I'm working on a film set right now and we did a set up where we tried to power a couple lights off a deep cycle marine battery. I'm more accustomed to using AC generators and admittedly don't know much about DC power systems/batteries, and I've run into some trouble that i'm trying to figure out. Initially we tried to power two tungsten 1k lights off a 3000w inverter wired directly into the picture car battery with the engine running. It didn't work at all, the inverter started beeping and the lights started to flicker. My theory is the voltage on the car battery dropped substantially under 12v causing the electronics in the inverter to suffer and the lights to flicker. But I really thought with the engine running it would work - too much of a load for a car battery even with the engine running? So I called a battery store and talked to their expert, he assured us if we got two deep cycle marine batteries and wired them together in parallel to boost the amperage they should power the two lights for quite a while. So we picked up two Everstart 122 amp-hour batteries, paralleled them, and hooked them up to the inverter/lights. This worked for about 15 minutes, then the lights went out. We charged the batteries and tried again, this time I think we got 5 minutes before the beeping and flickering started. So they didn't have the reserves to go for very long, and they didn't seem to hold their charge as well the second time around despite what the battery expert told us. So I'm just curious why none of this worked. Is 2000w just too much load for a couple of batteries even if they are big deep cycle marines? Were we shafted by a battery salesman into buying a couple non-returnable batteries? Our next plan is to step everything down a bit - this time it will be 2x 400w LEDs with a 1000w inverter, hopefully this time hooked up directly to the car battery if it works this time... any thoughts on how much success we might have with this?