Connect 1500W inverter to 2 batteries

[ScottBrownstein]

Originally Posted by boondocking

That would make sense if the two batteries were inter-connected together with 20 feet of small cable (voltage drop).
If the connecting cable is of proper size and 1 foot long the two batteries are going to drain at the same rate no matter which way the inverter is connected.

Well, 150 amps is a lot of current. even a #4 or #6 connection will make a difference, especially over a year or more. Balanced resistance is the way to go and is always recommended.

Where are all those guys who swear that the interconnect between batteries needs to be 4/0?

[larryandamy]

I usually agree with ScottB posts, but this time I have to jump in. If the 2 batteries are wired in parallel with heavy gauge wire between them and good slid connections, then the batteries will provide power based on the charge and voltage level of each battery and has little to almost no effect based on which battery the inverter wires are connected to.
With the two batteries wired in parallel the batteries are constantly equalizing the voltage between them with or without any load or inverter connected to them.
The battery with more charge or current capacity will provide most of the current regardless of which battery the inverter is wired to.
The voltage drop across wire gauges that size is so small (<0.010 VDC) that is to totally negligible impact on how the 2 batteries supply current. Whichever battery can sustain the higher voltage will provide the bulk of the current. But the other battery will constantly be providing current to it trying to equalize the 2 battery voltages.
Thanks

[WolfWhistle]

Battery to inverter should have less than 1% voltage drop. + on one battery and - to the other with correctly sized cables as short as possible.

[Herk7769]

Here is an excellent article on this topic.
I suggest everyone read it.

[mlatter]

Originally Posted by Herk7769

Here is an excellent article on this topic.
I suggest everyone read it.

Yep, but 2 gauge cable is not heavy enough between the batteries, so they are able to show this huge usage difference. Heck, even my booster cables are 1 gauge. On the trailer, I am running 4/0 cables inter-battery and I believe all of the batteries are getting the same usage regardless of where I connect. The bottom line is that you need to run heavy cables, and then you won't have to worry about these line-losses.

[Tom48]

Hi[QUOTE=ArcticLyn;1201274]We just purchased a Rockwood MiniLite 1905 last fall and are just starting our camping for the season. We will be boon docking most of the time at territorial camp grounds so we'd like a little bit of AC power for Keurig, toaster, etc. We have 2 x 12V AGM deep cycle batteries wired in parallel (I think, the dealer installed them).

My question is, as with our solar panels, do we hook up the positive of the inverter to 1 battery and the negative to the other?

We tried a 3000W inverter today and got the low power indicator by hooking it up the way the manual said, negative and positive to the same battery. I think perhaps the 3000W was trying to draw more power than the 2 batteries could supply. I don't want to take a chance wiring it wrong cause if I blow it up, I can't return it for a lower wattage one.

Any help would be greatly appreciated. Thanks![/QUOTE


I think when I do it again, I will run one from one battery and one from the other. I would interconnect the batteries with at least 2 GA if not 2-0 and be sure the run from batteries to inverter is as short as possible. Try to stay under 4 ft of wire run and again at least 2 GA if it is over 1000 watt inverter and 4 GA min to 1000. I have pure sine wave inverter for pure current and a remote on off panel so it can be off when not in use. No load draw on mine is nearly two amps and some as much as 4 amps. If you had continuous use 2 GA would not be enough. But before you can heat up 2 GA wires from use, a two battery system would be dead.

[spock123]

I don't know a lot about cables and in setting up my new bank of batteries I listened to people that said I should go with 2/0 cable. That 2/0 cable is hard to work with in small places. Going to the charger and the inverter in a short run was impossible. I went with 2 gauge cable to hook up the inverter and charger. My cable run is two feet to both inverter and charger. I used the 2/0 to hook the batteries together, I had them cut to the right length, it was easy to hook them together.


Sent from my iPhone using Forest River Forums

[WolfWhistle]

Originally Posted by spock123

I don't know a lot about cables and in setting up my new bank of batteries I listened to people that said I should go with 2/0 cable. That 2/0 cable is hard to work with in small places. Going to the charger and the inverter in a short run was impossible. I went with 2 gauge cable to hook up the inverter and charger. My cable run is two feet to both inverter and charger. I used the 2/0 to hook the batteries together, I had them cut to the right length, it was easy to hook them together.


Sent from my iPhone using Forest River Forums

I suggest not guessing; all I needed to know was in the Magnum or Trimetric manual (solar), and took the time to read and learn. So, that is my final answer and I'm sticking to it.
WW

[dmdomokos]

I used the 2/0 stranded "locomotive" cable which is much easier to work with. Designed for DC current.

[spock123]

Anyway when I go to solar next year I will put the TM 2030 unit on. Anyway all hooked up and ready to go. I stilled use my knife switches but in a different way. I'll trade them out next year


Sent from my iPhone using Forest River Forums

[larryandamy]

I have read all of the posts and the attached article and all of them are good and are basically saying the same thing... except for the one from the retired electrician saying that only the battery the inverter is wired to will provide power of batteries wired in parallel. I think that was a joke.

The article was good, but it was flawed in that the differences it reported of current sourced from each battery is insignificant considering that the batteries wired in parallel with heavy gauge wires are always going to self equalize within minutes.

I always use water analogy to explain this to non-electrical trained people...
Volume/flow of water is electrical current. (gallons/minute vs amps)
Water pressure is voltage (PSI vs volts)
Pipe/opening size is wire resistance. (CV vs ohms) CV is technical flow coefficient term that considers pipe/opening size.

If you have 4 buckets with pipes/openings between them at the bottom of the buckets then the water level in each bucket is going to be exactly the same. If you pull lots of water out of a bucket (draw current load), then the water from the other buckets will flow into that bucket and in a very short time all the buckets will be at the same water level again but a lower level than before (battery charge level). The bigger the size of the pipe/opening (wire size) between the buckets, the faster this leveling occurs.
If you add water to a bucket (charging battery), then the water will flow into the other buckets and they will all be at the same level very quickly.

Now to the point I was trying to make.
Drawing over 100 Amps from your batteries through a large inverter is only a very short term solution and has a tremendous toll on your battery charge level that takes many hours to recover.
If you have high load AC power requirements it is always better to just crank up the generator for 10 or 15 minutes to provide the AC power for 1000W to 2000W loads and save your batteries for when you can't run your generator.

It really doesn't make sense to run 2000W load on your inverter for 10 minutes and then have to run your generator for 3 hours to recover your batteries or solar for +4 hour with full sun.

But this is a situational decision. If you want your Keurig coffee or toast at 6AM before you can run your generator due to noise to your neighbors, then yeah... use your inverter and then charge up your batteries for 2-4 hours of full sun or when you can run your generator. But if running the generator is an option, then it is by far the most efficient way to use your power sources.

I have only been on this forum for a short while and it is clear that there are many schools of thought on wire sizes. Yes bigger is better, but there is an economy of scale and laws of diminishing returns that should be considered.

I just found this link that is pretty good with reasonable wire size recommendations.
https://www.windynation.com/jzv/inf/...ight-wire-size

Running 4/0 wire is very expensive, is is huge and is very hard to route (you need to be superman to bend the wires). It requires a special crimping tool that is basically a large punch and a hammer. And going to 4/0 gets you from a 97% efficient solution with 1/0 to a 99.5% efficient solution that you will never see any real benefit from. Actually the connection termination point resistances dominate the current flow paths. They should be clean with good metal to metal (copper) surface area contact, great crimps and as tight as you can get them. (And they do come loose over time from vibration)

Thanks

[Tom48]

Originally Posted by Livin the Life

Here's the answer to your problem with no math required:



Then enjoy your boon
docking with no worries of power.

Enjoy.

Amen What is wrong with people who need their huge electrical coffee contraption when a good old pot works so well.

[spock123]

Have a coffee percolator that sits on the stove. I like coffee hot and it's hot and black.


Sent from my iPhone using Forest River Forums

[mlatter]

Originally Posted by larryandamy

Running 4/0 wire is very expensive, is is huge and is very hard to route (you need to be superman to bend the wires). It requires a special crimping tool that is basically a large punch and a hammer. And going to 4/0 gets you from a 97% efficient solution with 1/0 to a 99.5% efficient solution that you will never see any real benefit from.

I agree with most of that, but engineers have produced charts of current draw vs wire gauge vs cable length, and in my case, the 4/0 is the ticket for inter-battery and to the inverter (bendable 4/0 welding cable). I run my trailer air conditioning all day long while travelling using the truck alternator and I don't have any issues with any cables heating up, etc. Perhaps thinner cables would work, but I don't think they'd work as well.

[mlatter]

Originally Posted by larryandamy

And going to 4/0 gets you from a 97% efficient solution with 1/0 to a 99.5% efficient solution that you will never see any real benefit from.

I tried, but couldn't let this comment go. Where do these percentages come from? You must realize that quoting efficiency percentages like that requires that we know what the current draw is. If you are running a single 1155 bulb using 4/0 or 1/0 cable, then you will see ~0% efficiency difference. However, if you are running a starter motor drawing 500 amps, that you most certainly will notice the difference.

[Skyliner]

I agree, blanket statements to use 4/0 for all battery connections is like saying that a 1 ton camper is needed to haul a popup camper.

For my setup where my inverter's max draw is 166 amps, I went with 2/0 gauge cable whose max load rating is 190 amps. My voltage drop over 4 foot distance at max load is only 0.1 volts or 0.83 volts. 4/0 wire in my setup would not have been an economical choice and my voltage drop was within my desired range.

[mlatter]

Yes, the bottom line is that you need to use the proper size cable for the job at hand. In my case, the inverter has a 320 amp draw, so the proper cable in that case is 4/0, and a cable length of less than 5'.
Using a chart like this makes it easy to determine what you need: https://www.solarledkits.com/media/r...age%20drop.gif

[ScottBrownstein]

Originally Posted by mlatter

In my case, the inverter has a 320 amp draw, so the proper cable in that case is 4/0, and a cable length of less than 5'...

Wow, yes that is what the manual says. When I calculate it you have to be drawing 3,750 watts of 110 volt with that and total currents would be 331, 330, 328 and 327 amps using 5 feet of 2/0, 3/0 and 4/0 cable respectively.

Don't know what you power a 3,750 watt inverter with to keep it running for a while but a 400 AH bank would only last about 36 minutes and the difference between the cables specified is only 1 minute of running time (86, 58 and 53 watts dissipated in the cables) or about 2-15 watt 12 volt DC incandescent lamps.

Big rig.

[mlatter]

Right. I look at it from a voltage drop perspective.
Using a battery voltage of 12.73v as an example (100% charge):
4/0 cable = 12.57v at the inverter
2/0 cable = 12.48v
1/0 cable = 12.42v

Let's assume we don't want to feed the inverter at any less voltage than 12.0v and we'll have the low voltage cutoff set at 12.0v.

Using 1/0 cable, we consume the battery until the LVC activates in the inverter. This will happen when the battery voltage hits 12.31v.

Using 4/0 cable, we consume the battery until the LVC activates in the inverter. This will happen when the battery voltage hits 12.16v.

It doesn't sound like much of a difference, but according to the charts, the difference between 12.31v (~65% charge) and 12.16v (~55% charge) is the equivalent of ~10% battery charge.

I understand that the capacity is still in the batteries of the 1/0 setup, but for any specific LVC for the inverter, you are rendering a significant portion of the battery capacity unusable using the undersized cables.

Also, you asked about a power source for this setup? I use this inverter setup as a "generator" while travelling, running the trailer air conditioning on the road, etc., with the power source coming from the 320amp alternator in the truck, and the battery banks provide the power for short stops, etc. When we boondock, the loads are obviously very light so we make the most of the 440 amp/hour bank.

Cheers

[ScottBrownstein]

Originally Posted by mlatter

Right. I look at it from a voltage drop perspective....Cheers

Absolutely correct. I looked from an energy standpoint but you are right, the voltage drop (which is due to the resistance and power dissipation in the cables) does cut you off earlier as it grows relative to a fixed bank voltage cutoff. We are both saying the same thing from opposite directions. There is almost just as much energy remaining with any of the cables, but the fixed cutoff dominates and the voltage drop comes off the top!

At 5 feet you can afford as big as you want, at 22 feet the issue is a little different. Running an AC is a big deal as you recognize, running a few assorted televisions, computers, CPAP etc are another thing entirely. I have two inverters, a 2000 watt (residential reefer 900 watts peak) and a 1000 watt (TVs, half of the outlets on the coach) along with a 480 AH stock battery bank. My inverters never approach more than about 50% load and the 1000 generally idles or outputs 250 watts. Run time is generally not a problem since I use my engine alternator to keep everything running on the road and supplement with my generator when boondocking.