On 30 Apr 2014 at 20:36, Michael Ross wrote: > So what should I be paying for? Weight? $/lb value? $/Ah value? I am > hoping at least for directed opinion.
Who was it who categorized all falsehoods as lies, damn lines, and battery pitches? I think it may have been Lee Hart. ;-) Especially with cheap, no-name batteries, amp-hour ratings can be iffy. There are many ways that battery salesmen can fudge the numbers. If you're a battery manufacturer, you can make a battery's capacity seem higher by rating it at a very slow discharge - perhaps C100 (100-hour discharge, common for PV systems for some reason) instead of the standard C20 (20 hour discharge). OTOH, some of the good batteries are actually rated at C5. Or you can measure capacity at some absurdly high temperature that would degrade the battery in a few weeks if you actually used it that way. Or you can measure highly optimized lab samples made with high quality materials, but conveniently ignore the poorly made junk that actually comes off the asembly line. Or you can just outright lie about it. So I'd say that price/amp-hour is pretty iffy. Weight is a better indicator of real capacity. Lead is your fuel. Again, make sure your vendor is telling the truth! This can also change if you're drawing large currents. You might find that a battery with a lower C20 capacity than another one actually has a higher C1 capacity. The factor which models capacity loss at high currents is the Peukert (pronounced POY-kairt) exponent. If you have a capacity rating for your battery at two different currents, or if you have C20 and reserve capacity (and you can actually trust the manufacturer's rating), you can calculate the Peukert exponent with this calculator: http://evdl.org/uve/battery.html The lower the Peukert exponent, the more capacity the battery will have at high currents. Some (but not all) AGM batteries can approach a Peukert exponent of 1.1. 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)
