Sunday, May 27, 2012

Trusting the Charger

Alert readers will recall that I was having doubts about the effectiveness of my charger.  I was alarmed to see it was driving my 120V DC battery pack at 153V DC, when the sticker said it should be 143V DC!

After I called the Zivan repair service and described what I was seeing, they told me to send it in for testing.  After they ran it on the bench, I got a call from Mark, saying it was fine.  Here's what he said, any errors or omissions are mine.  I've been learning about the mysteries of battery packs on the streets and shady blogs, so I was happy to have another data point.

Mark said that the pack should be overcharged.  Specifically my pack should be charged aggressively up to 157V with the T-125 battery setting "4" on the rotary switch.  If I'm concerned about charging it too hard, I can set the switch to "3" which offers a slightly gentler charge for T-105 type batteries.  After 12-18 months the batteries should be mature and I can switch back up to "4" and drive them a little harder.  110% - 115% overcharge is OK.

The lower-voltage cells have to be allowed to charge up to the level of the higher-voltage batteries.  This is what Jack Rickard calls a "top balancing" scheme, which he says is wrong for lithium batteries, but he doesn't believe in lead-acid so it may be valid.

For the charging-state LED, a fairly-long yellow is OK, which represents the 80% to 100% stage.  I thought it should be much shorter, adding to my distress over the observed voltage.

Now having explained all this, I still have exactly the same symptoms that another person at Zivan diagnosed the charger as faulty and I should send it in.  This exercise cost me $70 plus shipping with nothing to show for it but a little more knowledge.

After I redid the whole charging system with the J1772 connector wired in, I did a short 3 hour charge and took it to the car show.  I noticed that the performance was rather sluggish, but the voltage was right about 120V DC and the PakTrakr state of charge was 100%.  Yesterday I decided to trust the charger and let it go through the full cycle without panicking and pulling the cord.  Well imagine my surprise when it put 13.11 KWH into the pack over the course of 10:33.  I had been doing about  8 KWH over 6 hours previously.  The voltage peaked at 153.3V DC and dropped down to 132.2V DC shortly after disconnecting the charger.

13.11 KWH is far beyond what I calculated for the 80% to 100% charge capacity, so I'm hoping that the pack was just very thirsty because I hadn't been charging it all the way up.  I did a short drive around the back yard today and the acceleration was neck-snapping like I remember from my first drive.  I hope I haven't damaged the pack.  If you know me, you know I'm a bit of a pessimist and I have this sinking feeling in the back of my mind that I now own a 10 mile pack.  Well, I'll do some range testing and see how it looks.  I just don't have the spare cash for a lithium pack so I'll have to make this work for as long as I can.