I'm trying to figure out if my thinking is right on batteries and circuits. College physics was a long time ago.



Am I missing something here?

That sounds about right to me.

You can link any number of unlimited batteries in the same condition in parallel, theoretically.
If you start connecting in series, especially any more than two, you'll potentially blow up or melt something....

Parallel connections increase current potential.



Series connections increase voltage potential.

Can your wiring and fuses handle more than 400w? 400w @ 12v is 33.3 amps. Double for 800w. Going to need some heavy wire.

 
The manufacturer does state that you can chain up to 4 of the units together in parallel. I have heavy, MC4 solar panel grade cables for connecting the batteries, so I think I'm ok on that part.



 
 

For large loads you use higher voltage inverters to keep currents manageable.
You then put multiple batteries in series to generate the higher input voltage.


Unless you want to purchase higher voltage batteries.
We used 48 V equipment for 0 dropout on large UPS systems.
We could loose the AC feed and not have a single cycle dropped or any disturbance on the 120 V AC lines.

The AC and DC sides are not the same.

You have to go through watts and conversion efficiency to get from one to the other.

1 amp and 12 volts is 12 watts.
At 120 V 1 amp is 120 watts.

HOW you get from DC to AC matters.

The appliance will only pull what is needed, so a 6.6 W appliance running off a 400W capable battery system will only pull 6.6W

Transformers only work on AC power.
You use an inverter to pull DC from the battery and create AC (sometimes of not very 'clean' quality).

And loads for 120 V AC will not do much of anything on 12 V DC.

Something else you may want to consider i when connecting batteries together is the health of the batteries.  If they are at different stages of life (think different capacities) and connected in parallel, when your device pulls the full 800W, the weaker of the two will be unable to discharge at it's rated rate causing the other one to discharge at a higher than rated rate (an extreme example of 600W from one and 200W from the other).  For your case, it may be better to put them in series to distribute the power by doubling the voltage instead of the current.  This also forces the batteries to discharge at the same current.

But then the weaker battery becomes a resistor in the circuit, let alone the fact you have increased voltage instead of amperage.

I had considered this. My two batteries (the original GZ Yeti 400 and a second, identical battery) are within a month of each other in age and have the same number of charge/discharge cycles. They've been hooked together by Powerpoles ever since I bought the second battery.
 


ETA: I don't want to double the voltage. The GZ Yeti 400 has two 120 Volt outlet plugs. So, it's taking the 12 VDC and inverting it to 120 VAC. If I plug in a second, identical battery, shouldn't I be able to run a 120 VAC device for twice as long with the two batteries, in parallel, as compared to the time I could run it with a single battery.






 

?!?  How do you plan to run 12V items off your 24V series battery pack without setting something on fire?

The best answer is neither parallel nor series.  The best, simplest answer to the OP's situation is get a single (larger) battery that can support the loads he needs to operate.  Unless you're talking about some substantial battery capacity that simply cannot be obtained in a single battery.

You'll also need some substantial wiring to support the kind of amperage you are talking about.  66amps at 12VDC, 8ga would be an absolute minimum, 6 or 4ga would be better particularly if you have any length to deal with.  Larger gauge isn't out of the question either.

Yup it will be a resistor in both situations.  So it kinda depends on what is more of a power loss, driving current through a resistor (series) or charging the weaker cell while also powering the device (parallel).  And it doesn't matter if you double voltage or current to get the same power output.  Doubling voltage allows you to keep your current draw low so you don't risk damaging components (which isn't necessarily an issue in this case I think).

I may be missing something but where does he say it needs to be 12V?
 I just saw that he required a power rating and suggested another method.  As I understood, the final appliance being run is 120AC, requiring an inverter anyways.

Edit:  Ha I'm an idiot.      I agree with gamma, best answer would be to get a battery/inverter designed for 800W+ load.

I get it now....  GoalZero Yeti is a commercial battery backup.  
What is the power rating on the inverter that  the yeti has?  I would imagine that would be the limiting power factor and not the battery.

As I read my original post, I see that I've neglected to add some details. I have a GoalZero Yeti 400, which has a 12 VDC, 400Wh/33Ah battery in it. It has two 110 Volt AC plugs in it (each rated at 2.6 Amps, for a continuous wattage of around 300 Watts, with a surge wattage of 600 watts), along with two USB plugs (these are rated at 5 Volts [email protected] Amps each, which is enough to charge an iPad). Goalzero calls it a Solar Generator, which is wrong. It doesn't generate anything. It has a 12 volt DC battery connected inside a case with an AC inverter and some AC plugs, as described above.

The details, including specs, are here:



Up to this point, I've used the Yeti 400 as an iPad/phone charger and to power a string of LED lights at week long Boy Scout camp. It also has Anderson Powerpole plugs on either side of the case, which can be chained (in parallel) to another 3 identical batteries (4 batteries in total, including the Yeti itself).

From the specs, it looks like it can power an AC device at 300 watts continuously, at least until it discharges. If I connected a second battery in parallel, then should I be able to deliver the same 300 watts for twice as long (in theory)? In other words, adding more parallel batteries wouldn't let me deliver more power...it would simply let me deliver the same rated power for a longer period of time. Is that right? If my wheat grinder requires 400 watts, then I'd be out of luck.

As Gamma762 pointed out, I may need a bigger battery. I've looked at this one:



It's their biggest battery pack. It's rated at 1200 watts continuous out the AC plugs. So for a high wattage, AC appliance, I'd probably need to go with something like this. And similar to my question above, I could add a second, identical 1200Wh/100Ah battery in parallel and double (in theory) how long I could deliver that 1200 watts.
 







Before someone points it out, I realize that Goalzero products are expensive for what they actually have in them. The Yeti 400 battery only costs about $80 and the Yeti 1250 battery is about $300. I know that I could get the same capability with the same battery and a charge controller and an inverter.

But, I like having it all in one tidy, portable box. And the Goalzero factory and showroom is only about 5 miles from my house, and they sell their refurb units for about 40-50% of the list price. So it's worth it to me.
 
 
 
 
 
 

In that case the limiting factor may be the battery, or it may be the inverter that converts the battery DC to 120V AC.  You need both an inverter that can supply that much power as well as batteries that can supply that much amperage to the inverter.

You're probably moving into the realm of RV power systems or something similar rather than smaller portable integrated systems.