EVDL Administrator via EV wrote:
Alas, any time you fix a declining EV battery by replacing the worst cells or blocks, rather than all of them, you have no way to know what other cells or blocks might turn toes up next week, next month, or next year.
That's true. However, it is also true that the one cell that failed may have been "infant mortality". It could have died young due to some quality defect or temporary abuse (like excessively discharging the pack, which killed the weakest cell). If you replace it, you can keep right on using the rest of the pack for an extended period.
I've done what they describe with lead EV batteries, replacing blocks as they fail. With that situation it's mostly a losing game, because once a couple blocks die, the rest usually aren't far behind.
This is often true because the driver and the battery charger don't *know* there's a problem. For example, if one cell shorts, the charger blindly charges the remaining pack to the full voltage that was needed with all cells good. That overcharges the remaining good cells, driving them into an early grave.
Coincidentally, this is a NiMH question I'm facing too. I have a 36v 10ah NiMH e-bike battery here that's on its last legs. I've been debating whether to (a) replace just the bum cells and see how it goes, (b) replace all the cells, or (c) gut the battery and try lithium cells instead.
If the original cells were high quality, they would all have been well matched. When one of them fails, the rest may not be far behind because they are all so much alike. For example, I've tested Prius nimh modules; even my 10-year-old ones are as alike as peas in a pod.
If the pack is made from el-cheapo cells with negligible quality control, you could easily have a few lemons that died young, and are making the whole pack look bad. For example, I tested a set of ten 12v 7ah batteries from an anonymous Chinese manufacturer in a UPS; 4 were junk, and the remaining 6 varied from 1.7ah to 4.2ah. I was able to replace the 5 worst, and keep using the remaining 5.
The only way to know for sure is to TEST. Test each cell or battery, and see what you've got. If they are all low in capacity, then it's time to replace the pack. If only one or two are significantly worse than the rest, replace them and keep going. :-)
-- Obsolete (Ob-so-LETE). Adjective. 1. Something that is simple, reliable, straightforward, readily available, easy to use, and affordable. 2. Not what the salesman wants you to buy. -- Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm _______________________________________________ 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)
