A couple weeks ago, on a Monday morning, we were in off peak hours and I figured it would be a good time to top off the batteries before I ran the day's errands. You may remember that I recently destroyed my eXpert Pro meter by re-applying the pack voltage to it and not removing 12V power first. By the way, DON'T DO THAT! Any way, I'd received a new one, but I hadn't yet installed it. Without it, I have no idea how much current I'm pushing to the batteries, so I need to keep an eye on them to be sure I'm not over charging them. The fact is you learn things when you watch what the batteries during charging. Over time, I've learned quite a bit, and by watching them closely I can pretty much tell what's going on and predict when the charger will terminate.
At any rate, I know which cell will start to rise above 3.45V first, so I
put a meter on it and started the charger. For me, 3.45V is the top of the charge. There simply isn't enough energy put into the cells above that point to justify the potential risk and damage to the cells by getting them to the manufacturer's 3.6V. Ten minutes later, I walked
out to see the voltage on the cell had climbed from it's initial reading of 3.26V up to 3.380V. I know by experience that means
I'm about 35 minutes away from 3.45V when the charger should
terminate it's charging sequence. So you can imagine my surprise when I
walked out 25 minutes later and found the battery at 3.364V, and the charger still
running. The voltage had dropped, yet the charger was still running. I stood there for a moment, completely confused, it just didn't
compute in my head. Kind of like walking into your favorite BBQ place
and seeing that it's full of vegans. Not that there's anything wrong
with it, but it just doesn't make sense.
Figuring that the charger must have just gotten confused (after all it
does have a simple logic board in it), I figured I'd just reboot it. I
turned it off, waited 4 or 5 seconds and turned it back on. The moment I
turned it on, there was a succession of 3 pops, with the last one being
quite loud and producing a bright orange flash and rush of hot gas
shooting out the charger's vent port. This was followed by me yelling
"Oh shoot!", or something similar but perhaps more colorful. I turned the charger back on...
nothing. Wasn't too surprised by that. I checked the breaker on the
house and had not tripped, so whatever failed in the charger failed in
such a way that it simply consumed all the power coming into it and blew
up, rather than shorting. Or at least that's my take on it.
In either case, there was no doubt that I was going to need to remove
the charger and send it back to Manzanita Micro for repair. This was
the second time I've had to do this. The first was entirely self
inflicted when I mistakenly shorted one of the batteries to the chassis
with my multi-meter probe, while the car was charging. That blew up the
AC rectifier and melted the end of my probe. I had no idea what happened here, but I boxed it up, got
an RMA number and away it went to Washington.
It arrived in their shop on Friday and they sent me a note Monday
afternoon that it was repaired, tested and ready for shipment. Remarkably fast turn-around. I called
and spoke with Clarice, their office manager, to ask if they knew
what caused the failure. Clarice has seen enough repairs that she can
recognize the likely cause of the failure just by looking at the parts
that were replaced. In this circumstance, both IGBTs failed as well as a
capacitor. She said she'd honestly never seen anything like it, but
she'd ask Rich when he got it. Rich thinks that one of the IGBTs failed
and when it did, it took the other with it. He thinks heat may have
been a contributing factor. This is where things get interesting and
where there may be a lesson that all of you can learn at my expense.
As regular readers of this blog are aware, I live in Phoenix Arizona,
which is slightly north of, and roughly rock throwing distance from Hell.
It's not uncommon for the inside of my garage to be 105°F. If my wife
pulls her car in after getting home from work, as it ticks itself cool,
all the heat that was happily stored in all of that steel, gradually
works it's way into the garage raising the temperature well over 110°F.
Here's the thing. Two years ago the car lived through it's first summer
as an EV. A significant portion of that summer saw the car
dis-assembled because I had sent the motor back to Netgain because the
balancing putty had fallen off. Plus I took the opportunity to redo the
battery layout. So when it was back on the road in late July and I was charging it
on a regular basis, I would come out to find the charger's yellow light
flashing at me. I went back to read the manual to find out what that
meant, and I couldn't find a reference to what that was all about anywhere. I figured it was
just an oddity with this charger. What it really was, was the charger
warning me it was over heating. I blissfully ignored this warning for
the remaining portion of the summer.
As it happens, the car was taken apart for a good portion of the
following summer to again fix a balancing putty issue on the motor, and
to add air conditioning to the car. In fact, I didn't get it back on
the road until the first week of September, just in time to tow it out
to EVCCON 2011. But even then I would see the flashing yellow when
charging, but by then I'd figured out what it was so I would dial the current
back until the light stopped flashing.
That's been my modus operandi this summer. I'd turn the charger on and
dial in between 21 and 22 amps. I'd poke my head back into the garage
in 20 minutes or so and if the charger was over heating, I'd turn it
down. About 90% of the time, it was over heating. I'd turn it down to
16 amps or so. In retrospect, I think this behavior, and the initial instances of ignoring the warning, damaged the charger
over time. Heat is the enemy of all circuitry and when the charger was
overheating, it felt quite literally like a blow drier firing out of
that little vent port. So my advice to you is don't to this. I realize that
most of you don't live in such hot climates, but for the few that do,
pay attention to your charger. Heat kills.
At this point, I intend to charge at 16 amps or so during the hotter months to protect the
charger. It's remarkable how much cooler the unit runs at that current
level. The problem is the charger was configured at a higher current
level. That means if I charge at a lower current level, there's a risk
that the charger will overshoot the voltage I've set and consequently
over charge the batteries. Re-tuning the charger for a lower level is
possible, but such a huge pain in the butt that I don't want to do it
for the 1 month it's necessary before cooler weather arrives. But I would like to be able to charge at any level I like and not risk over charging the cells. I do
have a solution in the works which I will be implementing and writing
about soon, so stay tuned for that. In the mean time, I'm going to have
to watch the end of the charging curve very carefully to protect the