing...sure to have questions. And attached is a revised wire diagram, suggestions always appreciated. Thank you Bill and mcgivor!
I suppose it wouldn't make much difference where the ballancing is performed if the charging, along with the dischargeing, is protected by cutoff settings, however this is a risk one needs to consider without a BMS. Most commercially available controllers or inverters use a sum total voltage with a 100 mV resolution, which is pretty vague considering a BMS monitors individual cell voltages at 1 mV resolution and the higher the nominal voltage the larger the potential of having a single cell drift outside it's safe limitations.
Choose what you may, it's your equipment, your opinions are yours, my opinions are mine, if you don't like what I'm saying, that's fine, but just remember to keep an open mind, because a person who fails to listen is blind.
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.
I've read dozens of write-ups on balancing cells for solar, and I don't think I've read more than one or two that recommended something other that top-balancing.
Understood, thnx. And the expertise offered by forum members is very much appreciated!
This sounds almost like controlling "skin depth" current distribution in conductors--Generally connected with AC power:
https://en.wikipedia.org/wiki/Skin_effect
And the context of "Litz" wire add braided cables to make the most efficient use of the copper cross section:
https://en.wikipedia.org/wiki/Litz_wire
The above applies to "AC" power/current (alternating current).
In theory, does not apply to DC constant current applications... HOWEVER, I have always wondered if "skin effect" may apply to our DC input for AC inverters... The input current is generally a "sine squared" current wave form (sort of a sine wave or rounded pulse train--Current peaks when AC voltage/current peaks, and drops to near zero when AC sine wave crosses zero voltage).
-Bill
https://www.powerstream.com/Wire_Size.htm
skin effect on a #2 solid starts at 410 hz, on OOOO starts at 125hz
|| 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 ,
Here you go, you’re better at interpreting this PDF.
Excerpts-
“The paper discusses how introducing a slotted hole shape for the bolts in bolted busbar connections increases significantly the true contact area and reduces contact resistance.
The new slotted hole shape arises from [3] where
longitudinal slots of width 3-4mm and length 50mm are
introduced between the buses holes in order to expand
the true contact area.”
A question, what’s the easiest way to cut slots and is it worth the time.
https://www.researchgate.net/publication/237770786_Bolted_Busbar_Connections_with_Slotted_Bolt_Holes
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.
The cells were actually fairly close at 3 mV hense the short time taken to stabilize.
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.
Great. I read (for my application) to discharge 280AH cells individually to an equal low voltage then combine them and charge. I have a DC power supply providing CV and CC. 6V auto lamps to discharge sounds good. Can you describe your setup; number of lamps, total wattage, approx time to discharge? Thnx
To prevent a runaway cell limit the voltage to around 3.450 VPC or 27.6V, perhaps less as you plan on not utilizing a BMS, the individual cell voltage spikes happen in fractions of a second, faster than can be monitored manually, even with multiple meters. For this reason top ballancing with a low voltage disconnect at 20%SOC is a simpler way to operate, naturally a personal opinion.
The 6V automotive lamps I used were 21W, I used sockets to facilitate easy removal to adjust the load, but it must be noted I was only dischargeing individual cells, perhaps a high power rheostat would be a better alternative but the lamps were easier to obtain in my particular case.
Using a light load is recommended to allow minor differences to slowly adjust, it's the reverse in top ballancing, using low current especially as the knee approaches, my 3200Ah bank took close to 2 weeks using a single cell charger, don't attempt to rush things, patience will pay dividends.
Personally I wouldn't bottom ballance nor operate without a BMS, but to each their own.
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.