Tuesday, July 2, 2013

TTID Revisited And Calculating Ranges.

On my www.mynissanleaf.com blog I posted observations on an experiment I did which was motivated by heat degradation issues seen in the extremes of a Phoenix Summer. The objective was to see if the BMS was playing any role when Phoenicians were seeing a rapid loss of capacity bars last Summer.  If you want to see the blog post, go here and I do recommend it due to some pix that show a VERY unusual condition! but keep in mind; the blog there was more for personal notes to myself and some of it is a bit hard to follow which was a mistake because a year later, even I have had a hard time following some of it which is the reason for the rewrite here. But this post is better than most for clarity!

Living in the Pacific Northwest makes it VERY difficult to do anything that even resembles extreme heat but was lucky in that we had a forecast of weather near 100º so I decided to couple that heat with charging during the hottest part of the day.

Background; LEAF was 19 months old, had 20,000 miles on it and GID count that ran from 275-280. Up until this point, I was doing full charges about 30-40% of the time which a handful of QC's. GID count verification by 100% charges at night 5 days in a row.

Experiment guidelines; Try to time the charge to complete between 3 and 5 PM which is normally the hottest part of the day here. Take GID count 1-2 hours after charge completion whenever possible.

Results;  Heat plays a major role in how much charge your battery will take. I saw GID counts dropping to 263 which is a 12-18 GID drop or as much as 6.4% of new capacity.  The pack is large and cools slowly. It took several days for the pack to get back to normal. I seriously thought I had done permanent damage to the pack (although I think the experiment did hasten the loss I have now)

Conclusions; Inconclusive.  Never read too much into a very short experiment that could easily be a fluke. I do recommend reading over the original notes in the blog at MNL but add this observation to the issues in Phoenix and it does look like the BMS is doing its job in protecting the pack but at an expense of a range loss.  I think that the hotter it gets, the greater to loss so a 10 degree jump from 90º to 100º will not be nearly as bad as the same 10º temperature difference (or more!) that Phoenicians see at 110º+.

Right now its 65º and expected to be about 25 degrees cooler than the past few days and seeing reports on Facebook about  lower than normal GID counts. I think we will see those numbers rebounding but it will probably take a few days.  Temps expected to be in the mid 70's for the next several days.

Another thing I saw this morning that I had noticed before was an apparent change in capacity that I will observe further. In my job, I do a lot of driving and have to constantly weigh whether the LEAF will work (and it usually does for most)  and how I will make it.  The best way to do this is to illustrate.

So, I have a 72 mile trip I need to make (usually plotted thru Google Maps) I reset trip A daily along with my miles/kwh meter so I can start fresh.  Now, if I don't have to stop to charge, that is always my preference and if driving 55 mph will do the trick I am all for it. So I take off in the morning, GID count 260 and I consider 250 to be usable. Using today's example, I used 72 GIDs to drive 25 miles which means I am humming thru 2.88 GIDS/ mile which equates to 86.8 miles (250 GIDS/2.88 GIDS per mile) which means my pace is good. Had it been too close to 72 miles, then I would adjust my driving accordingly. I found that starting to worry about range when you have 20 miles left does not leave a lot of room to adjust.  So, plan your range early when you still have 80% SOC and plenty of room for adjustment!

Anyway, the reason I combined this with TTID (temporary temperature induced degradation) is that while driving back from office, I was bored so checked progress again and got nearly the same results of 2.87 GIDs per mile at 42.4 miles.  The first time I checked my performance at 20 miles, I used the estimated range of 86.8 miles for off the wall calculation of stored charge by using the miles/kwh reading on dash which was 4.6 miles/kwh  to get 18.86 kwh usable. which is pretty much in line with my 91% (of 281 GID) capacity battery. But this time the estimated range had dropped to 82.4 miles but my performance had risen to 4.9 miles per kwh giving me an implied capacity of  16.81 kwh.

Now, there is always been a discussion of the true value of a GID, miles/kwh (which is different from drivers display and the center display and if you don't believe me, reset both at the same time, drive 20-30 miles then check both of them again) reading and what its measuring, etc.

But if using the GID meter which is only reading what Nissan is telling it to say and the odometer which is supposed to be super accurate per federal guidelines, then I have to take those numbers as being much more valid than anything the car has to say when the GOM is involved.  the Miles/kwh is based on readings of the GIDs and then it throws in a algorithm to complicate (and screw up) the situation and we get a overly optimistic range calculation when the pack is full and one that is fairly close when the pack is empty.  So, what am I to think here?  Is it the power for  accessories, HVAC, etc., not being included in that calculation?  Well, the HVAC power would be easy enough to test come Winter...

Any one else seeing this? post your results and maybe we can create some sort of expectation of calculating in that parameter when planning trips.

No comments:

Post a Comment