Post-balancing Distance Calibration

I have been (properly) balancing my lithium cells for a number of months now. I hate to say it, but the time before that I wasn't pushing the upper envelope of charge high enough for all the cells to get fully recharged. That meant that if I went for a significantly longer distance than I normally do, I would risk an over-discharge alarm on some of my cells (< 2.5 V).

Luckily, I never went that far.

Now I believe my pack is happily balanced, being pushed to ~183V (~3.52VPC) on every charge for the last few months (full charge voltage is 3.3V). This weekend I started to take another look at my range, and start *really* using my amp-hour meter to see how far I can really go. With lead acid batts, you're supposed to only use up to 50% of the charge if you want them to last. With lithium batts, it's up to 80% (80 of the 100 Amp-hours in my case).

Some data points from this weekend:

1) 28.2 miles, half freeway driving, half not. 51.5 Amp-Hours expended. No BMS alarms, no performance differences at all at "halfway". Great! On to data point #2...

2) 32.5 miles, ALL freeway driving, quite fast in windy conditions. 65 Amp-Hours expended. Again, no BMS alarms and no noticeable performance difference at the end of the drive!

Inching toward the mythical limit of 80 Amp-hours! I want to be careful when establishing these 'confidence' points, because I don't want to get stranded or cause harm to the batts. Good news is, because of data point #2 I now know that I can confidently expend 65 AH without getting stranded - that's about three times my daily commute!

Has it been THREE years?

I just realized that I have been driving Electro for three years! The maiden voyage (first with lead acid batteries) was back in August 2008. 6,200 gas-free miles since then and going strong.

5,000 miles!

As of this morning's drive up to Mill Creek, Electro has passed 5,000 'electrified' miles. Woo-hoo!

This is more of a symbolic milestone than anything else. Pretty nifty, and I am looking forward to many more.

On the road again...

I'm not sure if Puxatawny Phil saw his shadow or not, but it has been pretty cold during the last few months here in the PNW. Cold enough that I just didn't want to drive Electro around (remember, no heater). Well, Spring is starting to rear its head, and today I felt it was warm enough to hit the road in Electro again.

I have also been tuning the charger to be *slightly* more aggressive on the charge voltage, running the pack up to 176V (with 52 cells in the pack that works out to just under 3.4V per cell), in an attempt to better balance the pack. I'll let you know how that works out!

Cell failure and replacement

It's been awhile since my last post - nothing of importance to note. Electro has been working great, although in the last couple of months I have driven the car pretty much zero time because of the really cold weather (no heater in the EV). So, Electro has been quietly hibernating in the garage, waiting for Spring. While working in the garage one day, I heard the BMS audible alarm go off. Thinking it was an open in one of the signal wires (remember, the cells are all wired in series back to the BMS, and an open in that chain signifies a problem from one of the cell modules.

So, this Saturday I went into debug mode, tracing down the cause of the open. I found it in one of the rear cells. Remember, the cell monitors show green when all is well with the cell voltage, red when the cell voltage is out of range, and nothing if the cell voltage is drastically low. Look at the middle cell in this image:

Something is up with that cell. So I measure the voltage across it:

1.3V? Nominal is 3.1-3.4V. The cell is clearly in trouble. When I picked up this collection of cells back in February, I happened to pick up an extra cell that has just been sitting in my garage cabinet collecting dust. I went to measure the voltage across it, and was pleasantly surprised that the cell is still at 3.1V! So I went about extracting and replacing the problem cell. Disconnecting the signal wiring and straps was a snap, and by partially inserting the lock down bolts, I could pull the offending cell out vertically, and drop in the replacement without too much fuss.

After putting in the replacement, the cell monitors installed all worked fine. I did a cursory check of the remaining bolt down connections, and buttoned the whole pack up. After resetting the BMS, the audible alarm stopped, which means all is well with all cell monitors.

It was handy having a replacement cell on hand for this exercise. I will be contacting the folks at EV Components (now at Current EV Tech) about sending this problem cell back (they probably would like it back) and about purchasing another spare or two.