MX60 Max Array Voltage? [RE-wrenches]

Tue Jul 1 11:58:32 PDT 2003

Ray Walters wrote:

> What is the collective experience with running the MX 60 with a high 
> voltage array to a 48 vdc nominal battery bank. The manual shows a Voc 
> max of 140 which looks like I could go with a 72 V nom. array. I'm using 
> 140 watt modules with a Voc of 42.8 - Times three puts me at 128.4 Voc 
> at 25C. I know I need to add some Voc for cold weather, but my 
> experience is that if the sun is shinning on the modules enough to 
> produce power, it will produce heat too. I usually have measured about 
> 30 to 40 degreesF higher cell temp than ambient. I know air speed 
> humidity etc play a part too. Basically, The lowest temp I would 
> anticipate in good sun would be 20F or -7C. Using Outback's chart (1% 
> increase inVoc for each 2.5C below 25C) and assuming a 20F warmer module 
> (cell temp=40F = 5C) then I should add 8% to 128.4Voc which would be a 
> Voc max of  138.7F. Boy that's close.

We have had OutBack MX60s running with three 24 vdc sharp modules in 
mild climates for over 8 months now without any problems.  As far as 
installations in cold climates - we will find out this winter I guess... 
The newer MX60s do record internally the maximum open circuit voltage 
ever seen within the microprocessor itself - so we can conduct autopsies 
if required.  

> What happens Chris, if the array does exceed 140Voc? Smoke and fire, or 
> temporary shut down? 

The 140 vdc limit has additional headroom still in it - we also will 
turn off the controller if the voltage gets too high and will wait until 
the modules warm up and the open circuit voltage drops.  We have tested 
the MX60 well above the 140VDC limit and have found the limit varies 
slightly between units - when you reach the ultimate limit the 
transistors fail - but not much exciting happens other than the 
controller won't work.    

> Did I mention the array will be over 300 ft away? Now I know supposedly 
> Voc is open circuit and that means no current therefore no voltage drop, 
> but wouldn't the meter still see some discernable volt drop in real 
> world conditions? ( meter has to pull at least a few micro amps to 
> measure voltage) Is the controller switching the array off temporarily 
> to check Voc every few seconds? Will the Voc measured at the controller 
> for this split second really be the same as at the array or would it be 
> a volt or 2 less?

We DON'T check the open circuit voltage every 8 seconds - we use a real 
maximum power point tracking method where we actually measure the power 
being produced by the PV array and maximize it by moving up or down with 
the operating voltage.  

The open circuit voltage will be the same at the controller as the PV 
array 300 feet away - no significant voltage drop will occur.  

> Also, in the MX 60 Manual, they show a chart with various array voltages 
> labeled 40, 65, and 95 V Nominal. Is that a misprint and should that be 
> labeled V at Max Power instead?

These values on the performance graph were just the simulated PV array 
voltage that we used for the test conditions for efficiency 
measurements.  The charts for system design/wire sizing are included in 
the manual have "real" PV array voltages - such as 15V, 30V, 60V and 72V 
nominal.

> We're even considering running two 24 v modules in series with a 12 v of 
>  similar current rating, but the racking and look would be awkward.

Mixing PV modules types or models within the same series string is not 
the best option in my experience (if that is what you meant...).  

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas at outbackpower.com  www.outbackpower.com
Tel 360 435 6030  Arlington WA USA

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