maanantai 8. huhtikuuta 2013

On swelling and battery life

Ok, I'll just steal that one from Jack's University of Batteries, but I do think it holds true. Swelling is telling and if your cells swell you are damaging them. Hence there is no need to bind them together tight, unless you want to destroy them by overcharging or, oh, let's say by Top Balancing, which really is just a form of doing that.

Another very, very interesting thing is the excellent paper from Sweden Jack is also kind enough to present to us. What's apparent is that to get the 2000-3000 cycle cell life (down to 80% capacity) you need to continously charge to 100% and discharge to about 10%. I think you'll get the same results by Top Balancing as well if you boil your cells at 100% SOC for extended periods of time. And every time you charge to 100% you're harming the cell, so it's really better to Bottom Balance and undercharge a little, say by charging to 3.5 volts per cell. It's the best way to take of your cells really. You only charge them to a voltage and stop immediately.

The test went on to 9000 cycles with LiPO cells in cyclic simulated EV use before the 80% capacity limit, if you didn't charge them over 50%. It didn't matter how fast you discharged. They lasted better if you went from 23 degrees Celsius to 35 degrees. I know it's LiPO cells, but they should be similar enough for all this to apply to LiFePO4 cells as well. Nothing whatsoever in there that would support anything the BMS people are claiming.

I'd go as far as claiming that if you only have 80% capacity left after only 2000 cycles you've probably been doing something wrong. If you are using a BMS then it's your BMS that is harming your cells.

Another thing to take home here is that discharge rate doesn't matter, which means I'm definitely going to remove any artificial current limits in my motorcycle right away. The SE40AHA cells can put out 400 A, that's 10 C, and I'm going to take it all. My AXE7245 controller could go up to 450 A. I will add some cooling though, I have a 8" fan and a 3" marine blower that I'll be experimenting with, but more on that later.

4 kommenttia:

  1. For some strange reason, I'm starting to be convinced that bottom balancing is the way to go. Too bad that the charger I ordered hasn't any calibration abilities.

    I've also thought about using computer water cooling if my Alltrax will get hot someday. Blocks are not that hard to made, controller has large smooth area for heat to transfer and pumps work with 12 volts.

    1. There's two ways you can go about this. You can either decide on the cell count and then order a suitable charger or if you have charger you can match the cell count to the charger. For example use 25 cells with a charger meant for 24 or 33 for a charger meant for 32. Of course, if you already have the cells as well then it gets complicated.

      You could also experiment with adding a separate volt meter with relay control so you could end the charge as soon as the cells hit 3.5 V. You'd lose the CV phase of the charge, but on the other hand you'd gain more life cycles because of the resulting undercharge. At least it's easy to measure how many Ah you'd get into the pack if you did this just by watching a charge and noting the Ah count when the voltage limit is reached.

    2. I have also gathered a couple of old CPU coolers to see if I could attach them to the bottom of the AXE controllor, with or without fans. I'm not sure, but I think I may have hit a thermal limit last summer. The symptoms were power being cut off and then coming back suddenly. I should probably ask Alltrax whether the AXE thermal throttling works like this.

  2. Cell count is 21 and I have no idea where I coult get 22nd cell as they're some old used prototype things. That's also why I'm really not that interested if they last thousands or hundreds of cycles. Maybe I'll give a thought to that voltage based relay control..