Monday, December 14, 2009

When Things Fall Apart

Yesterday was not a good day. My landline and internet connection went dead on Saturday and the lamp globe for my ceiling fan broke apart above my bed, putting glass shards in the covers.

This gave me an excuse to get off my computer and drive over to Home Depot yesterday (Sunday) to try and find a new globe for the ceiling fan. I had recently installed the PakTrakr system, so I cleared out the serial logging attachment and started collecting data. On the way to Home Depot, I missed my exit and ended up going another two miles to the next exit. I knew I had gone farther than my battery pack should have handled, especially in the cold, so I stuck to city streets and slowly drove home.

Halfway home, the battery gave out. I crawled into a Taco Time and asked if I could plug in. The manager pointed me to an outlet outside the building but he didn't think it worked. It didn't. After that, I walked a block away and found an auto parts place that was kind enough to let me plug in. After the car sat for 20 minutes, I had enough juice to get to the auto parts place. I also collected the serial data log on my laptop from the PakTrakr for later analysis.

After plugging in, my friend Ruth picked me up and dropped me off at home. I had an appointment last night near the auto-parts place. My plan was this: have a friend drive me to pick up the Civic (after four hours of charging), drive to my appointment, plug in there for an additional three hours and hopefully have 40% charge left to get home.

We'll, I picked up the car from the auto-parts place, drove to my appointment and plugged in. Three hours later, I came out and realized that the charger hadn't given the batteries any extra charge. After poking around under the hood, I also found a critical resistor that enabled the DC-DC converter had broken off. Without the DC-DC converter to charge my 12V battery, my car would die quickly with the headlights on at night. It was rather moot because I still had little charge in my batteries. It looked like the charger had given up the ghost on me.

I got a ride home from another friend (it's good to have friends when you own a home hacked EV) and called a tow-truck this morning through BWC (the environmentally friendly alternative to AAA). I felt rather sunk all night because the car is basically useless without the charger and the batteries will degrade rather quickly if they sit in a discharged state.

I got the car home later this morning, fixed the resistor for the DC-DC converter and tried the charger again. To my surprise, the charger actually started up. My batteries seemed to be quite out of balance and the charger just wasn't putting much amps into the batteries, even with the shunts not active yet.

I still have to analyze the system more, but here's what I think is happening: The charger can only generate about 175 volts because it gets it's input voltage by rectifying the 120V AC line. When cold, AGM batteries need a much higher charging voltage. If I look at the datasheet for the LifeLine batteries, I actually need to charge each battery up to 14.90 volts at 40 degrees F to get to 80% charge. Let's see, 14.90V x 12 is 178.8V, which is higher than the charger can actually put out. The battery shunts were actually working properly and only shunting the batteries when they reached 14.9V. Since the charger couldn't reach that level for all batteries simultaneously, only a few shunts kicked in before the charge current dropped precipitously, leaving an unbalanced pack.

In short, It doesn't look like the Belktronix charger can actually charge these particular AGM batteries under cold temperatures properly.

My next task was to look at some of the PakTrakr data I gathered. The serial log was heavily garbled, so I couldn't just load it into a spreadsheet. I had similar noise issues with the 914EV here, but adding a 100 ohm series resistor didn't clean up the issues.

I did find that under massive current draws that at least one battery was getting yanked down to 7.2 volts (ouch!). Either that battery is undercharged or I perhaps hurt it over this past year and it's (probably) damaged. It's too bad that the best battery data comes under a high current draw when the serial data is the noisiest.

This being the wintertime, it'll probably be a few months before things heat up to where I can effectively drive the car again. In the meantime I'm going to check out Nissan's Leaf this week and think about ditching all this unreliable homebrew stuff to buy a commercially engineered EV...

Happy Holidays!


David Harrington said...

You need to add a 0.1uF capacitor between the Serial Data and GND lines. I did that and my PakTrakr has been 100% reliable since them. Before that, any decent amount of AMPS I pulled would reset it and I would lose my SOC.

That stinks having all those problems. My Civic EV had been very reliable until my transmission blew, which I have finally gotten around to finishing the replacement.

TimK said...

Hi David,

Thanks for the excellent suggestion. I'll try it out this coming weekend.

Congrats on getting your transmission fixed. Why did it blow? Was the torque from the electric motor too much?

David Harrington said...

It blew due to age. It had 244,130 miles on it and I think the previous owner stopped changing the ATF at 160,000 miles so it just burned up the clutches.

Ross Cunniff said...

The charging and range issues with AGM batteries and cold concern me as well - so I'm installing warmers.

debhollenback said...

Hi Tim,

So, given a second chance, would you still have purchased the Lifeline batteries, or another AGM brand? I'll be buying AGMs in the spring...

A Leaf sure would be nice!

TimK said...

Hi Deb,

It's hard to say at this point. The batteries gave me a good year of operation. I discharged them 60-90% about eight times a week for a year, yielding about 800 discharge cycles. I also can't say that I treated the batteries that well. A larger pack would have lasted longer but would also be heavier.

I really like the copper (not lead) bolt terminals on the LifeLine batteries. I never had connection issues and the batteries put out a good punch of current when I accelerated.

If I had to start all over at this point, I would go with lithium-Ion given the smaller size, much lower weight and much longer life.

Since I'm focussing on getting a Nissan Leaf, I might just use the same batteries again, so I don't have to redo all the cables and possibly the mounting brackets.

So much for a direct answer...