Premature Wiring/AC question

[Mtrx804]

Howdy everyone! I'm picking up an awesome new-to-me skoolie next weekend and had a couple questions on how and why the AC might not be working, as I understand the way it should work.
The information below is the build they have, they mentioned the ac unit only works when the bus is off but does not when running. What are some things I can start with to maybe get this up and going while on the road? Living in Texas... its not only a comfort its a necessity.

2001 Blue Bird TC2000 TC3904-4100

Xantrex Freedom SW 3012 12V Inverter/Charger - Model 815-3012
with Xanbus System Control Panel

x6 Crown CR-235 Deep cycle 6V lead-acid @235Ah ea. Wired into 12v
Total 705 Amp Hours

Surge Guard 35530 Surge Protector - 30 Amp

Progressive Dynamics 30A Automatic Transfer Switch - 120 VAC

Emergency External Electrical Cutoff Switch
InstallBay 500A Battery Isolator for Charging House

Batteries While driving.

Champion 3500-Watt Wireless Start Inverter Generator

Dometic Roof top AC 15000 BTU unit.

Thanks in advance! (If this post needs to be moved to electrical please do so/let me know and I can repost)

[kazetsukai]

Could be a number of things...

• The AC is wired only for shore power, and not into the inverter
• The inverter may not be powerful enough to support the load even if the AC was wired into it
• Something disconnects the AC specifically when engine is running

...among others.

Notably, if the AC draws 15A (120V), that's 1800W, and consequentially, 150A from batteries at 12V. That's quite the draw for a 12V system... Even if it runs, 15k BTU is not going to cut it driving one way or another, if you ask me.

[Mtrx804]

Originally Posted by kazetsukai

Could be a number of things...

• The AC is wired only for shore power, and not into the inverter
• The inverter may not be powerful enough to support the load even if the AC was wired into it
• Something disconnects the AC specifically when engine is running

...among others.

Notably, if the AC draws 15A (120V), that's 1800W, and consequentially, 150A from batteries at 12V. That's quite the draw for a 12V system... Even if it runs, 15k BTU is not going to cut it driving one way or another, if you ask me.

So best short term thing is to connect directly to the generator. A stronger generator wouldn't help much in this specific situation.

Long term would be measure the draw/assess the demand our family will put on the system and upgrade some of the components as needed to handle a bigger load.
Any extra advice or "here's what I'd do" is appreciated. 😀

15k BTU won't cut it? As in its probably too small for the space?

Thanks again!

[kazetsukai]

Originally Posted by Mtrx804

So best short term thing is to connect directly to the generator. A stronger generator wouldn't help much in this specific situation.

If you can reproduce the problem and tell us the exact behaviors, we might be able to further help with the issue. My post was conjecture, I don't know if what I said was actually the problem or not.

Originally Posted by Mtrx804

Long term would be measure the draw/assess the demand our family will put on the system and upgrade some of the components as needed to handle a bigger load.
Any extra advice or "here's what I'd do" is appreciated. 😀

If you want to run AC off an inverter, I would suggest a battery upgrade (lithium) and move from 12V to a 24V or 48V system. Problem is, you already have a very nice, and very expensive inverter, so you're kind of invested in 12V at this point. You might be able to sell it to offset some of those costs, idk.

Originally Posted by Mtrx804

15k BTU won't cut it? As in its probably too small for the space?

At speed you need a _lot_ more cooling than the vehicle would need at rest. School buses with AC units installed have capacities in the 100k BTU range, for a reason.

That being said, our 9k BTU mini split is effective enough to cool the driver, since its blowing directly on the driver seat . If you can get your rooftop unit or otherwise to do the same, it may be an effective driving solution. But it isn't going to cool the whole vehicle, for sure.

15k is probably enough at rest. I want a second 9k BTU unit in the rear, but that's more because there's no good way to circulate air from bus rear to front.

[Mtrx804]

how do i delete this edit?

[Mtrx804]

Originally Posted by kazetsukai

If you can reproduce the problem and tell us the exact behaviors, we might be able to further help with the issue. My post was conjecture, I don't know if what I said was actually the problem or not.

Problem is I don't pick it up until next weekend so I cannot reproduce it right now. Im in the planning phase of what to buy for what if and when that time comes.

Originally Posted by kazetsukai

If you want to run AC off an inverter, I would suggest a battery upgrade (lithium) and move from 12V to a 24V or 48V system. Problem is, you already have a very nice, and very expensive inverter, so you're kind of invested in 12V at this point. You might be able to sell it to offset some of those costs, idk.

From what I've read you can chain 2 of these specific inverters together to share the load. Would this help? I also plan to get a larger generator if needed.

I totally understand all this is hypothetical, but we budgeted quite a bit for the technical upgrades that might be needed.

Originally Posted by kazetsukai

At speed you need a _lot_ more cooling than the vehicle would need at rest. School buses with AC units installed have capacities in the 100k BTU range, for a reason.

That being said, our 9k BTU mini split is effective enough to cool the driver, since its blowing directly on the driver seat . If you can get your rooftop unit or otherwise to do the same, it may be an effective driving solution. But it isn't going to cool the whole vehicle, for sure.

15k is probably enough at rest. I want a second 9k BTU unit in the rear, but that's more because there's no good way to circulate air from bus rear to front.

I plan on at least one more Domestic 15k, possibly the one with heater function as well in the back where the bunks are for the kids. Being 40ft theyd be further away from the first unit. The soft start for each unit should also help with the initial draw.

[kazetsukai]

Originally Posted by Mtrx804

Problem is I don't pick it up until next weekend so I cannot reproduce it right now. Im in the planning phase of what to buy for what if and when that time comes.

Well, we are in the dark sort of like you are. I would suggest however that regardless of the nature of the problem, it likely can be fixed/mitigated.

Originally Posted by Mtrx804

From what I've read you can chain 2 of these specific inverters together to share the load. Would this help? I also plan to get a larger generator if needed.

I totally understand all this is hypothetical, but we budgeted quite a bit for the technical upgrades that might be needed.

The problem with 12V and big loads/big builds like this is that every component of the system is subject to high current draws. If you have the solar to support AC, you will see high amperages from the charge controller, then from the batteries to the inverter... Double up your inverters and you might halve their loads, but you're still putting the same draws on the batteries, wires, etc.

Don't get me wrong, doubling up on inverters isn't a bad idea to reduce the load on the inverter. I'm thinking system load in general- a voltage jump reduces the current draw on every part of the DC electrical system. If you're fine with a camping experience, and this is a getaway vehicle or something, 12V will probably work just fine. If this is a domicile/full time and you want something resembling a residential energy experience, you will want lithium batteries and a 24V or 48V system.

To put it into perspective, lets say you run a microwave oven (1000W)... that's 83A from the batteries at 12V. The same load for me is a mere 20A. That keeps cables and connectors cool, losses low, and makes for a more robust system.


You have time to weigh the cost/benefit, do research, and make the choice that's best for you.

Originally Posted by Mtrx804

I plan on at least one more Domestic 15k, possibly the one with heater function as well in the back where the bunks are for the kids. Being 40ft theyd be further away from the first unit. The soft start for each unit should also help with the initial draw.

Heat pumps are nice...

[Mtrx804]

Originally Posted by kazetsukai

Well, we are in the dark sort of like you are. I would suggest however that regardless of the nature of the problem, it likely can be fixed/mitigated.


The problem with 12V and big loads/big builds like this is that every component of the system is subject to high current draws. If you have the solar to support AC, you will see high amperages from the charge controller, then from the batteries to the inverter... Double up your inverters and you might halve their loads, but you're still putting the same draws on the batteries, wires, etc.

Don't get me wrong, doubling up on inverters isn't a bad idea to reduce the load on the inverter. I'm thinking system load in general- a voltage jump reduces the current draw on every part of the DC electrical system. If you're fine with a camping experience, and this is a getaway vehicle or something, 12V will probably work just fine. If this is a domicile/full time and you want something resembling a residential energy experience, you will want lithium batteries and a 24V or 48V system.

To put it into perspective, lets say you run a microwave oven (1000W)... that's 83A from the batteries at 12V. The same load for me is a mere 20A. That keeps cables and connectors cool, losses low, and makes for a more robust system.


You have time to weigh the cost/benefit, do research, and make the choice that's best for you.


Heat pumps are nice...

Its really just weekend/week long trips. I mean fine with upgrading to a higher 24v system if it's going to make sense for our uses, we really like our AC haha. I really just don't know how to go about doing it.

The fridge is the only appliance there really that runs off the 12v system, but it also can run off propane. Unless outlets, lighting, on demand water heater, and water pump are also affected by this change.

I could use some sort of step down device for those appliance, but yeah a lot more research needs to be done on my end.

[WIbluebird]

Running air conditioning off an onboard battery bank is ludicrously expensive and imo a losing battle unless

(A) you're a full timer with money to burn and

(B) operate in a climate where you can really take advantage of it and AC is needed for more than 2-3 months a year.

The only real option for AC while driving is running it with compressors off the engine. Roof ACs and mini splits don't do jack while you're underway. I will rig my generator so it can run while driving but that isn't the main reason for doing so.

[Mtrx804]

Originally Posted by WIbluebird

Running air conditioning off an onboard battery bank is ludicrously expensive and imo a losing battle unless

(A) you're a full timer with money to burn and

(B) operate in a climate where you can really take advantage of it and AC is needed for more than 2-3 months a year.

The only real option for AC while driving is running it with compressors off the engine. Roof ACs and mini splits don't do jack while you're underway. I will rig my generator so it can run while driving but that isn't the main reason for doing so.

I've also considered this, but having the generator running for 5 to 6 hours at a time I was worried about the strain on it. Being in Dallas and most of our travel plans currently are more southern national parks and beaches along the gulf and maybe FL.

[Mtrx804]

Originally Posted by kazetsukai

If you want to run AC off an inverter, I would suggest a battery upgrade (lithium) and move from 12V to a 24V or 48V system. Problem is, you already have a very nice, and very expensive inverter, so you're kind of invested in 12V at this point. You might be able to sell it to offset some of those costs, idk.

This had me thinking, other than just being generally better overall, would getting 6 - 12v with around the same capacity help in this situation? I considered upgrading batteries in general from the get go.

I do not know enough about swapping over from 12v to 24v or even 48v, I am open to the idea of if its as simple as just swapping parts out. Like you said, I can resell the parts that i take off.

Thanks in advance for the wealth of knowledge you are bestowing upon me!

[kazetsukai]

Originally Posted by Mtrx804

This had me thinking, other than just being generally better overall, would getting 6 - 12v with around the same capacity help in this situation? I considered upgrading batteries in general from the get go.

More, or bigger batteries = less stress on each individual battery, but the stresses on the rest of the system (inverter, solar charge, etc) remain. If batteries were the primary problem, its a decent solution. When I say voltage bump its more of a full system change/upgrade.

Originally Posted by Mtrx804

I do not know enough about swapping over from 12v to 24v or even 48v, I am open to the idea of if its as simple as just swapping parts out. Like you said, I can resell the parts that i take off.

You can use lead acid or lithium to get higher voltages. Your bank right now is wired with 6x6V cells, 2 series, 3 parallel, to get 12V. If you had 8 cells, you could do 48V (8 in series), but you'd have a rather low Ah rating and its possible they could get out of balance.

In short, how you wire your batteries determines your voltage.

To get 24V with 12V AGM batteries, you need a 2S configuration- an equal number of batteries wired parallel, those groups wired in a series of two. With 48V, you'd need 4S, groups wired in a series of four.

You would keep any 12V appliances, you simply add a converter between the battery array and the fuse block feeding them:
https://www.amazon.com/LEDMO-Convert.../dp/B089XZKYCH

Only the inverter (and sometimes a fridge or other 24V/48VDC native appliance) would consume 24V/48V, and the solar charge controller needs to support 24V or 48V battery banks. 24V is pretty commonly supported, but 48V is a little more rare.

Originally Posted by Mtrx804

Thanks in advance for the wealth of knowledge you are bestowing upon me!

I'm not an electrician, take my and anyone else's advice here with a grain of salt. At the end of the day, you have to decide what is best for you. Whats good for me, might not work well for you, and vice versa, after all.

I've built 12V, 24V and 48V solar systems, and upgraded from 12V o 24V, 24V to 48V during my build. I went 48V in the end because I'm a full timer, this bus is my house, it needs to perform like an apartment or similar and is under normal residential use every day of every week. A trip-mobile / weekend getaway vehicle is a different use case. Will 24V/48V work for it, yeah. Is it worth the investment of time, effort, potentially money, that's up to you, your circumstances.

Glad to be of help in any way I can.

[Kentucky Dreaming]

[QUOTE=Dometic Roof top AC 15000 BTU unit.[/QUOTE]

A Dometic rooftop unit is a 120 volt AC unit intended to run on shore power when parked in a campground. It could possibly be operated off a generator while driving, but that is not the intended use. As Kaz has said, it would really suck a battery bank dry if run off an inverter.

I have an Atwood 13,000 BTU roof unit on my truck camper. I think it pulls 13.5 amps at 120 volts full load. You don't want that on batteries.

RV roof air units are best used only on shore power. If you can run your generator while driving, it might work, but I'm skeptical.

Does this bus have a system powered by an engine driven compressor?

That would be the ideal setup.

[Mtrx804]

Turns out it does work on shore or battery. As long as nothing else is on it doesn't trip the max amps being pulled.

I'll probably end up getting a larger generator than the champion 3500 one they gave me and see what I can manage with that....

But all that will have to wait. Drove it home to Dallas from Nother part of Iowa and this was the underside (fresh fluid caked to the bottom of the entire motor)...

It did have the service soon light come on and it went into a limp home kinda mode where it capped out at 40mph but snapped back to normal after a couple minutes... the other concerning thing was the oil pressure (analog) was showing 100psi the whole time. I was told it never registered correctly but who knows.

Stupid picture won't flip no matter what I try.

[musigenesis]

Originally Posted by Mtrx804

Stupid picture won't flip no matter what I try.

Try converting it to a PNG before posting it.