People talk in amp-hours because it's easy and/or they're being sloppy. Batteries are sold in standardized packages (1 cell @ 3.6V for lithium, 3 or 6 cells at 6V or 12V for lead) at which amp-hours is the metric used to differentiate different capacities. But when you build a big series string (as in an EV), the voltage is arbitrary, so you're 100% correct.
Amp hours are particularly useful for at least one measurement: state of charge. A lithium battery's amp-hour capacity is pretty much constant no matter what the discharge rate, while its watt-hour (i.e. energy) delivery depends on load (because at higher loads, more of its watt hours are wasted making heat in the cell). Lead's amp-hour capacity is somewhat more rate-dependent, but still more predictable than energy. Likewise, amp-hour capacity is independent of cell performance. Two different lithium ion cells can both be 100Ah at 3.7V nominal, yet differences in internal resistance can cause them to yield different energy outputs. These aren't directly specified by a manufacturer, so again, the Ah is an easier number to grab. Cheers! -Ben On Jul 9, 2014, at 2:17 AM, Larry Gales via EV <ev@lists.evdl.org> wrote: > I know that watts = amps * volts, and watt-hours (wh) is a measure of > energy, but over and over again I hear people talking about EV batteries in > terms of amp-hours. What does that mean if you don't know the voltage? > 1000 amp-hr is not impressive if the voltage is, say, 0.00001 volts. Is > there some sort of standard voltage, e.g., 3.6 volts, that is assumed when > one talks about LiON batteries? Does amp-hours tell you something more > than watt-hours? > > Thanks in advance,-- > Larry Gales > > -- > Larry Gales > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20140708/122acf6c/attachment.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) > _______________________________________________ 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)
