Wednesday, August 4, 2010

TS-LFP40AHA In-System Data

As part of the Cap Kart Summer 2010 upgrade, we switched from 79Ah @ 36V lead-acid batteries to 40Ah @ 39.6V lithium iron phosphate batteries (Thundersky TS-LFP40AHA). The old batteries weighed 159lbs. The new ones weigh 42lbs. We got them from Elite Power Solutions, and they even came with little carrying handles:

One of three 13.2V modules.

The loss in capacity is somewhat overstated since lead acid batteries are rated at fractional C rates, while the lithium ion batteries are rated at 1C. They also came with balancing boards, but they don't even fit on the cells. (The drain FETs don't clear the vent caps without additional washers to space them out vertically from the terminals.) Not worth the extra $100. Get yourself one of these instead for $13. They don't balance the cells for you, but they do have an audible alarm for over and under voltage, so you know to stop driving or stop charging and balance if necessary.

I was a little worried about the power-handling capability of these cells, since they are only rated for 3C continuous discharge (120A). The kart at full tilt sends 300A to the motor. I wasn't expecting 300A from the cells (except at high speeds, the motor current is higher than the battery current), but something around 5C or higher for peak discharge would be nice. The internet is fairly bad at documenting the capability of these cells. The closest thing to consensus I found was a per-cell resistance of about 3.5mΩ. Assuming a low-voltage cutoff of 2.5V and an open-circuit voltage of 3.3V, that gives a peak current of (3.3V-2.5V)/0.0035Ω = 229A, and a peak power of 2.5V*229A = 573W. With 12 cells, that's almost 7kW, which would be just about right for the kart.

Enough math. Here's some on-kart test data:


This is for the full 12S battery pack and includes the resistance of the bus bars, fuse, wiring, manual switch, and contactor. It's at about 75% SOC and shows both charge (regen) and discharge data. The output is very linear, which I guess makes sense for a cell with relative high DC resistance. The resistance per cell came out to about 3mΩ and it has no trouble putting out 6kW, with still a bit of room before the low voltage cutoff (30V for a 12S pack). In other words, works as advertised by the internet or slightly better. For the price, hard to beat these cells. But don't expect to win a drag race against A123s.

If nothing explodes, there should be new kart test drive video soon!

5 comments:

  1. Thanks for sharing actual experimental data on these cells. You're right about the internet being full of subjective info but no hard data. As far as the current limits, is there space for a parallel 12S string?

    Once comment about the TS vs A123... Yes, the A123 cells have impressive performance, however since you and I aren't able to buy them (except as part of a cordless drill powerpack), I'll take the cells that I can actually purchase. A123's business model really annoys me.

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  2. How do you test you batteries? A specific machine or by hand? By looking at the graph it looks like it was by a dedicated machine.

    Show us a picture of the balancing boards! I think I saw them on the manufacturer website but not 100% sure.

    Thanks!

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  3. Tested on the go-kart. (Actual test drive data via telemetry.)

    In point of fact, there is a stack of 10,400 A123 26650 cells sitting about 50 feet from the go-kart. They were donated to the MIT Electric Vehicle Team (not affiliated with the kart, but we share space). They are quality control rejects, but mostly totally fine. I call them B123s.

    But as a group of DIYers, the kart crew, too, prefers cells that the Average Joe can acquire at a reasonable cost. I think soon some Chinese cells will actually match the performance of A123s. (See my post on "HK123s".) Bad for intellectual property law, good for us.

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  4. I remember that post about the HK123. My friend who is in the hybrid SAE will do some testing on the chinese cylinder A123 batteries.

    I just bought myself the new Turnigy nano-tech 6000mah 6S 25~50C Lipo Pack for my brushless scooter.(Finally in stock) Expensive! Can't wait to try it and see the 1.2mΩ internal resistance in action.

    I will be sending the HK123s link to my friend.

    Thanks again!

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  5. The quadrotor we just built uses a Turnigy nano-tech battery (4S, 1800mAh, 35-70C). I've never felt it get even remotely warm. It's also capable of high-C charging. (It says Max Charge 8C on it...)

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