First-Time System Design Feedback

[heckler40]

I am planning a system upgrade in my 2022 Valor 37V13, which currently has a basic solar package. Basic solar is 2 320W panels, converter and 1 100ah battery (all Renogy). No inverter. Generator is a Onan 5500 RV Q.

System goals and notes:
1) Be able to run 120V loads without starting gen (TV, Microwave, A/C, etc).
2) Be able to run one A/C unit for 8 hours (or 2 for 4 hours) without starting gen (10-11A on high , 6A or so on low each, can run 2 no issues on 30A shore).
3) Or when cold, run 1 strip heater (in A/C) or fireplace and house LPG heater.
4) Put some power back into the system during the day.
5) Use primarily during the weekends. Will have no draw during week other than fridge and nuisance lights/items.
6) Gen will be used as backup as to recharge batteries when needed. Would like to cut hours down.
7) Two 24V 304ah DIY batteries in parallel.
8) 24V-12V 70A converter to service 12V loads.
9) Don't need alternator charging.

Primary outstanding items:
1) Will the 24V to 12V converter handle the high amp loads such as gen start and jacks? Specs indicate yes but couldn't find someone doing it.
2) Is there a better/cleaner way to fuse the 12V loads?

Been researching for about 1 year and have learned a lot from many folks here. Thank you!

Feedback and critique are appreciated.

UPDATED: Uploaded latest diagram. Since there isn't a ton of solid info I'm going to try the 2 Orions in parallel to see if it will handle the jacks and gen start. Back up plan is I have an extra sxs battery with 300+ CCA.

 

[time2roll]

Google:

How many amps does it take to start a Onan 5500?
I read the manual on my Onan 5500, and it specifically recommends a battery with no less than 350 Cranking Amps.


Here is a you tube with 2x BB with 400 amp surge capacity starting a Onan 5500. skip to about 1:45 to start.

70 amps from a DC-DC converter.... probably not.

 

[heckler40]

Google:

How many amps does it take to start a Onan 5500?
I read the manual on my Onan 5500, and it specifically recommends a battery with no less than 350 Cranking Amps.


Here is a you tube with 2x BB with 400 amp surge capacity starting a Onan 5500. skip to about 1:45 to start.

70 amps from a DC-DC converter.... probably not.

I did not find that in my search. Also, found a thread here: https://diysolarforum.com/threads/amperetime-chins-12v-100ah-enough-to-start-rv-generator.26895/#:~:text=I read the manual on,less than 350 Cranking Amps.
I knew it wouldn't take 400A or so to start the gen as I am doing it with no issues on the existing battery. Key takeaway is with a LA or AGM battery, voltage will drop and amps need to come up. That is not the case with lifepo4.

Another item I read, but there was less discussion, is the jack system taking 90A. More googling needed.

A potential solution to my system would be to get a dedicated battery or parallel two of the Orions.

 

[Vigo]

Technically the cells in the existing 100ah lithium would have NO problem starting a generator, it's more a question of will the BMS consistently allow it to happen, or will it get fussy if you go over its continuous amp rating for more than 1 second, etc. As far as i can tell no pre-built lithium battery manufacturer (ahem... assembler) really describes every nuance of what their bms does, but they do usually say what the continuous amp rating is and often what the 'one second' and '5 second' amp numbers are, etc. If your generator is under your continuous amp rating it should be fine.. should.

But considering that generator took a whopping 100amps to crank, i would just stick a $29 Walmart group U1 riding mower battery on that and let your stock converter or a small dedicated charger maintain it. If your generator decides it wants 200 amps, fine.. a $29 u1 is rated 230cca! Which is at 0 degrees F, mind you! No need to overthink a 100a cranking requirement, imo. But that's with the benefit of hindsight of KNOWING that it's 100a.. so we should be thankful for the testing.

As far as running one rooftop ac for 8 hours or 2 for 4... you're talking about a 48v 5kw+ inverter to do that comfortably, or 2x 24v inverters operating in parallel (and starting the AC units one at a time). Rooftop ACs are bastards to build solar/inverters around because they basically dictate you end up with big-house size components just to power a tiny house. But figure they each use about 1500w on high, if you want to consistently run both AND put energy into batteries at same time you'd need well over 4kw of solar to have any hope on that one. If you limit your expectations to running ONE ac and charging batteries, 2.5-3kw would work. Next question i suppose would be, do you have the square footage available to mount 2.5-4kw of solar?

 

[heckler40]

Technically the cells in the existing 100ah lithium would have NO problem starting a generator, it's more a question of will the BMS consistently allow it to happen, or will it get fussy if you go over its continuous amp rating for more than 1 second, etc. As far as i can tell no pre-built lithium battery manufacturer (ahem... assembler) really describes every nuance of what their bms does, but they do usually say what the continuous amp rating is and often what the 'one second' and '5 second' amp numbers are, etc. If your generator is under your continuous amp rating it should be fine.. should.

But considering that generator took a whopping 100amps to crank, i would just stick a $29 Walmart group U1 riding mower battery on that and let your stock converter or a small dedicated charger maintain it. If your generator decides it wants 200 amps, fine.. a $29 u1 is rated 230cca! Which is at 0 degrees F, mind you! No need to overthink a 100a cranking requirement, imo. But that's with the benefit of hindsight of KNOWING that it's 100a.. so we should be thankful for the testing.

As far as running one rooftop ac for 8 hours or 2 for 4... you're talking about a 48v 5kw+ inverter to do that comfortably, or 2x 24v inverters operating in parallel (and starting the AC units one at a time). Rooftop ACs are bastards to build solar/inverters around because they basically dictate you end up with big-house size components just to power a tiny house. But figure they each use about 1500w on high, if you want to consistently run both AND put energy into batteries at same time you'd need well over 4kw of solar to have any hope on that one. If you limit your expectations to running ONE ac and charging batteries, 2.5-3kw would work. Next question i suppose would be, do you have the square footage available to mount 2.5-4kw of solar?ds

Never had any issues starting with the existing lifepo4 battery. But, certainly an interesting topic. My hope with the converter was just to simplify the system and connect it to the existing 12V bus. Note that I'm trying to solve for the jack system too, which is on a 80A resettable breaker. Wouldn't start the gen and use the jacks at the same time

Regarding A/C, I believe you are a little off on the estimates. As stated above an A/C unit will draw appox 11A or 1300W on high. Obviously won't stay like that all the time. The inverter is 2400VA and the battery bank is 24V with 608ah. It will run that load for 8 hours no issues without any solar going back into the system. The solar, in a perfect setting, will put ~50A back into the batteries during the day. Hopefully get 30A. Would take approximately 15-20 hours to fully charge the bank from 0%.

But my use case isn't to run all 3 A/Cs on high for 10 hours.

Note that I am under-paneled. I am only putting 3 units up and realistically only have room for 4. The manufacturer did not plan the roof very well so I'm locked there.

Edit: my concern with the 2 A/C units is more around the VA as well as the startup current to get them moving. Even with a soft start, which I don't have, startup current is still high. If I can't run 2, I'd be ok with that.

 

[HRTKD]

8) 24V-12V 70A converter to service 12V loads.

This is one reason why it would be easier to stick with a 12 volt system. Zero issues when all the voltages are the same.

With enough LiFePO4 batteries in parallel, starting the generator isn't an issue. I would want at least 200 amps of throughput available from the combined BMS devices. I chose to keep a separate 12 volt lead acid battery to start my Onan 5500 generator. In the future I might eliminate the lead acid battery, but I will need to run new cables from the LiFePO4 to the generator.

Your need to run multiple air conditioners doesn't necessarily rule out a 12 volt system but it does put you into the 24 volt area. My rooftop air conditioner pulls more amps than you specified. The rating is 19 amps on mine, but it's usually a little lower than that, but not as low as you posted.

You put a third Class T fuse in the positive circuit between the batteries and the switch. I don't think that third fuse is necessary.

There should be a circuit breaker between the solar charge controller and the Lynx. The fuse on the Lynx is at the wrong end to protect the wire during charging. It's also handy to have the circuit breaker in that position to be able to easily/quickly disconnect the battery from the solar charge controller without bringing down the rest of the system.

 

[heckler40]

I chose to keep a separate 12 volt lead acid battery to start my Onan 5500 generator. In the future I might eliminate the lead acid battery, but I will need to run new cables from the LiFePO4 to the generator

If I stay 12V I can run the gen starter cables to the 12V busbar. Certainly is way easier than the road I'm going down now. I'm leaning toward the lead acid instead of paralleling the Orions. Still need 1 Orion for 12V loads.

You put a third Class T fuse in the positive circuit between the batteries and the switch. I don't think that third fuse is necessary.

Great point. My thought was that the T class fuses allow significant amps over the stated capacity and wanted to protect against that. The 304ah at .5C discharge is over the face rating of the battery fuse. The fuse allows reasonable over-current during normal and extended time.

There should be a circuit breaker between the solar charge controller and the Lynx. The fuse on the Lynx is at the wrong end to protect the wire during charging. It's also handy to have the circuit breaker in that position to be able to easily/quickly disconnect the battery from the solar charge controller without bringing down the rest of the system.

Did you see the breaker (rated at 62A) between the panels and the mppt? Note that the panels are in series and the panel series fuse rating is 25A with the panels pushing 13-15A at ~105V. My thought is that the panel fuse would handle any issues and then the breaker. In this instance, would it be necessary or recommended to put another fuse in?

Thanks for the input. Any other issues?

 

[HRTKD]

Did you see the breaker (rated at 62A) between the panels and the mppt? Note that the panels are in series and the panel series fuse rating is 25A with the panels pushing 13-15A at ~105V. My thought is that the panel fuse would handle any issues and then the breaker. In this instance, would it be necessary or recommended to put another fuse in?

The 62 amp breaker is too large to do any over current protection. 20 amp breaker is more appropriate. But the breaker in that position is often just a switch with over current protection benefits. Be sure it's a DC breaker.

You still need over current protection on the output side of the solar charge controller. Breaker or fuse, you decide. But most systems use a breaker.

 

[time2roll]

Edit: my concern with the 2 A/C units is more around the VA as well as the startup current to get them moving. Even with a soft start, which I don't have, startup current is still high. If I can't run 2, I'd be ok with that.

My energy hog Dometic 13.5 will easily start and run with my 2000w-12v inverter. Although it would have to be an extraordinary situation.

 

[Vigo]

As stated above an A/C unit will draw appox 11A or 1300W on high. Obviously won't stay like that all the time.

So the power requirement of the compressor is mostly dictated by the high side refrigerant pressure of the system, which depends a lot on both ambient temp, and how effectively it transfers heat from the condenser to that. So a rooftop ac unit will pull more power on a hot day, a little less in the shade, possibly a LOT less if you're pulling the thing down the road at 60mph. It is highly variable with outside conditions, which I'm not sure what conditions you tested under but unless it was 'hellscape' i would fudge the number upwards a good bit to be safe.

It's also somewhat dependent on how much heat you're adding to the refrigerant from your indoor temp, so power use will go down some but not much, as indoor temp falls. What will actually happen instead of power requirement going down, is the compressor will 'cycle' on and off, which with 2 units running, or 1 unit plus random possibilities of other loads, starts a game of roulette where you face the chance that two compressors try to restart simultaneously, or one compressor tries to restart while your inverter is already busy powering a substantial other load like a microwave. Since we don't micromanage compressor starts, we have to oversize the inverter to compensate for this chance. Same as we do with generators, thus your 5500w generator feeding the same system you are trying to feed with a ~2000w inverter. No snark intended here, it's just harsh realities with these damnable ac units.

I would honestly be surprised if a 2400va inverter could even start a lot of rooftop ACs. Of course it's a Victron so it probably does every inch of what they say it does as far as 'surge capacity'. I would not expect it to run 2 rooftop units under basically any conditions because even if you start them separately you still have to have the power available to RUN one, and START one, simultaneously. Even if a 2400va unit could start one, it probably couldn't run one, AND still start another one.

You do have enough battery! That's the most expensive part, so if you've already swallowed that pill i'd say you're 80% of the way there.

 

[HRTKD]

I would honestly be surprised if a 2400va inverter could even start a lot of rooftop ACs. Of course it's a Victron so it probably does every inch of what they say it does as far as 'surge capacity'.

My 3000va Multiplus does it, without complaint. 2400va would be cutting it awfully close and run far too close to the limit for my comfort.

 

[heckler40]

My 3000va Multiplus does it, without complaint. 2400va would be cutting it awfully close and run far too close to the limit for my comfort.

I should have stated this would be a MPII 24/3000 2x120V. I mistakenly typed 2400VA when I meant W.

 

[heckler40]

The 62 amp breaker is too large to do any over current protection. 20 amp breaker is more appropriate. But the breaker in that position is often just a switch with over current protection benefits. Be sure it's a DC breaker.

You still need over current protection on the output side of the solar charge controller. Breaker or fuse, you decide. But most systems use a breaker.

Thanks for the feedback. Unfortunately the mega fuses don't go down that low. Will work on it tonight and figure out a solution.

 

[heckler40]

So the power requirement of the compressor is mostly dictated by the high side refrigerant pressure of the system, which depends a lot on both ambient temp, and how effectively it transfers heat from the condenser to that.

Great stuff. I'm primarily using the data from Coleman, some fudge factor, and my experience on 30A shore power.

Same as we do with generators, thus your 5500w generator feeding the same system you are trying to feed with a ~2000w inverter.

It will be a MPII 24/3000 2x120V, so 3000VA. I mistyped.

You do have enough battery! That's the most expensive part, so if you've already swallowed that pill i'd say you're 80% of the way there.

I'll have the last four cells tomorrow. They are EVE 304ah cells. It's DIY and cheaper than buying GC3s but pricey. The batteries and BMS and the Victron gear are about equal in cost so far. Gotta pay the blue tax

 

[Vigo]

It's going to be an impressive set of hardware! I dont know if another blue inverter is in the budget but if you want to run 2 ACs simultaneously you could consider multiple options..

1: switch one or both of the rooftop units out to be inverter-based units, or leave both existing in place and install a regular 'residential' mini split somewhere. Reason being that inverter-driven air conditioners don't have startup surge and DO throttle down their power vs cycling hard on/off and playing 'compressor start roulette'.

2. Wire one of the rooftop AC units to be either shore power or gen power but never inverter power. Still usable, but at a small ongoing cost vs a big upfront cost. This is also possible if you do the mini split from option #1.

3. Wire one of the rooftop AC units to a cheap dedicated ~3000w inverter that has no features and is dedicated to just that. Amazon suggests that's a ~$300 option. Up it to 500 and you can get a cheap 'all in one' which, while certainly no victron, does give you a backup mppt and AC charger in addition to being a backup inverter. Not that victron stuff ever breaks.

I believe all 3 of those options are cheaper than the 2nd Victron inverter. Just putting it out there!

 

[heckler40]

The 62 amp breaker is too large to do any over current protection. 20 amp breaker is more appropriate. But the breaker in that position is often just a switch with over current protection benefits. Be sure it's a DC breaker.

You still need over current protection on the output side of the solar charge controller. Breaker or fuse, you decide. But most systems use a breaker.

Switched this from the 100A Mega to a 60A Mega as well as the converter.