That was a very good summary. Cell balancing is something that was done with other chemistry a and many have tried to apply it to Li Ion chemistries but it doesn't work with Lithium since cells do self discharge or drift.
If one wanted to balance them it has to be done below 3.38 volts otherwise you are still charging the cell. Thanks for your input. Sent from my iPad > On Jun 17, 2015, at 10:53 PM, David Nelson via EV <ev@lists.evdl.org> wrote: > > That is a reason I don't use a cell level BMS. With a cell level BMS > like the miniBMS there is a constant drain on the cells running the > BMS boards and it is nearly impossible to make sure that each board > uses the _exact_ same current regardless of voltage in the cell. > Furthermore, boards like the miniBMS expect that you will balance at > the end of every charge and last I saw, that balance voltage was at > 3.6V or so which is over the theoretical 100% SOC level for LiFePO4 > cells which leads to potential overcharging of the cells on every > charge cycle. If you stop charging without the balancing taking place > and/or let the pack sit for extended periods of time then the > different current draw of each board working 24/7 introduces an > imbalance in the SOC of the cells in the pack. > > Willie, > > If you had made sure that you had a set of good LiFePO4 cells (ie > discharge to about 2.7V and let them sit for a week or so and then > measure voltage to see if it is dropping or stable then remove any > cells which still drop in voltage). Then assembled the pack with only > connecting to the most positive and most negative end of the pack. > Next, charge the pack while monitoring individual cell voltages and > note when the first cell went over 3.5V and then had the charger hold > that pack voltage and taper current down to 0.05C (5A for a 100Ah > cell) and then shut off. Finally, have a master cutoff switch like a > BRB which totally breaks the battery circuit to eliminate any > parasitic loads that you would still have a trouble free setup. > > That is how I setup my electric push mower. I can let it sit all > winter and it is still charged when I go to use it in the spring. It > has sat for two winters now with no issues. Other than doing a top > balance on my Gizmo that is how I have my Gizmo setup except for the > master cutoff switch. It hasn't had any balancing for 4 years and when > I check it it still is in balance. My pack now has over 20k miles on > it and has been in use since January 2010. > > I know many here think I'm crazy and seem to want to discount my data > or say I'm just lucky but you aren't the only one with problems with > your LiFePO4 pack. All the ones I hear about with problems have cell > level BMS boards on them or they don't do a proper initial balance on > the pack. The only other common item is that they over charge their > packs. > > Basically, don't over charge or over discharge, don't put ANY > imbalanced load on the pack, undercharge slightly, and don't pull > extreme currents from your cells and they will last a long time. A > properly fully charged LiFePO4 cell like those from CALB will rest at > 3.38V when at room temperature. Any higher and the cell has been > overcharged. > > >> On Wed, Jun 17, 2015 at 1:15 PM, Willie2 via EV <ev@lists.evdl.org> wrote: >>> On 06/17/2015 01:55 PM, EVDL Administrator via EV wrote: >>> >>> Can you be a little more specific here? What makes them so expensive? >> >> The miniBMS modules suffer from exposure to the elements and they are not >> well protected in golf carts or the Zap. Or my Ranger. Rodents have chewed >> a lot of the signal wires, which have been too light a gauge. I've been >> told that the cells self discharge at quite different rates so that, when >> left sitting, the battery gets badly out of balance or some cells go to zero >> and self destruct. Also, I seem to have some vampire loads that lead to >> discharge in a few months or less of non-use. The real problem is that I'm >> not willing to jump on a problem as soon as it develops; I tend to let it >> sit for months. By that time, I'm likely to have lost some cells. I may >> use only one or two of these farm EV conversions at a time but I've been >> trying to keep about five ready to use at any one time. Right now, I have >> two golf carts, the Zap, and the Ranger all dead and I am able to use only a >> golf cart and the imiev. The imiev is WONDERFUL, completely trouble free. >> So far. I believe the imiev is going to be considerably cheaper than the >> Zap. Or even a golf cart. >> >> My experience is that the big cells with miniBMS have a mean time to failure >> of about two months. Maybe less. I'm looking for a year or more with no >> more maintenance other than charging every couple of months. >>> >>> >>> Golf cars are pretty economical and reliable with lead golf car batteries. >> >> I can not agree. In my experience, lead in golf carts is FAR more demanding >> in terms of maintenance. Monthly watering, bad battery connections, >> corrosion, rusted out battery boxes, general nastiness, ever declining >> capacity. >> >> >> _______________________________________________ >> 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) > > > > -- > David D. Nelson > http://evalbum.com/1328 > http://www.levforum.com > _______________________________________________ > 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)
