Low efficiency converting 48V to 12V around 500W?

Discussion about the MX60 Charge Controller

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debcar
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Joined: Sun Mar 13, 2005 8:45 pm
Location: Austin, Texas

Low efficiency converting 48V to 12V around 500W?

Post by debcar » Sun Mar 13, 2005 9:38 pm

Howdy from Texas.

I have just installed a PV system on our motorhome consisting of four Sharp 175 panels (700W nominal total) and an MX60 controller feeding our existing 12V battery bank. The panels are configured in a nominal 48V series-parallel arrangement (giving MPP voltages in the 60s).

The system seems to be working fine, with the various losses along the way about as I would predict, except for a seemingly excessive loss in the MX60 itself of around 15% based on the values on the MX60's display. For example, at one point late this afternoon I observed:

Displayed Input Volts = 63.0
Displayed Input Amps = 7.7

Displayed Output Volts = 13.0
Displayed Output Amps = 31.5
Displayed Output Watts = 410

Calculated Input Watts = 485.0 (63.0 x 7.7)
Calculated Output Watts = 409.5 (13.0 x 31.5)
Calculated Conversion Efficiency = 84.5% (409.5 / 485.0)


The owner's manual has graphs showing the conversion efficiency with 24V and 48V systems for various voltages and power levels, but no such graph for 12V systems like mine. So I don't have a way to determine exactly what efficiency I should expect to be seeing, although 95-97% seems to be typical for most operating conditions.

If the numbers above are accurate and the efficency really is that low converting 48V to 12V at the relatively low power levels I'm running, I'm wondering if the MX60 might not be the right fit for this application after all. The idea of buying a premium controller, of course, was to squeeze the most out of our panels as possible, so losing 15% in conversion is an unpleasant surprise.

For what it's worth, the cooling fan on the unit seems to be running most of the time (ambient temperature of around 75 degrees F today) so there does seem to be a good bit of waste heat being generated in there.

Anybody have any explanations/theories/insight about this?

A conversion efficiency chart for 12V systems would be interesting to see as well to know what to expect in this configuration. I'm trying to understand if I have a bad unit, maybe some incorrect calibrations, or if this is normal for this power/voltage range.

Thanks!

boB

Post by boB » Mon Mar 14, 2005 2:15 am

Hi Carey...
You're OK I think. The system sounds like it's working fine.

One problem is that you cannot calculate efficiency by using the
MX60 meters. The main reason is that the input current is not measured...
It is calulated from the battery (output) current and is not as good usually as the output current. The input current reading only semi-close. You are more likely getting somewhere between 93% and 96% efficiency.

Accurate efficiency is hard to find properly though and takes very careful measurent with external instruments and/or shunts and meters etc.

It is true though that the efficiency of the MX60 is not quite as good at 12 volts output as it is with 24V out or best as it is with a 48 Volt battery output.

The reason is that the losses of the MX60 are the same for a 12 Volt battery as it is with, say, a 48 volt battery. There is simply less output watts at 12V and so the losses are a bigger proportion of the output watts. The actual system efficiency (from solar panels to battery) can vary depending on things like wire size and PV wire length.

If your panels are real close to the MX60, the system might gain an extra 1/2 to 1 percent of efficiency by re-wiring the array for 24 Volts instead of 48 Volts. Probably not worth the time.

The MX60 is as far as I know, the highest efficiency MPPT controller of its kind for your application.

Thanks,
boB

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