but yeah ill keep at whatever 2 of those is. around 850ah.
Rookie mistake. Understandable. That's 850 Ah, but at 6 volts.
Just to be clear so there is no misunderstanding, when you connect two identical batteries together, depending on how you connect them, you either double the voltage and the Ah stays the same, or double the amp hours and the voltage stays the same.
For example, two 6-volt 400 amp hour batteries connected together either becomes one large 6-volt battery at 800 Ah, or one large 12-volt battery at 400 Ah.
When you connect two batteries together using positive-to-positive and negative-to-negative connections, that's a Parallel connection, and the volts stay the same and the Ah of each battery are added together. So, the volts stay the same and you double the amps.
A Series connection is when you connect the negative terminal of the first battery to the positive terminal of the second battery (think multiple batteries in a flashlight). That will keep the Ah of each battery the same, but will double the volts.
I have four 6-volt batteries (305 Ah each) connected in series/parallel. Two paris of batteries are connected in Series to make each pair into one large 12-volt battery of 305 Ah, and then I connect those two in Parallel to keep the 12-volts and double the Ah to 610 Ah.
When they get down to 50% ill idle to let them charge up again, no big deal, side question how long approx. would 850ah bank take to be charged from your alternator at 50%?
Breathtakingly longer than you think. OK, you've got 400 amp hours to put back into a battery. You fire up the engine to idle and at first you might get 30-50 amps coming from the alternator (assuming a 150 amp alternator). But that won't last more then a few minutes and the charging currently will quickly drop tp 20, then 10, for several hours. As the batteries get to 90% charged the current will drop to 4 or 5, for several hours. Realistically, putting 400 amp hours back into a battery from the alternator will take 18-24 hours, at a minimum.
As Roadtime noted and I think you've discovered, it's not practical to run any kind of resistive heat appliances off an inverter. Those include air conditioners, electric heaters, clothes dryers, dishwashers, refrigerators.
There's also something called the Peukert Effect, which is kind of like driving speed and fuel economy. The faster you drive the worse your fuel economy. The same with amp draws. The faster you draw amps from a battery, the fewer amps you have available in the first place.
Battery Ah ratings are (almost always) at the 20 hour rate. Meaning, the number of amps available if you draw the battery at a rate that will deplete the battery in 20 hours. Draw at a rate faster than that and you have few amps, draw at a rate slower than that and you have more amps available.
You might want to think that a 100 Ah battery would last 100 hours at a 1 amp draw and would last 1 hour at a 100 amp draw, but no. That would be too easy. At a 1 amp draw it would last 140 hours, and at a 100 amp draw it would only last 30 minutes. (it will last one hour at a 60 amp draw)
The simplest example is a 100 Ah battery. It's rated at 100 Ah because it will last for 20 hours at a 5 amp draw. A 400 Ah battery will last for 20 hours at a 20 amp draw.
But, if you take that same 400 Ah battery and pull 50 amps from it, the battery becomes a 329.98 Ah battery, the 50 amp draw becomes a Peukert Corrected Amps of 60.61, and the run time will be 6.6 hours (actually, 3.3 hours at 50% discharge).
The same 400 Ah battery with a 5 amp draw becomes a 535.17 Ah battery, and the run time will be 107.03 hours.
Peukert’s law expresses mathematically that as the rate of discharge increases, the available capacity of that battery decreases.To see if I can make your eyes roll back in your head, I post the actual formula thusly...
There are online Peukert calculators so you don't have to do them manually, and I've also attached a spreadsheet for Peukert calculations which makes it really easy to see different scenarios at a glance. The defaults (in the purple boxes) are for a Trojan T-105 battery. The Peukert Exponent for that battery is 1.21. For my J305E-AC battery the Peukert Exponent is 1.24. For Trojan L16H-AH batteries the Peukert Exponent it is 1.30.
You don't need to become a Peukert Expert (unless you're on a sailboat where your life depends on your batteries, then you kinda do), but it's something you need to be aware of. The spreadsheet helps with that.
Now back to the 900W draw, just for clarity and giiggles.
Remembering that amps x volts = watts, and therefore watts / volts = amps, 900 Watts is 900 / 12v = 75 amps. Plus another 10% for the inverter loss, that's 75 + 7.5 = 82.5 amps from the batteries.
With a 440 Ah battery bank, using the handy dandy attached spreadsheet, you can see that an 82.5 amp draw turns the battery bank into 320.29 Ah, and you can run that draw for a whopping 3.88 hours (or, 1.94 hours at 50% Dod).
You might think that 3.88 hours becomes 7.76 hours (and 3.88 at 50% DoD) if you double the bank. But Peukert says "NO!" It's actually better than that because of the bigger battery.
With an 880 Ah bank, the same draw turns the bank into a 756.77 Ah bank, and you can run that load for 9.17 hours (4.585 hours at 50% DoD).
It should be clear that you want as much battery lead as you're willing to haul and pay for, but at the same time be very, very frugal with your amp draws. That's why I said earlier to be as efficient as possible with things like a 12-volt refrigerator instead of a 120-volt fridge that runs through an inverter. I started out with a dorm/office fridge (Microfridge) that was very efficient for a 120 volt appliance. It only draws 1.3 amps AC (about 14 amps 12-volt DC), and it was brutal on the batteries. Brutal because it runs all the time. In a van, unlike in a house where a fridge might run about 8 hours a day, it'll run more like 18 hours a day. In the winter it's warm in the van so it runs all the time, and in the summer it's really hot in the van so it runs nearly nonstop.
If I had a new van and was setting up a battery bank, instead of raw charging the bank from the alternator, which is hard on the batteries, I'd absolutely get a
Sterling Power battery charger. Basically, instead of connecting the alternator to your vehicle's charging system, you connect the alternator to the Sterling Power charger first, and then reroute that to the vehicle's charging system with one cable, and then you use another connection from the charger to the house bank. The vehicle's charging system run unimpeded, but you get smart multi-stage charging of the house batteries. And if you want you can even use different battery chemistries for the house and starting batteries (wet for one, AGM for the other). I would also get the remote monitor to go along with it.
If you use that kind of a battery charger, you'll be consistently charging the batteries fully and properly, instead of raw charging them from the alternator. It's the difference between the batteries lasting 2-3 years versus 6-7 years. Using 12-volt appliances instead of higher amp draw appliances through an inverter will also dramatically increase the life of the house bank.
