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.

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.

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.

Winter update

That's 3.7˚C

It's been a little slow on the blog side, but in the real life I've been driving both of our electric cars regularly. Even exclusively, since I sold my diesel car and we're now all electric with two cars and a motorcycle. Hooray!

The DIY car got a little winter update in the form of a cabin heater, which I bolted on to the side of the battery box in the trunk. It gently heats up the aluminum box with the cells inside, but mostly the cabin of the vehicle. The little thermometer has it's sensor inside the box, in the middle of the pack and between the cells. It really hasn't shown temperatures below zero yet, since both charging and driving seem to keep the cells well heated. Instead I've seen temperatures close to 30˚C even with the outside temps hovering around or slightly below freezing. Currently the car has no batteries in the front that would need heating, so the single PTC heater takes care of everything.

With no batteries in the front of the vehicle, it should not come as a huge suprise that even though I bought some studded winter tires, they don't grip very well in the front. It could also be that they are "Aurora" branded tires, which are hardly recommended. Should get me through the winter though. 

Also in the picture behind the big charger is a smaller, grey 12V charger. My 12V AUX battery hasn't  been very happy with just the occasional 13.5 volts it gets from the DC/DC converter while driving, so I installed an automatic lead battery charger to keep it charged at all times. It's connected to the same 230VAC input as the main traction pack charger, so it automatically charges the AUX battery whenever the main battery is charging. Quite like the C-Zero, where the DC/DC converter also is always on while charging.

The car has two 230VAC plugs in the rear, which means that I can choose between heating and charging simply by choosing which plug to plug in to.

As for the C-Zero, it's also working great. I installed a Binar ethanol burning 5kW heater to keep it usable over the winter as well and also a 230VAC plug for a electric cabin heater. It's been a great vehicle and fun to drive. The only thing I'm really missing is Cruise Control. It would make keeping a steady economical speed a lot easier than it is now.

Baby got home

So I got our new baby home, finally. They had fixed the dents in the roof and repainted the whole roof. I'll have to make sure they did the brake vacuum pump recall or take it back for them to do that when they get the part. It had to be ordered from France.

So what happens when you get a new vehicle? Well, you start to think about ways to improve it, of course. So far I've acquired some Calix heating gear in order to pre-heat the car before driving. I'll just need to decide where I want to put the heater within the car. More on that later.

C-Zero

We decided to take the plunge and get a non-DIY electric vehicle as well. The main reason being that a store bought EV gives you the ability to quick charge. I figured that a black box EV is still better than a black box Diesel. At least you won't be burning the oil. And I honestly think that the EV should pay for itself. Since I got a really good deal on a 2011 Citroën C-Zero it might actually happen.

This'll be my gas station experience henceforth.

I took the car for a day of test driving. Did over 100 km and quick charged three times. It worked very well. Average consumption while doing 80-90 km/h behind a lorry on a freeway was about 128 Wh/km. Not bad. The car only has 10.000 km on it so it's practically new, even if it's a 2011. It had been the floor model that apparently nobody wanted. I'm not surprised, since the sticker price was over 30.000 euros. Fortunately I got it for less than half.

Good times.

Bloatware

I'm going to come out and recommend some pack level monitoring, which will cut off your pack completely from anything else in certain conditions. At least if you're like me and leave your vehicle for over a week with the high voltage turned on and therefore both DC/DC converter charging your 12 volt aux battery and your controller charging it's caps all the time.

Maybe if I had been a little bit more careful when adding 8 new cells into the total of 29 cells in the motorcycle and bottom balanced all of them together instead of just driving the old cells empty and bottom balancing the new, I might not have almost a complete set of bloated 40 Ah cells. Maybe. Maybe I would have lost them anyway. Maybe I should have just been a little bit more careful.

If there's a good news somewhere in this it's that these LiFePO4 cells are safe. You can discharge them to death and get a little bloat, but that's all. The bloat won't break anything, unless you've done what some people rather cluelessly recommend, which is tying them together so they won't have any space to expand. Usually that's how you make bombs. I don't do bombs.

Anyway. At least I'll try to change the bike so that I can't repeat this stupid mistake and I think you should try to do the same. At least have everything behind the key switch. A master cutoff could be a good idea too, but then again you can just as easily forget to turn that off too. I've already done that in the car a few times. Luckily in the car the only thing that can drain the pack without the key in the lock is the pre-charge resistor, which takes a really long time to drain a battery.

I think I'll concentrate on the car again for the winter and see what I'll do about the motorcycle in the spring when it gets warm again. Perhaps we'll have the new CALB CAM cells widely available by then... or maybe even something better. One can always hope. I'd really like to make my weekly 238 km trip between Espoo and Pori on electric, but we're not there yet. Unless you can afford a Tesla.

Bloated cells trying to get some Lebensraum.

Belly pan is back on

I grudgingly cut a piece off of the original belly pan of the GPX. I did look for a second hand pan on ebay, but they were all in such a crappy condition that I didn't want to buy any of them. Which really didn't make it any easier to cut into the one I had, which was in really good condition. I did save the piece so I suppose I could mend it again, or at least use it as a mold, in case I need to.

Anyway. The cut-off is not too bad and looks alright. Makes the charger blend in quite nicely. I'll need to do some more work to make it fit perfectly, but at least it's back on now and looking just fine.

Drove to work and back again today. It takes about 9 Ah to drive the 12 km distance. So I could get to work and back two times before needing to charge, but I charged at both ends anyway. Looks like the charger isn't very interested in stopping the charge at all. It may be because if I have the Cycle Analyst and contactor engaged, I also have my PSU DC/DC converter connected, which sips a little bit at all times charging the tiny lead acid battery I have. It does keep the voltage at about 102.5 volts, so it won't really overcharge, and drops the amperage to near zero, but it's not doing any good either. I'll need to see if it ever quits with everything else turned off.

At the apartment the outlet at the parking lot has a fixed 2 hour timer, so the charge will stop after the allotted time in any case, which in this case is a good thing. Two hours is also just enough to charge from fully empty to full at 21 amps anyway. The charger starts ramping up the amps very smoothly, which should prevent any slow 10 amp fuses from blowing, even though it's drawing 11 amps at full power.

I suppose I could contact the seller if there's anything that could be done about this problem, which is clearly a fault in the programming, but I ordered the charger directly from China and on the cheap, so any chances of anything happening are rather slim. It's what you get for being cheap.

One more thing. The bike is really nice to ride. Acceleration is very nice once you get moving. Right off the line it still hesitates for a bit. Then it's all pure fun until 120 km/h. Which makes it really nice in highway traffic. Good times.