Jim Brown <j...@audiosystemsgroup.com> writes: > I'm considering batteries in the 20-25 Ah range, with the Ah rating > based on 10A. So my question is, what sort of internal resistance > should I expect for a battery rated like this? Yes, I know if I had > the battery and a sufficiently accurate voltmeter I could measure it, > but these batteries sell in the $250 - $300 range, so I would like to > know before I buy. > > Here's a pack that I'm considering. > > http://www.batteryspace.com/polymerli-ionboxbattery111v252ah27972wh14arateli-ionwithcarsocketsmartcharger.aspx > > > A 11.1V (3-cell) battery is clearly the right choice to run the KX3, > but it's the KXPA100 (100W amp) that I'm concerned about. I suspect > that it isn't going to be happy much below about 10.5V. That's OK if > the published discharge curves for Li Ion batteries represent the > terminal voltage under load -- they show about 90% of capacity with > the rated discharge current, but if that voltage is reduced by the > internal resistance, I've probably got at least 10% less. > > The link below shows the discharge curves for a single cell, followed > by my own plots of the same data multiplied by 3X and 4X for 3-cell > and 4-cell packs. The KX3 and KXPA100 are rated for supply voltage up > to 15V, and I've been told that 16V is an absolute maximum. That means > the output of a 4-cell pack is too much, so I think it has to be 3 > cells. > > k9yc.com/Lithium Ion Discharge.pdf
In my experience, every plot labeled "discharge" has been the terminal voltage under a specified load, and they are often given as a family. For lead acid, it's typically at C/20h and then higher rates. A plot showing open-circuit voltage after some amount of usage should be labeled "stage of charge" rather than "discharge". In the case of the plots in your PDF, the first plot shows three lines, with 0.2C, 0.5C and 1.0C. This is sloppy, because C is in Ah, and 1.0C therefore has units of Ah, which makes no sense. But surely that means C/1h and is in amps. 0.2C is C/5h or 0.2 1/h * C. And I see this all the time; the RC people know what it means :-) It's interesting that the curves are so close, for 0.2C and 1.0C. As I read it, there's only a few % (from 100 to 95 at 3.2V cutoff) loss. For lead acid, there would be a far greater difference at these rates. For JT65, what power output are you running, and what input voltage is needed for the KXPA100 to be happy? I would think you'd be down around 50W for linearity and heat, but I don't yet have any JT65 experience. It's unfortunate that equipment designed around lead acid batteries (our nominal 13.8V notion) wants the first half of a 3S pack and the second half of a 4S. I don't think this is useful, but you could consider different batteries for the kx3 and kxpa100. But with the same max, the kx3 should be fine with what the amp needs. I find "25 Ah at 10A" to be unusual, but I think it's the difference between lead acid standby/cycle use, where capacities are ~always given in 20h rate, and LiPoly RC culture, where the 1h rate is a slow discharge. I wonder if 10A is a standard discharge current for multiple batteries - it's a fair metric if the question is "how long can I power a 10A load". Have you thought about DC-DC converters? I am about to order an AnyVolt3, which isn't big enough (3A) to power a KXPA100. But a converter could dodge the max/min issue, and may be worth it in weight/price, presuming it's quiet (which some are said to be). 73 de n1dam ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
