Fourth drive, and charging stats

So, we took the 120V Civic out for its fourth and longest run so far, driving 9.6 miles by going twice around the 5.1 mile course my wife would usually take to work and home. We shaved a few corners here and there, which accounts for the total drive being 9.6 miles instead of 10.2 miles.

Now, this course doesn't have any 5% slopes in it (except the hill to our house, which was one of the corners we shaved), so it was interesting to compare the energy use of trips 3 and 4. After each trip I put the car on the charger and let it sit for 24 hours, then looked at the Kill-A-Watt meter.

Trip 3 of 5.1 miles, showed 5.83 KWh on the meter after 23 hr 41 min.

Trip 4 of 9.6 miles, showed 7.90 KWh on the meter after 24 hr 19 min.

In my post on Jan. 9th, I estimated that the chargers and heaters pull about 240W on float charge. I'm going to shade that down because the weather has been warmer, and guess that 225W is probably closer. Let's assume that the batteries reach float stage after 6 hours on trip 3, and after 8 hours on trip 4. Total guesstimates, but let's just follow the line of thought.

That means that after trip 3, the chargers were floating for 17 hr 41 min @ 225W, accounting for 3979 Whr (4.0 KWh), making the total charge only 1.83 KWh. During active charging, the Kill-A-Watt meter shows a power factor of about 0.82, so I figure that means the chargers are about 80% efficient, which means 1.5KWh reached the batteries. Over 5.1 miles, that gives a Wh/mi measurement of 300 Wh/mi. Typical speeds on this trip were in the 20-30 mph range.

After trip 4, the chargers were floating for 16 hr 19 min @ 225W, accounting for 3671Whr or 3.7 KWh. That means the total charge was 4.2 KWh. 3.4KWh actually reached the battery at 80% efficiency. Over 9.6 miles, that gives 350 Wh/mi. This is probably reasonable, considering I "drove the car harder" being out on real city streets (not my little neighborhood streets) and at higher average speeds--typical speeds being more in the 30-40 mph range on trip 4.

Neither of those numbers demonstrate stunning efficiency for this vehicle, but they do seem to be inline with numbers posted by other people for Kelly controllers in similar small cars (look at Brian Blocher's blog about his Honda S2000 EV @ http://s2kev.blogspot.com).

Now the grand projection. The car has six 92 Ah batteries and four 115 Ah batteries (20 hr rating). That means the nominal pack capacity is 12.1 KWh. If the car consumes 350 Wh/mi in city driving, it implies a range of over 30 miles! Whoa. Of course, I would be wise to use a 1-hr rating instead of a 20-hr rating, if I knew it, but I don't. So let's assume that we actually get 60% of the published 20-hr capacity. That means the usable pack capacity is more like 7.3 KWh, and the range is more like 21 miles on city streets. I could believe that pretty readily.

I worked the numbers a couple different ways, making different assumptions for number of hours charging and watts drawn during float, and get similar results varying between 250 Wh/mi on the neighborhood streets and 480 WH/mi on city streets. In all cases, range was between 15 miles and 26 miles, depending where and how I drive. 21 is right in the middle, so gives me something to shoot for. :-)

Third Drive of 120V System

Well, I don't know why we put it off more than a week, but we finally gave the new & improved 120V Civic a 3rd test drive. Mostly I guess I was afraid something else would go wrong.

But finally, tonight Alisa and I drove a closed 1.7 mile course starting from our house, three times around for a grand total of 5.1 miles, with an elevation change of about 140 feet from the lowest point on the route to the highest. That's 420 feet of rise & 420 feet of fall in 5.1 miles, or an average slope of 3.1%. Of course, most of the uphill occurs in about 0.5 mile of the course, and the rest of the course is level or downhill (it's a closed course!), so actually it is more like an average of 5.3% incline over 1.5 miles and 2.2% decline over the remaining 3.6 miles.

The KDH14500B did pretty well, IMO. Yes, it is dog-slow off the line, but if you have it in first gear and really step into it, it does alright. Not as good as a golf cart off the line, but then it gets up to 20-ish in first gear pretty easily, and subsequently up to 35 mph in 2nd quite easily. Haven't taken it over 40 mph yet.

But coming up the steepest part of the hill, which is from the low point on the course up the one block to our driveway, we couldn't keep it in 2nd gear. The old 72V controller would hold about 18 mph in 2nd gear. But the 14500B really wants you to shift down into first, and then it will also hold about 18 mph up the hill. Now, we did add 300 pounds of batteries and chargers that weren't there before, and it is still a 500A controller. Probably if we'd added 300 pounds into the Civic with the 72V controller, it would have had more problem with the hill then, too.

Afterwards I went around and felt the connections and battery temps and etc., and measured voltages, and everything seemed pretty copacetic. The controller and the motor were both very warm, enough that if they were any warmer at all I would have said they were hot. The starting voltage while still on float charge was 133.0V, and the ending voltage was 127.1V at rest before plugging back in. The six (92Ah) C&D Tech batteries were between 12.60 and 12.65 at the end of the drive, and the four (115Ah) Discover batteries were between 12.90 and 12.94.

And that's my report on drive #3.

Up and Running again!

The story with the Kelly KDH14500B controller ended about 400 feet after it started. I took my first test drive the day or two after Thanksgiving, and the car sluggishly rolled about 400 feet then stopped with a low voltage error flashing on its front panel. The voltage at the controller was 128V.

People at Kelly Controller were helpful in doing some debugging, but in the end they concluded that all the measurements I made were at nominal values, so if the controller still wasn't working then I should send it back. It took me a while to get it out of the car, boxed up and shipped, but I eventually did, and then things slowed down for Christmas. I finally received a replacement controller on Tuesday (Jan 5th), and I found time to reinstall it this evening. The controller booted up just fine, so I took it out for a couple short test drives around the neighborhood streets, sticking to streets above my house where I knew I could roll the car home if it quit again. Overall, things seem positive at this point.

Performance-wise, this controller is much slower off the line than the 72V unit was (it was a KD72500). In fact, when starting on any sort of a grade, 2nd gear is absolutely anemic and seems like a bad idea... First gear may actually be required to get the car moving. But once rolling in 2nd gear, the KDH14500B seems pretty good, and I got it up to 30 mph (on a slight downgrade) quickly with what seemed like less effort than the 72V system required. Further observations will be forthcoming as I venture out on longer trips.

Finally, some stats on the battery heaters. The car has been sitting here without moving for the last 5 weeks while the controller was out of it, and I had the batteries plugged in on float charge that whole time... which means the heaters were running too, keeping the batteries warmed to about 95'F. In that time, the Kill-A-Watt meter measured 198 KWh. That's about 40 KWh / week, or about 240 W average power consumption. 90 W of that is the chargers floating the batteries, which means the heaters are drawing about 150 W on average, or 1.25 Amps. Given that the ten 35W heaters would draw 350W if they were running continuously, I surmise that they are running about 40% of the time. In 5 weeks, 200 KWh @ $0.09/KWh costs about $18 for 35 days, or about $15.50 for a 30-day month. That's $15.50/mo. to keep the batteries float-charging and the heaters running in average 38'F weather (avg temp from NOAA for Dec. 2009 in Seattle). Seems like the heaters are running a lot, so I might have to look at tightening up my insulated boxes a bit.

BMS design, based on AC techniques

At November's Seattle EVA meeting, Stephen Johnsen made a presentation and showed a video in which he short-circuited a 100Ah ThunderSky cell to test the claimed 2000A limit of the cell. During the test, the cell voltage sagged to 0.5V, but twenty seconds later when he threw the switch OFF, it popped right back up to 3.2V.

Since seeing that, I've been pondering the question of how to tell what a cell is really doing, while it's running in a system. The question that's been rattling around in my head is whether there's a way to tell the difference between a cell that's at 2.5V because it's under heavy load, and a cell that's at 2.5V ocv because it's fallen off the cliff and is in mid-plummet? I mean, obviously, there's the difference in the current going through the system under those two circumstances, but I don't think that's indicative. Let's ask the question a different way:

Is there a way to tell when one cell becomes 'reverse biased' in a running system? Let's say you've got 1/3rd of your pack charge left, and you start driving across Nevada on a flat level road at 50 mph. You're pulling a nice steady 100A off the pack, and you're just cruising along. 25 miles later, your lowest cell hits the proverbial cliff. How can you tell? You still see 100A going through the system, but now the rest of the cells are starting to shove current through one cell that's misbehaving. Is there any sign of this that can be detected before it's too late? Does the voltage across that cell and the current through it in any way indicate that the cell has become a resistor instead of a battery?

In the AC world, you could imagine looking at the phase of an AC device, and measuring whether its phase is leading or lagging the line voltage, to determine whether it's sourcing or sinking power. Is there anything like that on a DC battery? Can you tell by looking at "edges" in the current/voltage domain, to see where voltage edges lead current edges, and where they lag?

Would you learn anything by intentionally pushing a voltage ripple down the wire, and watching what it does as it flows through the system? Or would the capacitance and inductance of the cells just filter such a signal out? What if every BMS board had a PLL oscillator on it, that pushed a small ripple signal down the big pipe, and the next BMS board in the chain was watching that ripple to see how its own cell changed the shape of it? Could that tell you somehow what each cell is doing, and whether it's sourcing or sinking power?

I don't know the answer, or even enough about the field of batteries to predict what the answer might be, but it's an interesting question.

Charger / balancer co-design

In the process of upgrading one EV (lead acid-based) and planning a second (LiFePO4-based), I've spent a lot of time pondering the questions of charging and balancing the cells. I've come to the conclusion that these are not separate tasks. So I find it unusual in the EV community that the two topics are treated so separately. You buy your charger from company X, and then your balancers from company Y, and you pray that they play nice together.

Your charger comes pre-configured with a charge profile from the factory, and the balancer boards have the unsavory job of trying to fight against that charge profile to get all the batteries charged before the charger shuts off. What's going on here? I seem to be missing something.

Another possibility... which is the system being used on the lead acid EV I'm upgrading... is to install separate chargers on every battery. This is very practical if you only have 10 batteries to charge, and don't have to worry too much about discharge characteristics. But when you get into LiFePO4 territory, now you're talking about 45, 50, 90, 100 cells to charge individually? And still need a protection mechanism to protect them as they discharge? It seems like there must be some economies of scale that would be missed by installing separate chargers on every cell.

Yet, in many ways, having each cell charge individually is the ideal.

How can that ideal be accomplished with a series charger and balancer boards? What if the individual cell balancers know the charge profile of the cells, and direct the charging at a local level? The "charger" would become a slave unit, a cooperative partner that provides the total voltage requested right now by the balancers in the pack, with the current limited by the smallest current any single cell needs + the maximum current a balancer board can shunt (after some of them start ramping down).

So, it would be like you split the charger in half. Put "the dumb half" that has the AC windings and the DC regulator in a common spot, and spread "the smart half" that knows the charge profile out to each battery as part of the balancers. The dumb half shouldn't really be dumb... it should be Power Factor Corrected for efficiency, and it should actually be pretty sophisticated with tight control over the voltage and current being supplied to the pack. And the smart half could be pretty simple, using the simplest circuitry available to conduct CC/CV/float charging on a 3.2V cell.

The feedback from all those balancer boards to the charger base is the really tricky part in this equation, to make charging reliable and robust. I don't have a good suggestion there yet. I've been playing in my mind with differential op-amps, and wondering if they'd be sensitive enough to get it right without wandering off into wild oscillations or positive feedback loops.

Are there any chargers out there right now that match this design philosophy? I have heard suggestions that perhaps the Manzanita chargers do this, but their website is very "least common denominator" in this respect. About their chargers, the Manzanita website says (and I quote), "The actual power delivered is a function of input and output voltage." Whereas I'd like to see something that says, "The actual power delivered is a function of input and output voltage, the needs of the individual cells as they charge, and the capabilities of the balancer boards."

Wheels Spinning! With caveats...

I have 'succeeded' on the installation of my new 120V upgrade to the Honda! Almost...

Yesterday, I got the high-current wiring relocated and connected between the controller, the contactor, and the motor. Today I rewired just about 100% of the low-current wiring inside the panel box that provides power to the contactor and controller.

Then I hit the snag... I reconnected the battery pack, turned the system on, and all of a sudden... Nothing Happened. There was no error code flashing on the controller, just a steady green light. There was no satisfying "chunk" of the contactor closing. There was no spinning of the wheels when the throttle was advanced. There was nothing.

Hmmm.

I did a little debugging, and decided that for some reason, the controller is not pulling the contactor to ground. So as a test, I bypassed the controller and wired the low side of the contactor directly to ground. And that worked. Then the contactor went chunk, and the wheels spun when the throttle was pressed.

OK... so I just have to figure out what's going on that the controller is not controlling the contactor as desired. A question for the community, or the manufacturer, next chance I get.

Here's the engine bay of the nearly-completed 120V upgrade. White box tucked up in the grill is one of the heated, insulated battery boxes. Toward the rear, driver's side is the new controller and converted throttle. And to the passenger side of the controller is the panel box where many of the goodies hide such as the contactor, the fuse, the kill switch, the current shunt, the precharge resistor, etc. The panel box is kind of hiding behind the reflector of my shop light, sorry about that.

Looking inside the panel box. You can't really see much of the high-current wiring in this pic, except the 400-amp fuse (round white thing) and the front of the contactor (black box above the round white thing). The two thick red cables running in front of the panel box are the 2 wires from the controller to the motor, and the black cable cutting the lower right corner of the picture is the "most negative" cable in the car, returning the motor current from the controller to the low side of the battery pack.

All of the low-current wiring (skinny red, black, and green wires inside the panel box) is what I was working on today. The main issue was that the old controller used 72V pack voltage to run it, but the new controller does not use the 120V pack voltage, it uses 12V from the auxiliary power system to run it. In the process of rewiring that, I decided to change a couple other things, too. The relay that used to be pulled by the ignition switch to feed 72V to the controller, I am now using to feed 12V from the main fuse box to the controller, the contactor, and the power brake pump, instead of feeding all those components straight through the ignition switch. It's a cleaner power source... I measured 13.3V from the ignition switch, and 13.5V straight off the main fuse box.

A better view of the 120V controller (Kelly KDH14500B) and the rewired throttle box.

PB-6 Converted to 0-5V

For the Kelly KDHB series controllers, they only accept 0-5V throttle input. The Honda had a PB-6 two-wire 0-5Kohm throttle on it. So tonight, I converted it. Hooked up 5V to "the other lead" on the potentiometer, and also took the opportunity to wire up the throttle switch, which wasn't connected previously.

Back Side.

Front Side, with Kelly-provided "J2" connector and wiring harness.