Review of Capacity and Terminal Temperature of EVE LiFePO4 battery cells from Shenzhen Luyuan
We recently received a set of 280AH LiFePO4 cells that we purchased from Shenzhen Luyuan (Amy Wan).
We wanted to know three things:
What are the individual cell capacities and how evenly matched are they?
What is the capacity for a reasonably high-C (in our case this would be around 160A so around 0.6C)?
What temperature do the terminals get to at the “high-C” test? These are the new welded-stud terminals with worryingly small surface contact areas, so we’d like some peace of mind that they don’t heat up too much.
We have found in the past that anything up to around 120A is fairly forgiving - terminals not tightened correctly tend to not heat up that much. Above 150A and it's a very different story. Approaching 200A and your margin for error is fast disappearing.
The LiFePO4 Cell Set Up
These cells are 280ah, and we tested four of them.
We asked the supplier for the official torque for tightening the nuts down on the terminals and were told 10nm.
We used the bus bars that the supplier provided, which are 19.8mm x 1.9mm = 37.62mmsq. We would have been keen to double them up but we only received four bars with these cells, so went with single bus bars between each of the cells. We wanted to know if the terminal was up to the task, not the bus bars. So I guess we got to test both.
The ambient temperature of the room was 21°C. We'd normally test with the cells in a temperature-controlled cabinet at 25°C but we wanted to make it easy to observe the temperature.
The Capacity Tests
We top-balanced the cells and then did tests of individual cells at 15A. We used cables that were very well sized for the current that was being drawn & fastened well to prevent too much voltage loss.
After the individual tests, all four cells came out at between 289ah and 290ah. All exceeded capacity by about 10ah. This was well-pleasing as we’ve had negative results from similar LiFePO4 battery cells in the past.
We connected a Daly Smart BMS 200A and charged at 20A, within the “knees” the delta was very low - about 0.001v, again we're pleased with the results. We set the BMS safety thresholds to High Voltage Disconnect = 3.65v and Low Voltage Disconnect = 2.5v.
Once the cells were fully charged, we let the battery rest for about two hours and then connected a 2kw Giandel inverter that had a 1.7kw heater attached. This drew about 160A for most of the test.
We used two shunts to measure amps and ah just out of curiosity to compare a non-cheap Victron SmartShunt (£120-ish) versus an inexpensive Juntek shunt (£30-ish).
Because we were drawing high currents, there was a reasonably high voltage difference between the two shunts (12.54 closest to the battery vs 12.42 furthest from the battery at some stage), but amps were within 1% of each other and very similar to the BMS reported amps. Due to the voltage difference, there was a reasonable difference in kwh measured by the shunts.
The capacity got to 281ah before the BMS prevented further discharge. Again, happy with the results.
The test ran for about 01:49:00.
The Terminal Temperatures
Now it’s time for the terminal (and cable) temperatures.
We had four temperature sensors in an ambient temperature of 21°C:
On the negative terminal. This got to a maximum of 38.1°C towards the end, it got to around 28°C quite early in the test and slowly worked it's way up.
On the positive terminal. This got to a maximum of 38.8°C towards the end, it was always a few °C warmer than the negative terminal except towards the end when they became very similar.
On other terminals and on the bus bars. We moved this around a bit. The temp was always a couple of degrees cooler than the main terminals. The middle of the bus bar was nearly a degree warmer than the terminals it was bolted to.
On the positive cable, 35mm assembled by ourselves. This started out as several °C warmer than the terminals but eventually, they all came to about the same temperature. We didn't officially measure the negative cable (came with the inverter) but it was definitely warmer than the positive. At its hottest, the positive cable was just under 40°C
Prior to the test we were concerned about terminal surface area. After the test, we're not at all concerned. The 5-6nm extra toque more than compensates for the reduced surface area. We've had other cells terminals heat up too much higher temperatures.
Not many people need to draw anything like 160A. In our own vehicle, the most we usually draw is around 90A for our Nespresso coffee machine.
Final Thoughts
And finally, we performed a low-C test (at 30A), which yielded 287AH. All in all, we're very pleased with the cells and would recommend them. We also had a very pleasant experience dealing with Amy Wan from Shenzhen Luyuan. We would highly recommend her.
