April 16 Friday Show

This week we digress a bit from vehicle issues and take a look at the progression of LiFePo4 cell offerings over the past two years.  We also do some full 1C discharge and charge tests on the cells.

The first cell in this series is a Thundersky TS-LFP160AH cell we purchased about 18 months ago.  This cell is a Lithium Ion Iron Phosphate cathode cell.  The spec sheet for this cell is available here.  Note that the cell measures 182 x 276.4 x 71 mm in volume and weighs 5.6kg.  It would contain 3.4v x 160AH = 544 watt hours of energy.  With shipping, this cell cost almost exactly $244.

Second cell was a Sky Energy SE-180AHA cell. You can download the spec sheet here. It is only slightly taller at 279.5 mm but is also 182x71 mm and 5.6kg, but offers 180 amp hours at the same voltage - essentially 612 watt-hours of energy. It was purchased at $241 with shipping 9 months ago.

The third cell is a brand new Thundersky TS-LFP-200AH cell. Spec sheet here. This cell features the addition of Yttrium to the cathode material and offers longer cell life cycles - 3000 cycles vice 2000 at 80% DOD and 5000 cycles vice 3000 at 70% DOD. This is a pretty significant extension of life cycle claims. It's dimensions are actually 183 x 276 x 101 mm  or 362 x 256 x 55.5 and quite a bit heavier at 7.3kg. It is also about $240 once delivered to Cape Girardeau MO.

The latest Thundersky cells are quite interesting. Most significantly, they manufacture an unadvertised  variant of this cell at the original 183 x 276 x 71 mm size and 5.6kg weight. But they can sell enough of these without advertising it that there is NO MENTION OF IT on the Thundersky web site.


Because of the quantities of these cells they are now selling, EVCOMPONENTS can get these 276x183x71 mm cells. They do test out to 200AH, though not any more than that. That's 680 watt-hours per cell. http://www.evcomponents.com.  You do have to know to ask

This is rather an amazing occurrence.  In the space of 18 months, we've gone from 160AH to 200AH in the same form factor and at the same price.  That's a 25% increase in energy density, and at least a 50% gain in cycle life.

We developed an Arduino based circuit with some LEM-HASS hall effect current sensors to measure current and AH into and out of the cells. We used our GIgavac contactor box, described in last weeks show, to switch the batteries in and out.

We used a pair of ACME 1500 watt constant current loads to load the cells to a 1C rate, 200AH, 180AH, and 160AH respectively for an hour and recorded the voltages and temperatures at each 2.5% increment by counting 5, 4.5 or 4 AH between each notation.

And then we reversed the process and used a Lambda 10v/200A power supply to do the charge curves the same way.

The result is a series of graphs.  Here is the discharge/charge curve for the original TS 160AH cell.  Click on the image for a larger version.

Here is the results of the Sky Energy 180AH cell.

Here is the results of the new Thundersky 200AH cell.

Because we measured these cells at the same percentage of their total AH capacity, we can compare them. Here is the charge curve comparison of the three cells.

We can do the same thing with the discharge curves.

The interesting area of this test is of course the temperature gain during charge and discharge. The discharge process caused the greatest temperature gain.

But there were some additional temperature gain during the recharge process.

Finally, we're making the entire Microsoft XLS spreadsheet file with all the collected data available for download here.