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donradcliffe · 02-07-2009, 13:23

I've done the math, and something doesn't add up...

I agree with Xantrex's numbers on required input currents for their inverter chargers, as I see 22 amps AC of dockpower into my Freedom 20 when it is putting out a solid 100 amps of DC at 14 volts. Converting over to KvA, my charger is using 22A x 120V or 2.64 KvA to put out 100A x 14V or 1.4 KvA. This translates to an AC to DC conversion efficiency of 53% at peak output, which is miserable by today's standards, and comparable to EPA's minimum requirements of 80%. If I say that the Freedom's power factor is 0.7 based on shorepower, which is more tolerant of capacitance/inductance mismatches, then maybe the inverter is really only using 0.7 x 2.64 or 1.8 Kw, with a resulting efficiency of 76%--not good, but perhaps believeable.

Then I look at their requirements for gensets, and they want a minimum of 6.5 kva from the genset in order to achieve 1.4 kva from the charger. Even if I assume the same AC-DC conversion efficiency of 76%, this means the Freedom's power factor with the genset is only 0.15!!!

The conclusion is that either Xantrex or the genset guys are lying about their specs, and it would be good to get some real life data.

Anyone have more data on how many DC amps their battery charger can put out when running on a genset, and how many AC amps it takes to do it??

02-07-2009, 13:23	#25
donradcliffe Registered User Join Date: Mar 2007 Location: Santa Cruz for summer Boat: Beneteau First 456 Posts: 650	I've done the math, and something doesn't add up... I agree with Xantrex's numbers on required input currents for their inverter chargers, as I see 22 amps AC of dockpower into my Freedom 20 when it is putting out a solid 100 amps of DC at 14 volts. Converting over to KvA, my charger is using 22A x 120V or 2.64 KvA to put out 100A x 14V or 1.4 KvA. This translates to an AC to DC conversion efficiency of 53% at peak output, which is miserable by today's standards, and comparable to EPA's minimum requirements of 80%. If I say that the Freedom's power factor is 0.7 based on shorepower, which is more tolerant of capacitance/inductance mismatches, then maybe the inverter is really only using 0.7 x 2.64 or 1.8 Kw, with a resulting efficiency of 76%--not good, but perhaps believeable. Then I look at their requirements for gensets, and they want a minimum of 6.5 kva from the genset in order to achieve 1.4 kva from the charger. Even if I assume the same AC-DC conversion efficiency of 76%, this means the Freedom's power factor with the genset is only 0.15!!! The conclusion is that either Xantrex or the genset guys are lying about their specs, and it would be good to get some real life data. Anyone have more data on how many DC amps their battery charger can put out when running on a genset, and how many AC amps it takes to do it??