road-runner wrote:
After doing some research I've concluded than dual flooded GC2s are a poor choice for use with a large inverter. The deep cycle characteristic, a good trait for general RV use, goes hand-in-hand with higher internal resistance. High internal resistance limits how much voltage the battery can supply under a high load. Two batteries in series serves to increase the internal resistance even more. For calculations I'm assuming a 1,500 watt AC load, which would lead to a 139 amp battery load at 12 volts (90% inverter efficiency assumed). Resistance values are in milliohms.
What's the internal resistance of a GC2 flooded battery? Hard to find an answer! Trojan publishes this value for their AGM batteries, but not for the flooded deep cycle batteries. I found one online source that says the value for a brand new single GC2 is between 30 and 60 mOhms. Another source says a value of 10 mOhms or lower for a good condition 12 volt starting battery. The Trojan spec for a GC2 AGM is 1.9 mOhms, or 3.8 mOhms for 2 in series. Another source reported between 2 and 4 mOhms for a 12 volt AGM battery. Using the small-load/big-load test, I calculated 15.6 mOhms for my series GC2 setup. Pretty far from the 30 to 60 value for a single battery that I found online.
Using my measured value of 15.6 mOhms, the voltage drop from battery internal resistance will be 2.17 volts at 139 amps. My inverter's lowest cutoff setting is 10 volts even, and at 10 volts the current will be up to 167 amps with the voltage drop even more. A death spiral.
So unless I've made a math error or measurement error (always possible), running a 1,500 watt inverter load from dual flooded GC2s just doesn't work, math-wise. While researching I accidentally ran across a few posts that said (paraphrased) "I upgraded to dual GC2s and now I can't run the microwave from the inverter".
I didn't mention MSW/PSW or cable size because I think it's not the issue for me. The inverter is PSW. The cable size is awg 2, and is a minor factor considering the 2 foot cable run from the battery. awg2 is roughly twice the resistance of 2/0, and triple the resistance of 4/0. So the 2 foot awg 2 run is equivalent to a 4 foot 2/0 run, or six foot 4/0 run. Adding a foot for the series connector cable, 5 feet of awg 2 copper has a resistance of 0.78 mOhms, a minor factor compared to the internal resistance of the battery. At 139 amps this contributes 0.11 volts to the total voltage drop.
basically hit the nail on the head. Even with 4/0 cable 2ft that I have 2GC are not well suited to heavy inverter load. If the batteries are near full charge you can run a load for a reasonable period of time, but if they are below about 80%SOC often can't even get the load started. So unless you want or can always start with a fully charged battery bank, a pair of GC isn't a good choice. Been there done that.
4 do great, can start the load (100+A) even near 50% SOC. Now different inverters have different low voltage shut down points so some work better (or worse) than others.
The other downside is that for similar AH you will typically have twice as many paralleled 12V jars. Hence 1/2 the current from each jar. All adds up to 12V much better for high inverter load.
At a RV show I spent time talking to a trojan rep about batteries and loads. First question he asks users is "what do you want to run off the batteries?" If it ends of being high current draws, and that being most important his advice is 12V deep discharge. if the answer is low to moderate loads and wanting high cycle life and deep discharges then GC wins.
