Two 6 volt batteries or two 12 volt batteries ??

otrfun wrote:

ktmrfs wrote:

otrfun wrote:

ktmrfs wrote:
. . . if the pair of GC drops below about 75-80% SOC, you can hit the inverter shutoff limit pretty easy with loads of 800-1000 W. Some may depend on the specific inverter and what it's lower limit is.

Now if the batteriesa are above 80% or so, you in good shape. but that means keeping them near full charge when you want the inverter. So.... run the furnace at night then get up and turn on the toaster.... likely be a nope! low temps= higher internal resistance, lower capacity, coupled with partial discharge.

IMHO if your happy with your current 12V setup, don't mess with it. Or, get GC for most of the stuff and run the inverter off 1 or 2 12V AGM's or similar.

We had a situation where we had to run 35 ft. of 2/0 cable from a 1000 PSW watt inverter to two GC2 batteries. On a number of occasions we placed a 65 amp load on the batteries with the inverter (~700 watt a/c load) and regularly ran the two GC2 batteries down to 50% SOC with no inverter alarms or shutdowns. Output from the inverter was always stable at 117 vac. DC voltage drop (due to 35 ft. 2/0 cable) with the 65 amp load was approx. .35v (3%). This particular inverter was supposed to alarm at approx. 10.6v and shutdown at approx. 10.0v. It never alarmed or shutdown.

your inverter has a much lower threshold than mine does. mine alarms at 11.5V ad shuts down not much below that. From what BFL experienced I suspect his was similar to mine. You've got about another 1V margin, which would make a big difference. So, the GC work well with your combo. Another story with mine. So, yes sucess (or failure) is highly dependent on both the batteries and inverter.

Before this thread, I never gave alarm and shutdown voltage a lot of thought. Guess we lucked out with an inverter that supposedly shutsdown at 10.0 volts. Our previous inverter shutdown at 10.5 volts. Didn't realize some inverters shutdown at 11.0 volts or higher. Depending on system capability (batteries, cable length/size, etc.), a 1 volt difference like this could potentially be a huge deal in some dry camping situations. Going forward, we're definitely going to be looking much closer at alarm/shutdown voltage specs.

yes, shutdown voltage definitely can have a big effect, the difference between yours and mine highlights that.

at 10.6V open circuit (no draw) a flooded lead acid battery is at or near 0%SOC. Even with a C/10 discharge 10.6 is a very low state of charge.

I suspect different inverter designers have different philosophies on when to alarm and shut down and what to expect the typical user has for voltage drop on any line between the battery and inverter.

The best system would be to use a "4 wire" sense, hot and neutral carrying current and two wires back to the battery to measure the battery terminal voltage going to the shutdown circuit. that way you remove line drop from the equation. Don't know of any inverters that do that.

Now if it shuts down at 10.0 V under a heavy load, (C/3 or more) a GC the battery is likely above 10.6V, and has some margin. If it is this low with parallel 12V, IMHO not good, way to close to deep discharge. And if it doesn't shut down till 10V under a light load you risk running the battery way down near 0% SOC, not good for even good GC batteries.

But the 11.5ish shutdown on mine is going towards the other extreme of shutting down well before it needs to with GC, and somewhat above where it could discharge parallel 12V down to.

Would be nice if inverters had a selector, GC or 12V and then adjusted the shutdown voltage to account for battery internal resistance. It could even by a dynamic voltage, look at the current draw and determine internal drop, shut down lower at high draw, higher voltage at low draw.