The other video ‘SFK-260 vs SFK-260HP Watt Testing’ show what good cells should test at. They are cherry-picking for their higher priced 'HP' version of battery.
‘SFK-260 vs SFK-260HP Watt Testing’ video
The good battery.
Test Instrument ‘Tonghui’ HP8182 DC Electronic Load
12v battery, SFK-260HP. Likely 280AH used cells they are giving 260 AH derating. Same 54 lbs weight between 280 AH model and 260 AH model indicating use of used cells with some derating for used cells. As mentioned in video, the ‘HP’ model has better quality selected cells for its build.
Approx. 3.5 to 4 ft #2 welding cable pair + 150 amp Anderson connector + BMS for total resistance drop to tester. No remote voltage sense used on tester to battery terminals (sense connection would still have BMS resistance). 4 ft #2 welding cable + Anderson connection is about 1.6 milli-ohms resistance to instrument. Estimated BMS and internal battery wiring is about 3-3.8 milli-ohms. Total cable, connectors, and BMS and internal wiring would be about 4.6 – 5.4 milliohms.
They have written their own PC BMS application with BT link to BMS. Looks like JDS BMS.
PC BMS application shows individual cells as read by BMS, but only to two decimal points, 10 mV resolution. Test instrument voltage readings have cable and BMS loss. BMS cell readings have direct cell voltage readings.
1:06 timestamp
All cells at start at 3.32v per BMS readout with no load, approx. 70% SOC, top balanced. Battery reads OCV on instrument of 13.286v/4 = 3.3215v per cell. Instrument matches BMS readout with no load.
1:21 timestamp
600 watt load applied.
Instrument shows 600 watts 12.986v @ 46.190A.
Estimate actual battery cells at 12.986v + (46.2 amps X 5 milliohms) = 13.217v, /4 = 3.304v/cell.
3.3215v-3.304v = 0.0175v , /46.19A = 0.38 milliohm Rs per cell. 0.38 milliohm cell Rs seems a little high.
Another way to estimate cable, connector, and BMS loss. Assume about 0.30 milliohm Rs per cell x 4 =1.2 milliohms for 4 cells.
13.286v – 12.986v = 0.300v, /46.2 amps = 6.5 milliohm – 1.20 milliohms for cells = 5.3 milliohms total cable, connectors and BMS drop. 5.3 milliohms for Cable, connectors, BMS/internal wiring looks fair. Will use this when only instrument reading is shown without actual BMS cell readings. BMS resistance will rise 30-40% when it gets hot with high current.
1.33 timestamp
With 46.2 amps, BMS shows 619.93 watts. 3.30v each cell. X 4 = 13.20 vdc total (higher than instrument which has wires and BMS voltage drop included and 10 seconds of decay time into overpotential terminal voltage slump.
1.45 timestamp
54 seconds since load applied, should be about 70% of leveled out equilibrium voltage slump at cell terminal voltage slump at 46 amp load. Did not show BMS cell voltages so have to estimate based on instrument with cabling and BMS voltage drop resistance.
Instrument now shows 12.946v @ 46.330 A, 0.279 ohm Res, 0.04v Vpp, 0.3298 A Ipp.
Assuming about 5.3 milliohms (BMS MOSFET’s now warm) for cable, connectors, and BMS loss, 0.245v drop, batteries would be at 13.191v/4 = 3.298v per cell. That would be 3.322v – 3.298v = 24 mV cell terminal voltage slump at 54 seconds of load at 46.3 amps. 26 mV/60% to equilibrium = 39 mV estimate of terminal voltage drop at equilibrium. This is very good, like new cells, which would expect about 40 mV terminal voltage slump for a new 280 AH EVA cell with 46 amp load (16.4% CA loading).
1:46 timestamp
Remove load, instrument shows 13.241v, 0A. /4 = 3.3103v per cell
1:53 timestamp
Instrument shows recovery to 13.253v after 7 seconds from load stop. Still not recovered fully yet. /4 = 3.313v/cell.
1:56 timestamp
1000 watt load applied.
Instrument shows 12.752v @ 78.413A, 0.163 ohms Res, 0.04 Vpp, 0.3298 A Ipp
/4 = 3.188v per cell.
2:04 timestamp
8 seconds since 1000 watt, 78.4 amp load applied.
BMS shows all cells at 3.28v with 78 amp load current. These are well matched cells !.
Timestamp 2:24
BMS reads cells as 3.27v, 3.27, 3.27, 3.28v with 78.4 amp at 28 seconds time with load. Still well matched. These are very good cells with near new cell performance. Wish they read out cell voltage to 3 decimal points on BMS applications display. 28 secs is about 45% to equilibrium.
Overpotential terminal voltage slump from original 3.3215v – 3.27v = 51.5 mV slump at 28 seconds.
At 78.4 amps, 0.28 CA load on new 280 AH EVA cell would expect about 55 mV equilibrium slump. These are excellent cells in this battery pack.
Timestamp 2:27
Instrument reads 12.693v @ 78.774A.
31 seconds after load applied. 3 seconds after cell voltage shown to instrument shown.
BMS shows 13.09 vdc total, Instrument shows 12.692v. Looks like about 0.398 voltage drop for instrument cables, Anderson connector, and BMS. Likely BMS dominates. 0.398v/ 78.781A = 5 milliohms loss.
4 feet (x2) #2 welding cable about 1.4 milliohms, x78.8 amps = 0.110v drop. Anderson connector approx. 0.2 milliohm x 78.8 amps = 0.016v, 0.4 milliohms for instru banana plugs = 0.032v
Summing cable/connector drop = 0.110v + 0.016v + 0.0.32v = 0.158v. 13.09v-12.69v = 0.398v
0.398v-0.158v = 0.240v drop for BMS. 0.240v/78.8A = 3 milliohms for BMS and internal battery wiring. BMS MOSFET’s likely fairly warm at 79 amps so their resistance is a little elevated.
Roughly, cable, connectors, and BMS resistance is about 5 milliohms total.
Timestamp 2:32
36 second since load applied
Instrument 12.689v @ 78.802 A].
Load switched off.
Instrument reads 13.196v / 4 = 3.299 v per cell. Just beginning terminal voltage recovery.
Timestamp 3:05
33 seconds since load turned off. Not fully recovered in 33 seconds but getting close.
Recovery in 33 secs to 13.243v /4 = 3.311v per cell (started at 3.32v/cell)
Timestamp 3:07
Engage 1500 watt load.
Instru 12.472v @ 120.263A.
12.472v / 4 = 3.118 v/cell on instru with cable & BMS losses.
Cabling and BMS drop at 120 amps would be about 120 amps x 5 millohms = 0.600 v.
Series cells would be 12.472v + 0.600v = 13.07v /4 = 3.268v. Load just applied so not reached equilibrium.
3:16 timestamp
BMS shows 3.26v, 3.25v, 3.26v, 3.26v. Still well matched cells with 120 Amp load !!!
120/280AH (they are spec’g battery at 260 AH with used cells) = 0.43 CA cell load current.
3.32v original no load – 3.26v with 120 0.43 CA load = 60 mV cell drop. This is great performance.
Time since load applied is only 9 seconds so not at equilibrium.
Timestamp 3:27
“Only drawback, you are running up there. We don’t like to see for extended period of time”
This statement and only at 0.43 CA rate of current load. Matches my recommendation not to use 280 AH cells at sustained current above 0.5 CA rate. Less for used cells.
Also may have some issues with BMS heating.
Timestamp 4:37
Instrument showing 12.403v @ 120.939A. No BMS cell voltage shown so will have to estimate cell voltages from cabling and BMS losses.
Loaded for 30 secs with 120 amp. Cable, connectors, and BMS loss is 120 amps x 5 milliohms = 0.600v.
13.286v start – 0.600v = 12.686v-12.403v = 0.283v drop /4 cells = 71 mV slump per cell @ 30 secs of 120A load.
Has not reached full equilibrium in 30 seconds and BMS is probably getting hot with 120 amps of load so its resistance is going up by 30-40% of room temp resistance. BMS series MOSFET’s getting about 30-35 watts of heating. Wish he had shown BMS temp on the app.
This is very good cell terminal voltage slump for 120A load. Expect equilibrium slump for new 280 AH at 0.43 CA current of about 70 mV.
Timestamp 4:56
Turn off load
Instr 13.209v on immediate turn off recovery without recovery time. /4 = 3.302v
Conclusion: These are new or nearly new cells.