LiFePO4 Battery Charging Analysis : E-Lektech 12V 300Ah Batteries Not Holding Charge

Blue Skies Cabin at Rally Creek switched over from lead-based AGM batteries to LiFePO4 batteries at the start of 2024.

The main reason for the switch is that the lead-based batteries can only use 50% of the rated storage capacity to maintain battery health. Lithium batteries can use 80% of the rated storage for long term battery health. That means a 300Ah battery provides 240aH usable storage on LiFePO4 batteries versus 150Ah on AGM.

The other reason is that lead batteries like AGM are expected to last 2 years while lithium like LiFePO4 are expected to last 10. Overall the price per usable watt of power over the life of the battery is literally 10x lower. In addition LiFePO4 takes up far less space and weigh less than half the equivalent lead battery.

However, after investing in three 300Ah LiFePO4 batteries on Amazon — things are not going as well as we’d like. Our mini split is running for literally 1/3 of the amount of time we would expect from this 900Ah 12 volt stack of batteries. Something is up.

This is my personal notebook as I try to figure out why “my math is so wrong” — or something else is going on. Time to do a little deeper dive on LiFePO4 Battery Technology.

And not in the image.. now down to 93% and 1.3v another 4 hours later.

LiFePO4 Technology Short Notes

Notes are from April 2024.

  • LiFePO4 is used in marketing, but most of the tech world labels this LFP for lithium ferrophosphate
  • Weight – energy density
    • CATL LFP batteries are 125 Wh / kg
    • Can be 160 Wh / Kg with newer manufacturing methods
  • Cell Voltage
    • Minimum discharge (V): 2.0 – 2.8
      • Below 2.5V severe damage can occur
    • Working (V) : 3.0 – 3.3
    • Nominal Output (V): 3.2
    • Maximum charge (V) : 3.60 – 3.65
  • Construction
    • Internal cells are wired in series and parallel.
    • Usually uses a solid tinned copper busbar
    • For “12 Volt “batteries
      • (4) 3.2V cells are wired in series
      • Nominal Voltage: 12.8 V
  • Brands
    • Home Energy Storage
      • Enphase
      • SunFusion Energy Systems
      • SonnenBatteries
      • Tesla

Voltage Charts

State of Charge (SoC) is the percentage of capacity remaining based on the open circuit voltage (OCV) of the battery.

VOltage can tell us the SoC (percent of battery left), but not how much actual storage capacity the battery has. Most sources that explain how to determine the capacity of a LiFePO4 battery ASSUME you know the actual full capacity to begin with and that the battery is properly labelled. It does not account for degradation over time.

Single 3.2 Volt Cell Voltage Chart

The voltage chart from full to empty, graphed curve for a typical single 3.2V cell:

SoCOpen Circuit (V)
Charging at 100%3.65

This is the voltage chart provided by post-sales support at E-Lektech.

E-Lektech 12V 300Ah LiFePO4 battery voltage state of charge chart.
The E-Lektech battery voltage to state of charge chart. This is for a 12.8 volt (12 volt) battery which is four 3.2 volt LiFePO4 cells wired in series and should be 4x the values in the single-cell chart noted above.

A Real World LiFePO4 Battery Charging Experience

Some things that have been observed about charging LiFePO4 batteries — and the main take away — charging is not a linear process. When the battery is “low”, state of charge (SOC) is under 20% it SEEMS to charge faster. When the battery is “near full” with SOC > 80% is charges slower. When the SOC is > 88% , as reported by the on-board battery management system (BMS) the battery charges SIGNIFICANTLY slower and takes a LOT more power (measured in Ah going into the battery) to gain just 1% charge.

We are using the HTRC P20 Smart Charger to charge our E-Lektech 12V 300Ah Large Format battery sold to us by WHJC Energy

  • Start 10A or 20A – Start of a charging session at the specified Ah setting on the P20
  • SoC – State of Charge (%) per the BMS on the battery itself, if volts are shown it is the BMS voltage reading at that SoC.
  • +88Ah – How many Ah the P20 is reporting having output to the battery from the START of the session

Controlled Charging Session

Brought the E-LekTech battery from Rally Creek to our home base in Charleston, it has a starting charge from the solar array at the property.

Session 1 : 54% to 90% to 88% after 5m Rest

  • Sun 11:10PM Start 10A charging session: SoC 54%
    • Mon 8:10AM Check: +88Ah SoC 90%
    • Mon 8:55AM Check: +95.3Ah SoC 90%
    • Mon 10:25AM End Session: +110Ah SoC 90%
    • Mon 10:30AM After Rest: SoC 88%
  • Session Summary
    • Charge Time: 11h 15m
    • SoC: +34%
    • Ah Used: 110Ah
    • 110 Ah / 34% = 323 Ah
Takeaways from Session 1

For the first charging session we ran from 11:10PM to 10:25AM (11h 15m), pushed 110Ah to the battery. The charge went from 54% to 88% for a 34% gain. According to this math (110Ah / 34%) the battery is rated at 323Ah. Note that the charge cables are short so there is little loss there, but the battery does heat up a bit while charging so there are some losses there.

Note the SoC stays at 90% for the last couple hours of the session. This is while the charger is attached, which does influence the voltage and thus munges the SoC readings. Also note that LiFePO4 batteries need to “rest” after a charge or discharge session to stabilize voltage and provide a good reading of the actual state of charge.

Session 2 : 88% to 100% to 95% after 5m Rest

  • Mon 10:30AM Start 20A charging session: SoC 88%
    • Mon 1:00PM Check In: +38.5Ah (@14.1V / 15.5A) SoC 98%
    • Mon 1:30PM End Session: +45.8Ah (@14.2C / 15.4A) SoC 100%
    • Mon 1:35PM After Rest: SoC 95% 13.3V
  • Session Summary
    • Charge Time: 3h
    • SoC: +7%
    • Ah Used: 45.8Ah
    • 45.8 / 7% = 654 Ah
Takeaways from Session 2

Clearly calculating the battery capacity by using the charging Ah used is not a viable metric, at least not toward the end of the charging session. There is a lot of loss in heat , especially during charging beyond the 80% and notably the 88% charge point of the battery. This battery is rated 300Ah, so that last push from 88% to 95% is a bit absurd coming in at 654Ah if the same charge rate with no change in resistance.

Session 3 : 95% to 100% to 93% after 3h Rest

  • Mon 1:35PM Start 20A charging session: SoC 95%
    • Mon 3:11PM Check In: +24.7Ah (@14.2V 15.4A) BMS 100%
    • Mon 4:36PM End Session: +46Ah SoC 100%
    • Mon 4:41PM After Rest: SoC 100%
    • Mon 7:24PM After Rest: SoC 93%
  • Session Summary
    • Charge Time: 3h 06m
    • SoC: +5%
    • Ah Used: 46Ah
    • 46 / 5% = 920 Ah
Takeaways from Session 3

If is very clear that as the battery charge increases the internal resistance increases significantly. The power needed to add just 5% more charge at the last session came in at a 920 Ah rating versus 654 Ah for adding 7% to push from 88% SoC to 95% SoC. The first session, in comparison, took only 104 Ah to add 34% or a 323 Ah rating.

Session 4 : 93% to 100% to 98% after 24m Rest and 95% after 9h Rest

After letting the battery rest for several hours a random check of the battery showed the SoC at 93% and 13.3V. What in the hell. The ambient temperature dropped from 80F to 72F, but a 7% loss of charge? Hmmm…

Let’s do this charge thing again…

  • Mon 7:24PM Start 20A charging session: SoC 93%
    • Mon 10:04PM End: +40.4Ah (@14.2V 15.2A) SoC 100%
    • Mon 10:28PM Rest: SoC 98% 13.4v
    • Tue 07:27AM Rest: SoC 95% 13.3v
    • Tue: 09:24AM Rest (11h 30m) 72.5F: SoC 93% 13.3v
    • Wed: 07:24AM Rest 68F: SoC 93% 13.3v
    • Fri 3:15PM 87.6F : BMS SoC 93% Multimeter 13.32v (= 90% SoC)

Something is clearly not right with this battery. Losing 2% SoC in less than 30 minutes while in an ambient temperature of 72F does not seem right.

E-Lektech 12V 300Ah Large Format Specifications

Given the variations in real world charging tests, it is time to dig into this battery. Since they are an “Amazon Only” seller and do not have a presence anywhere else (why would they, Amazon protects third party sellers fairly well) the only info we can get is on the Amazon listing here:

Details from their marketing:

  • Voltage: 12V
  • Number of Cells: 4
  • Capacity (Rated): 300Ah
  • Usable Energy: 3840Wh
  • Max continuous discharge current: 250A
  • Max Load: 3200W
  • Water/Dust Rating (Ingress Proteaction): IP65 , 6=Dust Tight highest rating, 5=water jet limited ingress (5 of 8)
  • M8 Terminal – 16mm
  • Weight: 71 lbs
  • BMS: 250A, protects from:
    • Over current
    • Over discharge
    • Over voltage
    • Short circuit
    • Overcharge
    • Overheating
    • Low Temp Cutoff
  • Automotive A Grade Cells
  • 6P6S configuration supported
  • Cycle up to 8,000 times over 10 years
    • 4,000 cycles if depth of discharge (DOD) is 100% every time
    • 6,000 cycles at 80% DOD
    • 15,000 Cycles @ 60% DOD
  • Warranty: 5 years
  • On The Box…
    • E-LEKTECH : Lithium Battery Expert
    • UN 4G / Y46 / S / 23
    • Meas: 61.6x33x31CM
      Made In China
    • SKU: EL12-300-B


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