Caveat of battery bank on Grid tied SW inverter [RE-wrenches]

[Dan Duffield]

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What's the smallest 24 or 48 V battery bank out there on a grid-tie SW 
inverter?
         About five years ago our company was contracted for a PV 
installation (only) on a National Historic Site in New Mexico, (so the 
system design was out of our hands), but the prime contractor selected a 
SW5548 for this grid-tie application and only 4 - group 27 batteries were 
spec'd in approx (96 A-Hrs @ 48vdc). The array was 1530 watts, we should be 
able to sell 10, 11 or 12 amps of 120VAC power.
  Two interesting notes
         What happens if the grid goes down when all of or something close 
to 5500 watts are being pulled from this size inverter during a power 
outage, - loads will be of be transferred to the battery bank, let see 
5000/48 = 104 + amps, that not a safe C/3 rate for this many 
batteries:  that's more like a C/.92 rate - battery meltdown -  however, 
the voltage may drop soon enough to turn inverter off when pulling this 
high a wattage from our battery bank, but what if were pulling some where 
in between, say at C/1.5 rate or around 2300 watts - the voltage will stay 
up long enough to let the battery get real warm,  their exists no way to 
easily "be confident" that this is not a safety issue, so I put an 
appropriately smaller  Heinemann breaker  to prevent this from occurring.
         As a result of this, we installed the mentioned circuit breaker 
sized to allow a C/3 rate but no more - if utility power failed;  to let 
2300 watts pass out of the protected battery. So a 35 amp Heinemann was 
installed. Everything OK ? Nope.  When the grid was "up", and the site was 
selling/buying, this breaker was experiencing nuisance trips.
         We checked the float voltage, We were not selling battery 
power    We checked Maximum Sell Amps AC which was well below the DC 
side  circuit breaker rating, but it did'nt seem to be working - were we 
selling battery power - it was tripping - why?.
         It turns out- that the SW5548 was trying to help or smooth out the 
voltage drop spikes using the battery bus voltage.   Way out at the end of 
the rural community where this system was located  -the grid voltage 
regulation was a little "loose", from other customer's in the area and 
their inductive loads like well pumps and compressors. We swapped out the 
35 amp Heinemann for a 40 amp circuit breaker, still had these nuisance trips.
         We had turn lower  the  Maximum Sell Amps AC setting,  to reduce 
pulling high instantaneous amp values from the battery bank to do this 
local voltage regulation.  "Maximum Sell Amps AC"   on the SW series - this 
control is relatively slow. Where we shoulda, coulda, woulda sold at 10 or 
11 amps, we had to bring max sell amp value down to 8 amps - to prevent 
this nusiance tripping.  It was our only solution,  On future applications 
this could affect the sizing of  a PV arrays- why have the extra PV wattage 
if you cant sell it? i.e., (Maximum Sell Amps AC) and time of day usage. 
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