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[SOLVED] Intentionally undersized battery array for RV air conditioner?

cdevidal

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My RV has two 13,500 BTU air cons and I would be wiring in a SoftStart to handle the startup surge. The air cons are expected to have a run wattage of about 1300W so a 3000W inverter should cover them.

If I use an inverter such as an MPP PIP LV2424 and a small battery array--say, 100 watt-hours--would I be able to meet the following goals? Looking at the manual, I believe this is the case if I use Solar-first priority. As I understand it, as long as the sun shines, power goes directly into the air cons and when it's not shining, grid. (Or nothing at all.)

Goals:
  • Run RV air conditioner on solar while the sun shines
  • Switch to grid when it does not
  • Allow for boondocking with partial air conditioner usage during the hottest part of the day--so no grid tie
  • Running boondocking after solar hours is not required, to keep down the battery weight and investment
  • Not too high of a price
  • Reliable
I may be better off with a Growatt SPF 3000TL LVM-ES as it has an explicit batteryless mode. I'm just wondering if this also is an option.
 
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That and how many, what wattage do you have or are planning to have for solar? I understand the “battery as stabilizing buffer/capacitor” idea and (assuming 100Ah) that seems very skimpy.
 
Did you mean to say 100ah battery?
No, meant to say 100 watt-hours. But 100Ah would also work. I just mean to say it's a deliberately tiny battery array, so that it only satisfies the needs of the inverter to sense a battery, but pulls the majority of the power from the panels. Seems that according to the manual this is how it would work, I'm just asking for a sanity check.
 
That and how many, what wattage do you have or are planning to have for solar? I understand the “battery as stabilizing buffer/capacitor” idea and (assuming 100Ah) that seems very skimpy.
Skimpy on purpose.

Oh, nevermind. I just noticed this inverter can only accept 2.4kW of panels, which wouldn't be enough for two air cons. Would need two inverters, jacking up the price. The Growatt with the explicit batteryless mode is much better for this application.
 
As I understand it, as long as the sun shines, power goes directly into the air cons and when it's not shining, grid. (Or nothing at all.)
It does, however it does not have a setting for, "suns up but not making enough power for all the load". So say your pulling in 500w from PV, load is 2000w, it pulls the extra 1500w from the battery. That is not going to work with a 4ah battery.

You can move to the 48v 3kw version that will support up to 4.5kw of panels. I also don't think the battery less models will work for your application.
 
My RV has two 13,500 BTU air cons and I would be wiring in a SoftStart to handle the startup surge. The air cons are expected to have a run wattage of about 1300W so a 3000W inverter should cover them.

If I use an inverter such as an MPP PIP LV2424 and a small battery array--say, 100 watt-hours--would I be able to meet the following goals? Looking at the manual, I believe this is the case if I use Solar-first priority. As I understand it, as long as the sun shines, power goes directly into the air cons and when it's not shining, grid. (Or nothing at all.)

Goals:
  • Run RV air conditioner on solar while the sun shines
  • Switch to grid when it does not
  • Allow for boondocking with partial air conditioner usage during the hottest part of the day--so no grid tie
  • Running boondocking after solar hours is not required, to keep down the battery weight and investment
  • Not too high of a price
  • Reliable
I may be better off with a Growatt SPF 3000TL LVM-ES as it has an explicit batteryless mode. I'm just wondering if this also is an option.
Bad idea. A cloud passes over and the AC abruptly shuts off. I don't think it's ideal to be cycled on and off every time a cloud passes over. Maybe with some sort of automatic transfer switch or something. Just get some batteries sounds like you're over complicating things.
 
No, meant to say 100 watt-hours. But 100Ah would also work. I just mean to say it's a deliberately tiny battery array, so that it only satisfies the needs of the inverter to sense a battery, but pulls the majority of the power from the panels. Seems that according to the manual this is how it would work, I'm just asking for a sanity check.

Okay, doesn't sound practical to me.
 
What is the motivation to have the minimum battery? Why try to run off grid when you have utility power available?
 
What is the motivation to have the minimum battery? Why try to run off grid when you have utility power available?
That's my question too - Since you say you want to run at least one AC when boondocking, why not add 200 to 400AH of battery to the system so you can keep things running when a cloud passes overhead, or when you need to run other DC loads in addition to your AC and your solar isn't putting out enough. I just don't understand starting out from scratch aiming to design a system which will do everything without a battery

Don
 
No, meant to say 100 watt-hours. But 100Ah would also work. I just mean to say it's a deliberately tiny battery array, so that it only satisfies the needs of the inverter to sense a battery, but pulls the majority of the power from the panels. Seems that according to the manual this is how it would work, I'm just asking for a sanity check.
100wh is miniscule.

That's only an 8ah.

Even assuming you have no startup surge and the rooftop unit is 1300 at maximum. If you have 2kw of solar and it's good and sunny you might possibly maybe cover it with panels alone.

But if not, and/or a cloud rolls over, the inverter will pull from that tiny battery and you're looking at up to 130 amps.

That's over 16C current from an 8ah battery. In case you're not aware of C rate, 1C is current equal to the AH rating of the battery. So 1C for 8AH is 8 amps.

No go.
 
Yeah,. we can blame the media for this, they give the illusion you can run your house on a half dozen panels so most people think surely 2 panels will power up an entire RV.
 
Yeah,. we can blame the media for this, they give the illusion you can run your house on a half dozen panels so most people think surely 2 panels will power up an entire RV.
Lack of understanding of electricity isn't going to go away though.
 
Yeah,. we can blame the media for this, they give the illusion you can run your house on a half dozen panels so most people think surely 2 panels will power up an entire RV.
I'll put half the blame on the live-in-a-hut people and the other half on the resulting media coverage of that. Avg drive down a suburban neighborhood in the southwest and you'll see roofs covered with panels (10-20Kw). But it doesnt grab headlines when it reads, "spend $40K for solar". Its much more trendy to produce media on makeing it work with 1000w of solar on $300 worth of stuff. Even Will who started this forum started there.
 
I'll put half the blame on the live-in-a-hut people and the other half on the resulting media coverage of that. Avg drive down a suburban neighborhood in the southwest and you'll see roofs covered with panels (10-20Kw). But it doesnt grab headlines when it reads, "spend $40K for solar". Its much more trendy to produce media on makeing it work with 1000w of solar on $300 worth of stuff. Even Will who started this forum started there.
As I recall though he does a great job of explaining the limitations and drawbacks.
 
trendy to produce media on makeing it work with 1000w of solar on $300 worth of stuff. Even Will who started this forum started there.
Not really. He was doing a purpose for a known need and known budget.
That’s different than the tinyhome psychosis entirely.

If anything his nasa-level gigawatt arrays today are more theoretical and theatrical than the small builds for a purpose he started out with. And barring the aliens investigating why the energy is being sucked out of the solar system from Nevada, his theatrical tesla charging and whole-house supports are still a practical exercise with real world functionality.

Will’s videos and links and book will get anyone into realistic success- no 1000W will run the house expectations can be derived from his instruction.
 
Battery needs to be big enough to supply starting surge (reduced by soft-start), and to power the load while MPPT algorithm ramps up.
I have a "small" AGM battery bank for my whole-house backup system. It is sized for about 1 hour capacity at full draw of the load. Would only last 20 minutes at rated output of inverters considering Peukert effect. Just meant to be a buffer, with PV carrying 100% during the day, and small loads all night.

I have a LiFePO4 jump starter from HF that is only a couple pounds and a few cubic inches. That can supply a few hundred amps at nominal 12V (probably cranking voltage, about 8.5V).
Something like that might supply OP's purpose, except for self-heating issues.

You will never have capacitors on inverter input sufficient to hold voltage up for 1/2 cycle of AC.
My measurements at 40% load (10kW electric heater powered by 4x 6kW inverters) showed significant ripple current on battery.
At 100% load, I expect battery current to drop to zero and follow a rectified sine wave.


That's for a system with no PV input, or DC coupled PV. Mine actually has AC coupled PV (was turned off for this test) so I might get away with very undersized battery during steady-state load. However, mine requires some battery capacity so battery inverter can deal with "load dump", when AC load suddenly goes away but AC coupled PV continues to deliver power for a couple seconds. Similar to supplying power when a load turns on.

One of these days I'll try grid-backup using a single 6kW inverter, 6kW of PV (or 12kW strictly off-grid), and 100 Ah 48V battery (I have an old one with < 40% capacity left.)
I'm curious how well that works, with a small battery for buffer and otherwise direct PV to load.
 
Going with a Growatt 5000ES European 220V (93% efficient), wiring it for the mini-split legs (no neutral needed), locked rotor on the mini-split is 16A @ 230V. For the battery bank, using 13s4p lithium-ion SPIM08HP 8Ah cells rated 200A continuous, each series has a 13s 100A BMS. Each cell is expected to see no more than 21A surge current while the motor starts and about 7.8A while running so I'm very comfortable that will be within spec. Like @Hedges I don't need a lot of battery runtime, just enough for the surge and MPPT ramp-up. Run-time would be a few hours when I need mere seconds, so I should have plenty. And if not I can just add more SPIM08HPs.
 
Going with a Growatt 5000ES European 220V (93% efficient), wiring it for the mini-split legs (no neutral needed), locked rotor on the mini-split is 16A @ 230V. For the battery bank, using 13s4p lithium-ion SPIM08HP 8Ah cells rated 200A continuous, each series has a 13s 100A BMS. Each cell is expected to see no more than 21A surge current while the motor starts and about 7.8A while running so I'm very comfortable that will be within spec. Like @Hedges I don't need a lot of battery runtime, just enough for the surge and MPPT ramp-up. Run-time would be a few hours when I need mere seconds, so I should have plenty. And if not I can just add more SPIM08HPs.
So around $2000 or so for the inverter, batteries and soft starts? What will power your DC circuits, fridge, CO detector, device charging, etc.? Do you have a separate battery bank for this?
 
So around $2000 or so for the inverter, batteries and soft starts? What will power your DC circuits, fridge, CO detector, device charging, etc.? Do you have a separate battery bank for this?
Yes, entirely separate system for the 12V stuff.
 
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