Hi Ben, I agree with Tom. My first thought is that your cells are cold. As my Thundersky 100Ah cells get below 50F, they start to sag pretty badly. The colder they get, the more they sag. When outside temperature is around freezing, I have trouble pulling 1C (100A in my case).
My solution was two-fold. First step was to insulate the batteries. Previously, they were simply in frames exposed to open air. I added boxes around the frames created from 1/2" foam insulation and 1/4" plywood to protect the foam. I really wanted more insulation, but that is all I could fit in my limited space. Second step was to add heaters. I used aquarium / terrarium heaters with a thermostat, under a 1/4" aluminum plate under the cells. That lets me hold the cells at 70F. While driving, they will be cooling (but not too much because of the insulation), and then will be plugged in again while I'm charging. I do agree with David that your cells seem a little on the small side for your vehicle and its current draw, but I think they would work OK, if the cells were warmer. I try to keep my Thundersky cells below 2C draw (200A), but they are less known for current capability than the CALB. If you have the cells around 70F already and still having this problem, then you probably need to follow the testing method that David describes below. Mike > -----Original Message----- > From: [email protected] [mailto:[email protected]] On > Behalf Of EVDL Administrator > Sent: Friday, January 25, 2013 12:04 AM > To: Electric Vehicle Discussion List > Subject: Re: [EVDL] Jeep woes > > According to your evalbum page www.evalbum.com/4001, you have 50 CALB > 130ah > LiFePO4 cells. With 50 cells, 120v is 2.4vpc on average. I'm not a lithium > expert, but that sounds pretty low to me. > > In fact the CALB datasheet I found online said 2.5v was the low limit for these > cells. It also recommends a 0.3C nominal current, which is only 39a, with a > max of 1000a for less than 10 sec with no duty cycle specified. > > The datasheet has discharge voltage curves for up to 1C (130a) suggesting, > but not stating, that the manufacturer might consider that an acceptable > current. That chart doesn't show the voltage falling to 2.6v until the cell has > delivered a little over 140ah at that current, but it also doesn't say anything > about temperature. > > All that said, let's assume that the prudent EVer will try not to go much above > the recommended nominal current of 39a. However, your evalbum entry > says that on the highway at 60+mph you're seeing current in the 200a range. > > You also say you're using 500Wh/mi. Taking an average of 45mph and solving > for motoring current, we get 173a. That's still pretty high. > > Again, I defer to the experts on this, but my first thought is that your > prolonged use at relatively high currents may have accelerated the > depreciation of your battery. You may have chosen too small a battery for > the vehicle. The solution is either larger cells, or more of the current size. > > You also mention on your evalbum page that when you charge, the charger > only puts 117ah into the battery. The way it's worded suggests that previous > charges put more than that in. For good battery cycle life, you shouldn't be > discharging past 80% DOD (104ah). > > However, amp-hours charged may not indicate actual DOD. Do you have a > measurement of your actual consumption? If you're routinely discharging > below 80%, that could also be a factor in "battricide." > > To carry out diagnostics, you don't need a dynamometer, but rather a > dummy load connected in place of the motor (or possibly motor and > controller). > You want something that draws the current you need for testing at your > battery's voltage without having the EV move - in this case, between your > nominal road current (173a?) and your peak road current of 300a. > > One "classic" EV dummy load is an electric water heater element suspended > in a barrel of water. Others who've done it may be able to give you more > details. My dummy load uses resistance heating elements from derelict heat > pumps, split in half with the halves wired in parallel for lower voltage, and > cooled with a fan. It has its own contactor. (If you connect your dummy load > ahead of the controller, or if you have AC drive, you definitely need a > contactor.) > > Charge the battery. Apply the load and measure the voltage at each cell. > Also check across connections in the hope that maybe one is poor (though > you'd probably be seeing symptoms of severe heating if that were > happening). > > Continue to monitor individual cell voltages as the battery discharges. > > This test should show you whether your problem is a few "stinker" cells, or > the entire battery. > > David Roden > EVDL Administrator > http://www.evdl.org/ > > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
