[RE-wrenches] offgrid system question

[Darryl Thayer]

Hi Max
AC surge
I would never put that large an AC on that small a system.  The inverter surge must be considered, and the 104 amps surge from the AC does sound real.  I think you are correct, the "soft Start" is no more than an extra capascitor.  (I approximate the AC at 1kW/ton so the running load about 3.5 kVA (12 to 15 amps)  The surge varies on each start, but expect 10x running current,  expect up to 120-150 amps.  

DC surge
Different batteries handle surge Differently.  Some have a much better surge capability.  The battery voltage will dip upon surge, and some batteries dip much further than others.  As a battery system ages its surge capability drops.  The amount of cycling and strength of surge determines the life and surge capability.     A 120 amp 240 volt AC surge is about a 650 amps! and the battery must hold about 44 volts.  

Sorry for such a long post, and just to end, I think the batteries need to be looked at, use a peak hold meters to get voltage min, and current maximum.  

Darryl 


--- On Mon, 8/18/08, Max Balchowsky <max at seesolar.com> wrote:

> From: Max Balchowsky <max at seesolar.com>
> Subject: Re: [RE-wrenches] offgrid system question
> To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> Date: Monday, August 18, 2008, 3:00 PM
> Dear wrenches, going to look at a system installed by others
> and would like some input from some of you with more off
> grid experience than I have ( 4 systems over the years). the
> system was put on a new home - off grid 20 sharp ND123UJF
> panels 4 modules in series - 5 parrallel strings, 2460 total
> array dc voltage. mppt 60-150 charge controller, 12 sun
> extender sealed AGM batteries, 305 AH at 12 vdc, wired for
> 915 AH at 48 volt DC. The system is tripping off when the AC
> unit trys to start - AC is comfortmaker 3.5 ton 13 seer.
> They say start up inrush is 104 amps (sounds high to me) and
> they put a DST 10 soft start on the unit (I think that's
> just a big capacitor). I haven't been on site yet but it
> sounds to me that they just didn't design enough
> inverter capacity into the system...... What are your
> thoughts?
>    
>   Max Balchowsky
>   SEE Systems
> 
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