On Thu, May 28, 2015 at 11:01 AM, Cor van de Water via EV <[email protected]> wrote: > Nope, > I have a little experience with charging older style Li-Ion batteries > by hand and the resting voltage is typically a rather fixed amount (delta) > below the charging voltage, > no matter how high you charged them. > If you charge to 3.8V then they rest at say 3.65 > If you charge to 4.0 then they rest at 3.85 > If you charge to the max recommended 4.25 edge then they rest at 4.1 > If you overcharge to 4.5 then they rest at 4.35 (they will self-discharge > faster but not immediately) > > So, from measuring the rest voltage it is not clear that they are balanced - > you really need to measure each cell to make sure, > that is why a BMS is important. >
What chemistry are you talking about? If you charge LiFePO4 cells like the CALBs that Damon has to 4.25V and the current tapers to near 0A then you are definitely overcharging the cell and damaging it. If you charge a CALB cell and let it rest for 24 hours at room temp and it is resting over 3.38V then the cell was overcharged and you need to cut back your charging a bit. Damon would be best served to bottom balance each cell to about 2.75V. Then hook them up in a pack and stop charging when the first cell goes over 3.65V depending on current. With my 4 pack of 80Ah CALBs I have a 40A charger that is set to charge to 14.2V and shut off when the current drops to about 3A. In one of my packs the smallest cell goes to 3.8V but settles to 3.37-3.36V with no load so I know I'm not over charging it. In my Gizmo with 20 cells and a charger that tapers to essentially 0A the target voltage is 3.455V/cell. All settle to under 3.38V with no load on the pack. No balancing over nearly 4 years and over 12k miles. They haven't drifted as many predicted they would. There are several people who have been running LiFePO4 cells for several years now with no BMS and haven't run into problems. I have heard, however, of several with cell level BMS setups who have had problems from damaged cells to short lived packs. -- 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)
