Sunday, June 7, 2009

Sleuthing the Soneils

Against my better judgment, I took apart one of the failed Soneil chargers yesterday afternoon to see if I could find out anything about the failures. There weren't any burned components or obvious damage. After powering up the unit with the cover off, I could verify that the isolation transformer between the AC side and battery side wasn't getting pulsed, and thus passing no power to the battery (or the LED). It seems that the UC3842A chip DC-DC converter chip used to drive the FET which pulses the transformer was dead. It's power supply had a resistance of about 3 ohms to ground (bad).

After mulling over the circuit in my head last night. I tried to compile a list of facts I know about the failure:

  • The batteries were all fully charged in the morning, despite the charger failures
  • The batteries with working chargers had ~13.7 volts on them, the failing ones, ~13.4
  • There were far more failures in the batteries in the front of the car
  • The front batteries are closest to the negative pack voltage
  • I plugged in the rear batteries first when testing the chargers
  • The rear batteries typically have slightly more charge on them because they get warmer
  • I started all rear battery chargers simultaneously on a power outlet strip
  • I started all front battery chargers one-by-one by plugging them into power cords with three outlets on the end
  • The components crossing the isolation barrier in each charger are: a transformer, an opto-isolator, and three ceramic capacitors (.01 uF at 250V)
  • Since all batteries were at nearly full charge, the failure must have happened at the end of the charge cycle, perhaps when the charger transitioned from acceptance charge (14.7V at 3.5A) to float voltage (13.7V)
  • Since the front batteries typically finish charging last and I also plugged them in last, perhaps a surge from the rear battery chargers (when shutting off) caused failures in the front battery chargers
  • Is suspect that the capacitors at the isolation barrier in the chargers failed because they have a breakdown voltage of 250V. When charging one battery, this is fine. When charging a long series string of batteries (oh, say, 144V), the charger output stage can float +/- 144V relative to the AC voltage on the input stage. 120V AC really becomes 170V DC when rectified. Add this to 144V and you get 314V, which is clearly above the 250V rating in the isolation capacitors.
Based on this, I don't think sending the chargers back for replacements would really help the situation. If all the chargers have the same 250V isolation caps, then they would all fail eventually. I suppose I could take all the chargers and replace the isolation caps with similar values but give them a 1KV voltage rating.

I'm trying to figure out the best option for now. To drive the car, I'll probably need to re-install all the Belktronix resistive shunt balancer modules. The car would be driveable but it would still take a long time to charge. I don't like the idea of hand-modifying twelve Soneil chargers, but that would probably be what it takes to get them working.

I could send all the Soneil's back and get the Joule chargers again. That is, after checking with the designer that they support 1KV of isolation. The Joule chargers draw 35mA of current when sitting there unplugged. This hypothetically translates to 2857 hours of discharge from 100Ahr batteries or about 4 months until the batteries are completely dead. If I went on a long vacation, I guess I would leave the chargers plugged in on float charge anyway. The Joule chargers also only charge with 6A, which means (despite a bad PFC rating), I might be able to plug in all chargers to a 15 amp AC circuit (The Soneils pull about 18 amps for all 12 chargers).

This is a really bad time because we have so many electric vehicle events happening now. Having a dead EV is not too appealing. I'll ponder this for a bit.

Happy Sunday morning.


enganear said...

Hi, what model Soneil is this? I have 3 1212SR units that I plan to use on my 36V electric lawn tractor. The 1212SR has a metal case.

TimK said...

This is the 1214S model.

enganear said...

I am thinking about adding some conformal coating and a fan to my Soneil plan....

Ross Cunniff said...

Have you considered a mid-pack disconnect to reduce the possible over-voltage condition while charging? Maybe a big contactor... although that would become another potential failure point while driving.

TimK said...

Hi Ross,

Thanks for the idea. I see this as a bigger issue with the Soneils because they aren't truly isolated. Even with pack separated into two halves, the differential voltage might creep up to 72+170 which would be 242V. While this is under the 250V rating of the isolation caps, most designs should allow for 30-40% of guardband to increase the life of the caps. Of course, the failure mechanism could be something else, but having the caps out of spec make me concerned that there aren't other issues (possibly just an irrational customer response). Thanks again for the split-pack idea. I'll consider it.

Ross Cunniff said...

The other advantage of a mid-pack split is that the overall voltage is lower when you are doing pack service. A minor advantage - a 12V potential difference is plenty to vaporize battery terminals and cause severe burns.

The disadvantage is if you have both a mid-pack contactor *and* a contactor in front of your controller, you may have sequencing issues with turning them on.