K2 LiFePO4 Battery Testing

Ian Hooper, 4 January 2008

 

Introduction

Back in October 2007, I put together a report with results from testing some of the leading Lithium Iron Phosphate (LiFePO4) batteries on the market (click here to view). Since doing the report, many other manufacturers and suppliers have been brought to my attention. Of particular interest were the cells from K2 Energy Solutions (formerly Peak Battery). To follow are results from the set of charge and discharge tests on four of K2's cells - their high energy and high power cells in both 18650 and 26650 sizes.

The Cells

Four different cells were tested, as follows:

Make/Model
Size
Capacity
Cost (each)
Energy Density
K2 18650E
18650
1400mAh
US$5.99
109 Wh/kg
K2 18650P
18650
1250mAh
US$7.99
97 Wh/kg
K2 26650EV
26650
3200mAh
US$11.50
125 Wh/kg
K2 26650P
26650
2500mAh
US$13.00
97 Wh/kg

Pricing shown is for individual cells. Pricing in quantity of 1000 cells is around 60% of the single cell price, and may be slightly less in EV quantities of 2000-4000.


The cells tested, from left to right: 26650P, 18650P, 26650EV, 18650E.


The Test Equipment

Please view the original report for information on the test apparatus.

Results: Discharging Performance

 
18650E
18650P
26650EV
26650P
1C
3C
5C
10C

Click on an image above to view larger version here

Notes and comments:

  • The resolution of the ACS754 current sensor when used with the LabJack U3 was not brilliant (hence the noise on the current plots in blue), so amp-hour and watt-hour totals may not be precise (especially for lower currents). But, they should still be a pretty reasonable representation and comparison.
  • The rows are matched by approximate C rates only - since discharging was done with a resistive load and some batteries have more voltage sag than others.
  • The scale for current on the Y-axis with the 18650E 10C test was incorrect so the current plot was off the page, but the heating was so severe that I didn't really want to repeat the test! Basically I wouldn't recommend these cells for 10C applications.

Results: Charging Performance

The main reason it is useful for cells to handle high charging current is actually regenerative braking (aka "regen"). The fastest rate of charge for vehicles without regenerative braking is usually a fraction of 1C, since single-phase chargers are limited to around 3kW and most EVs have packs well above 10kWh.Most EVs use series DC motors and don't have regenerative braking, however I believe it will become an increasingly important feature in the years to come as it improves efficiency and range by about 10%. So, it's worth seeing which cells can handle high rates of charge associated with powerful regen..

 
18650E
18650P
26650EV
26650P
0.5C
1C
2C

Click on an image above to view larger version here

Notes and comments:

  • I am still of the opinion that a shorter Constant-Voltage phase (the second part of the charge, where the current is falling) with a steep drop off is a good indicator of how "happy" a cell is at a given charge rate.
  • The wacky temperature plots on some graphs (e.g 18650E 0.5C charge) is due to the air conditioning system. It's mid-summer here in Australia and we've had some scorchers - but I've tried to keep ambient temperature for testing around mid to high twenties (celsius).

Conclusion

The discharge performance was a bit of a mixed bag, so I'll go over each of them individually:

  • The 18650E was the lowest power LiFePO4 I have tested, though all cells in the previous round of testing were of the high-power flavour, so the comparison is a little unfair. Nevertheless in my opinion the 18650Es are a little weak for EV use.
  • The 18650P were remarkably stiff, maintaining just below 2.8V for the majority of the discharge @ 10C. This is about 0.1V higher even than the A123Systems M1 cells! That said, heating was significantly higher too. But overall, they seem to be a great cell for high discharge applications.
  • The 26650EV is not designed for high performance applications, nevertheless it performed fine in general and - considering the 125 Wh/kg energy density - would be a superb cell for modest power commuter vehicles and the like. (Not many people need 10C continuous!)
  • The 26650P showed only slightly greater voltage sag at 10C than the benchmark from the last round of testing, the A123 M1, though it did have significantly more heating (about 10°C higher).

Generally the charge performance was underwhelming - none of the cells exhibited a "correct" Lithium charge curve at 1C or above, though they were all fine at 0.5C. In normal single-phase charging applications (i.e plugging your car in to recharge overnight) this will not be a problem, but if using a controller with powerful regen, it is a bit of a shortcoming.

My final thoughts? If the 18650P or 26650P could be purchased in quantity at a price competitive with the PHET PE-1150 cells (e.g <=$2.50/Wh), they are definitely a worthy alternative, at least in vehicles without powerful regen. And similarly, if the price was right the 26650EV cell would be great for commuter vehicles, as the 25% higher energy density is pretty significant.

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