We progressed to higher discharge currents today. First, a 3C (120 A) discharge. Remember that TS cells are supposed to be able to be discharged continuously at 3C:
As you can see, the voltage dipped only to 2.9 V, then to 2.85 V, then actually increased all the way to the end of the test (to 2.914 V @ 113A). The discharge current averaged about 115A, just a bit shy of the 120 A we were aiming for. The test terminated at about 33% DOD, since this is a new cell, and we are trying not to discharge it beyond 33% for the first few cycles. The temperature ended up peaking at just over 30°C, or just over 10°C rise. It certainly looks like TS cells can handle 3C comfortably when fully charged.
The load was bifilar wound to cope with the extra current. The temperature of the load water at the end of the test was in the fourties Celsius.
What we are more interested in though is 6C, or 240 A. This is about where we estimate our peak motor current would be, and we'd like the cells to dish out that current for at least 20 seconds for acceleration.
So we beefed up the cables and the load (now two bifilar wound loads in parallel). We decided that the 3C load will come in handy when the other cell arrives, so we made another similar load and paralleled it. This one had to be somewhat lower resistance, because the battery voltage will be lower (we estimated close to 2.5 V), and also the resistance of the rest of the circuit, including the shunt, becomes significant. Here is the setup:
Note the pairs of 16mm² cable; each is rated to carry 63 A continuously (as is each half of the breaker). So that's OK for long 3C tests, or short 6C tests (hopefully).
I was wondering what to do if the breaker couldn't interrupt the load. I had visions of this:
But then Weber pointed to the number "6000" in a box on the breaker, indicating that it could interrupt 6000 A at 48 V. I was a bit more relaxed about things after that.
That's a 200 A shunt in the clear container at the front right. It got hot enough (some 60°C) at 115 A that we decided it could do with water cooling as well as the load.
The cell is wrapped in some thick rubbery stuff (I think it was a garden kneeler). The clamp is just to make sure that the cell doesn't expand, and to make sure that the digital thermometer makes reasonable contact. The insulation of the foam will hopefully be worst case, and about the same as an infinite set of batteries of the same temperature surrounding the cell.
The two-layer load: [Edit: added later]
To make sure that the new load had about the right resistance, we initially put the two loads in series (around 1.5C load, or 60 A), and adjusted the new load so that it dropped the right proportion of resistance. We wanted it to be 115/125th (so the new one would pull 125 A for a total of 240 A), and also 2.5/2.9ths, since the battery voltage would be lower. The fine maths was somewhat wasted, however, and we ended up taking off half and even full turns to get the current about right. Here is a graph of the first discharge:
Edit: Where the current (red) goes off the bottom, it goes to zero.
The upper graph is a zoom of the first part of the lower graph (over the actual discharge). Time is in seconds. Some of those currents were actually averages of the earlier and later currents; we weren't as organised as we became later. As you can see, the cell voltage is barely above 2.5 V, but the temperature is around 25°C. We watched the temperature for a while to see what would happen; it kept rising for about 20 minutes, but not by very much.
The next three discharges were only a few minutes apart, to better simulate real driving conditions. Sorry about the very wide image:
You can see that we got bolder on the third discharge; although it doesn't look it, the cell voltage remained above 2.5 V, so we continued the test for 70 seconds. Here is the detail around the long test:
Note how the voltage actually rises, presumably as the internal temperature rises. (Most of the temperature figures during this discharge are interpolations; we didn't figure out that "blank divided by 10" isn't a blank according to Excel at that stage. Blanks get ignored; blank divided by 10 gets treated as a zero). The voltage actually rose again at the 70 second mark, but we'd exhausted our boldness at that point.
So the conclusion so far is: Thunder Sky cells handle 6C discharges pretty well when warmed up a little. (Warmed up a lot if you live in Antarctica). The voltage sag is somewhat severe, though.
It will be interesting to see how the "high discharge rate" China HiPower cells compare.
Edit: added more graphs and text
Edit: Expanded the vertical axis on most graphs
Edit: shunt -> load