Confused about inverter sizing as it relates to panels and batteries

[LifeStudent]

My friend was told that she needed to upgrade her 2500W inverter to a 3500W inverter just because she’s upgrading her batteries, and someone else said it had to do with her solar panels. I have watched a lot of Will’s videos, and surfed this forum and other resources (but I’m not saying I know anything, as I’m learning something new as I go along).

So, here’s a little info to help out with any answers y’all feel like supplying.
On a trailer camper, she has 3 Seraphim 305W 60-cell Mono panels.
She has a Reliable 2500W 12V 120VAC Inverter
She also has an Epever 80A 200V (though I suspect the paperwork is wrong and it’s actually a 150V) Charge Controller.
She started with 4 golf cart batteries and had trouble running her microwave at full power.
She has now purchased 3 Li Time 12V 200Ah LiFePO4 batteries with a 100A BMS to replace the others.

She’d like to be able to run the microwave at full power, and she’d like to be able to use her air conditioner past sundown occasionally. Those are the only power hogs that will be used (sometimes together). I’ve added pics of those two appliances for reference.

So, based on this, does she need the 3500W? And, if that is a YES, does her system have to be converted to a 24V system? I’ve heard 3500W inverters “should” only be used on a 24V system. What other info would you need to answer? Thanks!

 

[Texas-Mark]

So, based on this, does she need the 3500W? And, if that is a YES, does her system have to be converted to a 24V system? I’ve heard 3500W inverters “should” only be used on a 24V system.

Due to the microwave and AC, the 3500W would not be a bad upgrade.

I run my 3500W inverter 24/7 on a 12V system, so yes it will work. Going to 24V might be better for cable sized (lower current draw), but depending on what other devices run off the existing 12V system, converting to 24V may be more trouble than it's worth.

 

[FilterGuy]

So, based on this, does she need the 3500W?

The answer to that is a definitive *Maybe*.

Microwaves are notorious for not playing nice with inverters, particularly 12V inverters. However, it is not always strictly the inverter's fault.
With a 12V system, there is a relatively narrow DC voltage range that the inverter will operate in. At the same time, a large wattage load requires a very large current. With such high currents, if all of the connections between the inverter and the batteries are not perfect, the voltage drop is likely to cause an under-voltage event and shut down the inverter.

a 90% efficient 3500W inverter on a 12V system will draw (3500W/.9)/12V = 324A when the battery is low. A start surge could be twice that. (648A). Just 5mOhms total in all the wires and connections would cause 648A x .005Ohms = 3.24V voltage drop.

However..... Things may still work without a new inverter. The implied statement is that the microwave works, but not at full power.
Since lead-acid has a significantly higher internal resistance, the LiFePO4 batteries may reduce the resistance enough to let the microwave work at full power. Just make sure all of the wires are big, the crimps are tight, and the connections are clean.

Those are the only power hogs that will be used (sometimes together).

Running them together may become problematic... The pic does not give enough info to determine what the AC wattage is so it is difficult to say what the total is. Even if the run-time power is less than 2500W, the start surge may make it difficult to get them both running. Can she run the AC and the Microwave (on low) at the same time now?

My recommendation would be to install the new batteries (With oversized cables) and try it out before deciding if a larger inverter is needed.

If she does go to a 3500W inverter, there may be another problem she runs into. Each battery has a 100A BMS. That means the total continuous current from 3 in parallel is 300A. As pointed out before a 3500W inverter can draw more than 300A so if she maxes it out it might pop the over-current protection in the BMSs.

The good news is that the new batteries can probably handle the start surges. I found this in the specs for the LiTime batteries:

Max. Discharge Current 5 Seconds: 280A

That means the 3 batteries will allow up to 840A of surge.

 

[LifeStudent]

The pic does not give enough info to determine what the AC wattage is so it is difficult to say what the total is. Even if the run-time power is less than 2500W, the start surge may make it difficult to get them both running. Can she run the AC and the Microwave (on low) at the same time now?

Sorry, that pic wasn’t the best. The sticker shows a 20A draw (assume that’s peak?) @ 115V, so 2300W. And, I forgot to mention that she is also installing a soft start control kit that says it is “designed to reduce the starting current of the compressor. it increases the efficiency and the longevity of the unit by reducing the inrush current needed for compressors to start by 65 - 70%”

Not sure if the two have successfully run together. Will find out tomorrow. Also, forgot to mention the old golf cart batteries were 6V (though, for those in the know, that may have been a given).

 

[LifeStudent]

My recommendation would be to install the new batteries (With oversized cables) and try it out before deciding if a larger inverter is needed.

Was thinking the same thing. She’s got 2/0 cable. Should be good for the 2500W, right? Been getting mixed messages as to whether that is big enough for the 3500W though. Wondering if she should start with bigger, in case she upgrades the inverter.

 

[LifeStudent]

Due to the microwave and AC, the 3500W would not be a bad upgrade.

I run my 3500W inverter 24/7 on a 12V system, so yes it will work. Going to 24V might be better for cable sized (lower current draw), but depending on what other devices run off the existing 12V system, converting to 24V may be more trouble than it's worth.

What size cable are you using for the 3500W? I’ve gotten mixed messages about it… anywhere from 2AWG to 4/0.

 

[Texas-Mark]

What size cable are you using for the 3500W?

2/0 I run a 10K BTU AC without issues. But I only run a few lights at the same time, so I'm never maxing it out continuously.

 

[FilterGuy]

The sticker shows a 20A draw (assume that’s peak?) @ 115V, so 2300W.

That will be the max run-time, but the start surge will be greater.
The actual run-time will probably be lower. Let's assume 2000. Add 1.3K for the microwave and we are over the 2500W for the inverter.

However, it would be good to clamp an ammeter on the AC and see it's draw while the compressor is running. It may be considerably less than the name-plate.

Having said that, even if can work, a 2500W inverter is undersized for the demand.

What size cable are you using for the 3500W? I’ve gotten mixed messages about it… anywhere from 2AWG to 4/0.

Let's do some math

Let's assume a 93% efficient inverter. That means it will draw 3500/.93=3763W. The highest current will be when the battery is low, so lets use 12V. That gives a current of 314A. It needs to be fused so the fuse should be 314A x 1.25 = 392A. The next fuse up would be 400A so technically the cable needs to handle 400A.

Since this is a mobile install, Let's use the ABYC Standard for wire size:



According to my math, if you use the normal max current, the min size wire would be 2/0 105C wire. If you use the fuse rating (as I prefer), you would need to go all the way up to 4/0!!

This is why people don't generally recommend going above ~2.5K inverters on 12V systems. It can be done, but it takes big-ass cables and extreme care on all the crimps and connections. (400A will quickly overheat a bad connection)

 

[LifeStudent]

According to my math, if you use the normal max current, the min size wire would be 2/0 105C wire. If you use the fuse rating (as I prefer), you would need to go all the way up to 4/0!!

This is why people don't generally recommend going above ~2.5K inverters on 12V systems. It can be done, but it takes big-ass cables and extreme care on all the crimps and connections. (400A will quickly overheat a bad connection)

Thank you for such a clear and complete explanation! Now I know why, though the charts have said 4/0, people who work in the profession have said 2/0 would suffice. I, however, feel much more comfortable with the 4/0 in this case!

 

[Texas-Mark]

Let's assume a 93% efficient inverter. That means it will draw 3500/.93=3763W. The highest current will be when the battery is low, so lets use 12V. That gives a current of 314A. It needs to be fused so the fuse should be 314A x 1.25 = 392A. The next fuse up would be 400A so technically the cable needs to handle 400A.

IMO the bigger question is what is the highest sustained current that needs to be handled. Who will be drawing a continuous 3763W in an RV?

 

[FilterGuy]

IMO the bigger question is what is the highest sustained current that needs to be handled. Who will be drawing a continuous 3763W in an RV?

True. But....

* I always design for the worst case
* With 12V systems the operating band for the DC voltage is relatively narrow and having larger wires helps prevent problems.

 

[LifeStudent]

IMO the bigger question is what is the highest sustained current that needs to be handled. Who will be drawing a continuous 3763W in an RV?

The only sustained draw will be the air conditioner (though, of course, that fluctuates throughout the day). Others would be periodic, but some (such as the microwave) could go as long as 30 minutes (though that’s probably also in smaller segments of time).

 

[LifeStudent]

* I always design for the worst case
* With 12V systems the operating band for the DC voltage is relatively narrow and having larger wires helps prevent that.

I’m of the same mind.

 

[LifeStudent]

The actual run-time will probably be lower.

Okay, I found an online manual for the AC to get more info on the power usage. (I can’t put an ammeter on it at the moment because of the battery/inverter upgrade going on.) Does this information change anything we’ve previously discussed? Thank you!

 

[FilterGuy]

Okay, I found an online manual for the AC to get more info on the power usage. (I can’t put an ammeter on it at the moment because of the battery/inverter upgrade going on.) Does this information change anything we’ve previously discussed? Thank you!

63A start surge!! That is a perfect example of why it is a good idea to put in a soft-start.

 

[LifeStudent]

That is a perfect example of why it is a good idea to put in a soft-start.

That is definitely happening as well! Says it is “designed to reduce the starting current of the compressor. it increases the efficiency and the longevity of the unit by reducing the inrush current needed for compressors to start by 65 - 70%”

 

[Tomthumb62]

IMO the bigger question is what is the highest sustained current that needs to be handled. Who will be drawing a continuous 3763W in an RV?

If you don’t size a system appropriately (balanced and appropriate fuses, cables, batteries, inverters, etc) then how can you ever relax? You’ll be constantly wondering if you’re using too much power at once and if it’s going to blow a fuse, trips breaker or worse burn a cable hot enough to start a fire or trip the BMS on a lithium battery.

Size and build the system right, such as remove or upgrade inefficient or oversized appliances if needed or build a bigger system, and you can enjoy your home without worrying about fires, blown fuses or shut down inverters.

And that’s considering you’re the only one operating the system. Will you have to train each guest or family member to remember to use the system correctly? What if they forget. A recipe for stressful times since most people are accustomed to the seemingly endless power of the grid. But at least if you have a properly sized system, it’s one less thing to worry about.