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Background:

I have a Magnum Energy MSH3012RV 3kw inverter-charger that runs on 12VDC and I'd like to be able to discharge around 250 to 300 AH between recharges (from 120VAC input) without the batteries experiencing much more than 30% depth of discharge. Rates of discharge will be around 20-30A continuous with periodic short (~5 minute) bursts of up to 250-300A. Originally I had planned to purchase four 200AH/12VDC AGM batteries and put them in parallel, but upon further reading I've come to understand that batteries in parallel have a tendency to eat each other from charge oscillating back and forth between them due to slight differences in battery voltage. Now I'm looking at instead putting six 2V batteries in series.

Questions:

1) Is this inverter/battery topology sound? Is there a better way?

2) The particular batteries I am looking at are these. I need sealed/VRLA batteries for this application and my sense of things is that AGM batteries will serve me better than gel, but my understanding of battery technology is limited. Assuming my topology is good to go, are these batteries likely to be a good choice, or is there another type of battery or even a specific manufacturer I should consider (or avoid)?

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250–300A for 5 minutes? Sounds like a welder.

Power draw during that burst = 300*12 = 3600W = 3.6kW.

/glancesDubiouslyAt "3kW inverter"

I think you'll be tripping the safety shut-down on the inverter on a regular basis if you try this. The startup/inrush/surge current that occurs before the arc stabilises is much higher than the sustained draw. Always better to have a generous amount of headroom.

If it's an option, may I suggest bypassing the inverter and running the welder off the battery bank directly using DC (assuming that is an option with the welder you plan on getting)? Alternatively — if you already have the welder — get a (min) 5kW inverter.

If you are concerned — at all — about warranties, then pay the premium, buy your batteries locally instead, and write the extra cost off as 'peace of mind' insurance.

The long-term impacts of imbalanced voltages (for parallel setups) is a mere rounding error compared to the amount of damage your bank will incur from being routinely undercharged (by your solar panels, presumably). Yes, it is real, but it is not large enough to justify moving to an 'unusual' solution.

If I were you I'd just buy the batteries in-person, locally, and visually inspect each battery's markings to make sure they all came from the same batch, then call it done.

Assuming you are charging these off solar, then the average temperature of the batteries will determine whether AGM or gel is a better choice. For most folks in temperate (or warmer) climates, AGM ends up the better choice. In cold-temperate, high-altitude, or sub-arctic climates, gel is likely to be the better choice. If you plan on spending a lot of time welding in the snow, go gel — otherwise go AGM.

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  • Thanks for the answer. It's actually for a large fifth-wheel camper that will often be used in an off-grid / dry-camping situation and for various reasons I don't want to rely on a generator too heavily. The wattage numbers sound about right; this is based on a hypothetical situation in which a hair dryer and microwave are running simultaneously, along with some smaller AC loads like a TV. In addition, there are some direct DC loads (furnace blower motor, lights, etc.) that the batteries will have to power as well. – Josh Apr 20 '20 at 10:52
  • I've run similar loads off the 3kw inverter in my truck without an issue (I have a power meter that measures the real time amperage and/or wattage); in fact, by accident I once pulled 4.3kw and it was humming along just fine until the 300A fuse blew. So I'm pretty confident that the inverter will handle the task. – Josh Apr 20 '20 at 10:54

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