lauantai 28. helmikuuta 2015

Update on cell testing

Just a quick update regarding my previous posts on the SE40AHA cells that seemed to be fine after all. So far, in testing the cells by draining them to about 2.6 volts I've found that out of the 33 cells I have, nine seem to have internal shorts. I can tell this by leaving the cells at 2.6 volts and monitoring their voltage. The bad cells have gone down in voltage, some already below 2 volts. They must therefore have internal shorts. The better cells have bounced back a little and then kept their voltage.

So it looks like I might have 24 usable cells. Unfortunately this doesn't really match anything I have in use. The car, Xsara or kWsara, has 25 cells in series, and the motorcycle, kWsaki, is set up for 29 cells with it's 102.2 volt charger. So it's either figure out something else to do with the cells, get a new charger perhaps, or try to find a couple of SE40AHA cells. Unfortunately GWL Power doesn't list them anymore. They are a couple of generations old now, so it's understandable that they'll be unavailable. Sinopoly has similar black 40 Ah cells, which might do just fine, though.

keskiviikko 18. helmikuuta 2015

Regarding winter range

One of the questions I often get asked at the quick charge stations during the winter is how does the winter affect your range. It depends. That's the answer. Turns out, it doesn't just depend on one thing either. It depends on a lot of things.

The most obvious one, and the one people are most aware of, is the fact that current batteries perform better when they are in optimal temperature. That's usually around room temperature, or perhaps body temperature. Much above and you get into trouble. Freezing and below charging becomes slower and they're not quite as willling to part with their charge either. How much this matters on my C-Zero I don't really know. I've done most of my long trips in less than optimal conditions, so I don't really know well it will perform come summer. If I'd have to guess, I'd say it's in the 10-20 % range.

The second rather well known factor is heating. Electric cars tend to use the electricity in their batteries not just for driving but also for heating. In the C-Zero the heater can use around 5 kW. With a 16 kWh battery, with some usually left in reserve, it's not too hard to figure out that the heater can suck up a third of the range in an hour. Generally when I have 100 km range on the gauge, turning heat on will drop it to 70 km, which is probably quite accurate. So I'd say 30 %. In my car I've installed an ethanol heater, so I don't have to suffer this drop in range or the cold either. Some newer vehicles also have heat pumps, which should be able to reduce the consumption to around 1 kW in optimal conditions while still providing enough heat.

Some people are also aware of rolling resistance. Winter tires are designed to grip on snow and ice. And you can't have perfect grip with perfect rolling resistance. In other words, your summer tires will roll easier and winter tires will decrease your range. My guess, 5-10 %.

What I haven't seen anyone recognize is air resistance. If I've done my math right, going from room temperature to something like -20˚C can increase the air resistance or drag as much as 10 %. Simply because cold air is heavier than hot. That's quite a bit for something most drivers don't even think about.

I'll leave it to you to figure out how much all of these could dimish your range.

sunnuntai 15. helmikuuta 2015

Lessons learned

A couple of things I learned from the events described in my previous post.
  1. Cells at zero volts may not be dead after all
  2. Cells at zero volts will not charge from a full cell connected in parallel
  3. Bloated cells may actually shrink back to size
  4. Bloating does not automatically mean internal soft shorts
  5. CALB LiFePO4 cells can take hell of a beating
Point number two also casts a shadow on parallel bottom balancing. I think it probably still works if you start with cells in roughly equal voltage, definitely all cells above the bottom balancing voltage, but it will not cause lower voltage cells to rise to a higher voltage (or charge from it's buddies).

Magic recoveries

Remember the 29 cells in my motorcycle I destroyed last summer, when I accidentally left it turned on and the DC/DC converter has drained all the cells to empty and beyond? Something interesting has happened since. Actually, two interesting things.

I was actually going to take most of these cells in for recycling, but I just hadn't got around to it. Instead I had just left them in our basement. Before storing them I had connected them all in series and drained them down to 2.6 volts, figuring that they would bottom balance together and I'd see which cells had internal soft shorts.

Now the eight newest cells I had just bought to increase the cell count to 29 were at zero volts after my little mishap. I had also connected them in parallel with the others, figuring that they'd recharge from the other cells if they had any life in them. They hadn't and I had left them for dead at zero volts.

But before I get to those eight newer cells, let's get back the 22 older cells which had bloated. Actually 12 of them hadn't and I had connected them into three 12 V packs for whatever use I'd come up with later. I even tested them and each 12 V pack had more or less 500 Wh of capacity, so I thought these 12 cells can be saved for later use and the rest scrapped.

The remaining 10 old cells were pretty well bloated, but they seemed to retain voltage, at least for a while. To my no small amazement, they still had the same voltage after about six months of storage. All ten cells were around 2.55 volts. What that means is that they can't have any significant internal shorts. If they had, they'd keep going down until they're at zero volts. Alas they hadn't. What's even more amazing, they had lost their considerable swell and are now pretty much right size and shape! The had actually shrunk back to shape.

Back to the eight newer cells I'd left sitting for half a year at zero volts. I had even been so sure of their fate, that I had liberally sprayed some gold paint on them to mark them as the ones I'd definitely take to the recycling center.

After discovering that the older cells had gotten back to shape and were still holding their charge, I thought I'd give these cells one more chance too. What if they just didn't want to charge from the other cells while being in parallel? I made two 12 V packs of them, measured a voltage of about 0.5 V for both whole packs and connected a 14 V 5 A power supply.

What happened was that the voltage started to steadily rise as one might expect, so I just left them charging for four hours. When I got back, I disconnected the power supply and checked the cell voltages. They seemed normal, about 3.33 V, as they should for just charged, half full cells. Again I left the cells, figuring they'd probably start draining themselves. I was, again, quite amazed to find out they had not, but kept a voltage above 3.2 V instead. I repeated the same for the other pack with identical results.

These eight SE40AHA cells which I had left for dead and stored at zero volts for over six months were back as well! Now I don't expect the cells to have their full 40 Ah capacity left and I haven't tested them with a load, but I will bottom balance the rest of the cells now and connected them as a 29 cell pack. Charging them together will reveal if any cells shoot higher too early and what the remaining capacity of the pack is.

Right now it looks like bottom balancing saved the day after all.

Check your connections

I've been aware of the possibility of cell connection bolts loosening over time. There are ways to combat this, such as using braided straps and nord-lock washers. Having been reminded of these techniques by several recent YouTube videos, I thought I'd check my cell connections.

I've been cheap, as usual, so I've just used the plain copper interconnects supplied by GWL Power along with the bolts and washers which come with them. It's also been nearly a year since I put everything together, so I was interested to find out if they had indeed loosened. I had also noticed that the pack temperature does rise quite a bit while driving. No visible signs of extra heat were noticeable though.

Turns out not only where several bolts quite loose, I had even forgot to put in all the washers. So yeah, don't forget to check your battery bolts regularly, especially if you don't use nord-lock washers.