Inverter with ACR Dilemma

[ESGWheel]

I am helping a fellow boater wtih his ‘98 Maxum 2400 SCR to do an install and ran into a dilemma. Or at least I think it is one. Please allow me to explain.

The boat came with Shore Power (SP) and several AC powered items which he would like to take advantage of while underway or on the hook. Hence the Inverter. But to make life a little simpler he is upgrading to an Inverter / Charger (I/C).

And to preclude drawing down the start battery, he is installing a separate House Bank which will be changed up while on SP with the I/C. Engine will charge the Start Batt while underway and the I/C will change the House Bank when connected to SP.

But wait, they make Automatic Charge Relays (ACR) that can keep the House Bank charged while underway and alternativity keep the Start batt. toped off when in port on SP. Folks familiar with Add-A-Battery ACRs recognize that these devices will connect the Start and House batteries under certain conditions on either bank > it monitors both sides and will close if conditions are satisfied on either side. This means can charge the Start Batt. from the I/C and in turn charge the House Bank with the Alternator (Alt.) when the engine is running. Thus, for this scenario the Alt is connected to the Start side, not the House side.

The included pic is a representation of how this would be wired up. Note that it does not include wire or fuse sizing. For this discussion assume its correct.

Now the dilemma. For the sake of this discussion assume the I/C is a 2000W and the single Alt has a max output of 85A. Also assume that the full capacity of the I/C would be used while underway (it’s understood should not run an inverter at max but for this thought exercise please consider it will be). So, DC draw on the I/C would be 2000W / 12VDC = 166 Amps. This is 2x the capacity of the Alt.

While underway with the I/C going full tilt, the ACR will sense that it can close > recall the Alt is connect to the Start side, not House. Thus, now have a situation where the I/C is trying to continuously pull 165A on the House batteries (which it can do to a point) but now the ACR relay closes coupling the House Bank to the Alt (via the Start Batt). What will happen?

With the ACR is closed I think the Alt, with its higher voltage than the batteries, will be loaded to its max and potentially burn out. In other words, with the ACR closed, the Alt. will attempt to provide the full inverter load until it cannot and subsequently drop its voltage under the strain until the rest is provided by the House & Start batts (Start as well since ACR closed).
And while the Alt voltage output may drop to the ACR’s Open Low threshold of 12.75VDC (30 seconds) or 12.35 (10 sec.) it seems that this is unhealthy for the Alt. And it seems this cycle will continue > ACR open, Alt. voltage goes back to normal and thus ACR closes, but voltage drops again causing ACR to open. Repeat.

And based on fuse sizes, the Start Batt to ACR may blow.

Is my thinking correct? It seems so based on what I have read.

I assume folks have put in inverters with a greater capacity then their Alternator AND have an ACR to charge up their House while underway, what do they do? Leaving the inverter off or disabling the ACR under certain conditions are two possibilities but it negates the purpose of having them to a large degree. Limiting the load on the inverter to much less (like 50%) of the Alt. output is another option that seems reasonable.

What I am looking for is (1) is the concern valid and if so (2) what have others done when trying to take advantage of an ACR but also have an Inverter that exceeds the Alt output? Thanks for your input.

[mmwjr]

Slow down and breath.

What will be the true load on the converter? A 2400 should have a dual voltage refrigerator, water heater which can use engine coolant instead of electricity while underway, microwave which is only used in short cycles and an electric range. Most people never use the last two while underway just because it isn’t practical or safe to do so. There is also an optional air conditioner for some years. The alternator will only put out its max amperage period no matter what the load wants, now operating at that max output for extended periods will result in a premature failure. A higher output alternator could be installed along with upgrading the wires for this increase in power. Also the house battery bank needs to be sized appropriately for the load.

You may want to talk with an expert from Blue Seas once you calculate the real load and house size.

[ESGWheel]

Thank you for responding. Idea of this thought experiment is a “worst case” scenario. I cannot help but believe that someone else has run across this situation and determined that unless the I/C can be constrained to only supply 40 to 50% of Alt. capacity, you will have trouble. In addition to a larger capacity Alt as you are suggesting, one way to do this with the “Pass Through” type I/C he is using is to wire it as a split AC bus > the I/C is now limited by hard wiring to only supply certain AC loads that do not exceed that 40 to 50% of Alt capacity (as well was being well within the I/C capacity). This split bus is what I am leaning towards but also hoping others have tackled this and come up with other solutions or validate the split bus concept. Or perhaps my concerns are all wet, but I do not think so…..

Note: the split bus concept is associated with preventing exceeding the capacity of the I/C; so I am springboarding of that idea to also be tied back to the Alt because of the ACR.

[yahoo82]

Here’s another side of the thought experiment. If he’s drawing 166 amps continuously, he would need 3 group 31 batteries in the house bank to provide that and after an hour they would be toast. Is there space in a 2400 for that? Group 31’s are good for about 100 ah each but only 50% of that capacity is useable. So you need 300 Ah to provide 150 Amps for an hour. He could go lithium, but that adds another level of complexity.

So I guess my point is if he’s pulling that much load continuously, then the whole system is a moot point because practically speaking he would need a huge amount of batteries to support that.

[ESGWheel]

Thanks, and the limits of battery capacity, etc. are understood. In retrospect I should have used a 1000W I/C and a 55 Amp alternator to make it more realistic. But the whole idea is to see what others have done in this Inverter + ACR space with an Inverter Capacity much greater than Alt Capacity.

[mmwjr]

I use my 2400 as a day cruiser so I don’t have any need for an inverter. My Silverton has a generator so no shortage of power.

[shrew]

Your worst case scenario is that you draw down the house bank faster than the alternator can charge it. The ACR will isolate the house draw and from the starting bank.

More importantly, the 2000W limit of the inverter is 120VAC. What in the world are running off of 120VAC while the boat is underway?

The short answer to your 'thought exercise' is to upgrade to a high output alternator and an external regulator. Balmar makes great products to do just this. However, on a 24 foot express, unless you're attempting to run the Air Conditioning off of the inverter while underway, the discussion is about plausibility vs practicality.

You don't have a problem here.

[yahoo82]

After rereading this post and thinking about this a little more, there are two parts to this;
1) peak load / average load / house capacity.
For this to be practical, your average load and house capacity have to be reasonable and well matched. For DC, my boat draws about 8-10 amps with 2 fridges, the radio and stereo running. Under way, the 2 blowers add another 10 amps in total. If I’m being frugal, I can do 3-5A if I’m out on the hook. 2 group 31s would give about 20 hrs for the frugal case or 10 hrs for the day-use case with stereo (remember only 50% of lead acid rated capacity is useable). Adding a few intermittent AC peak loads thru the inverter (like microwave, blender) is still doable from a capacity standpoint. Adding a small LED TV or similar load that’s probably 5-10A DC, still ok for just a day on the water. But if you are running AC or something else that’s 1000w on a continuous basis, your house bank will never have enough capacity.

2) alternator protection - so let’s assume there are just a few intermittent high amperage AC users.
Option A) process control … just don’t turn them on while underway. The inverter power consumption is equal to output load plus a little extra for the conversion. So you can run a TV on AC power just fine and not worry about the alternator.

If you (or your crew) might accidentally run a high load device continuously while under way, you are right to assume the standard alternator may burn out (but remember your battery will also be dead by the time you get to your destination).

Option B) regulate the load draw on the alternator. You can do this with a DC to DC charger instead of the ACR (such as this https://www.victronenergy.com/dc-dc-...attery-charger). The starter battery with alternator is on the input, house bank on the output. This will only allow a max of 50A to come from the alternator. (Note: this device does not have an ignition protection rating.) This is the solution I used when installing my lithium house bank, because they can charge at a high enough rate to exceed my alternator rating.

You may also be able to get this next device to work in your application. It’s a little cheaper and is ignition protected and regulates current. But it could be a more technical integration.

https://www.victronenergy.com/batter...urrent-limiter

Hope that helps a bit. Good luck!

[ESGWheel]

@shrew
I like to think in extremes as it helps me understand if going to have an issue. This exercise is not about the boat or the AC loads to be run, etc., but rather trying to comprehensively understand to develop Rules of Thumb.

Examples of what these RoT could be:

• Never combine an ACR with an Inverter.
• Only combine an ACR with an Inverter if the loads on the Inverter are much less than the full output of the Alt.
• Combing an ACR with an Inverter is fine.

I too was initially thinking as you in that will not be an issue BUT then I started thinking about the higher voltage the Alt is putting out and thus it would be the primary source of current until its voltage drops. Thus the Alt will be at 100% of its output vs RPM curve potentially for an extended period, which I did not think was good > my understanding of Alt is they have about a 50 to 60% duty cycle and running at 100% anywhere along its curve for an extended period is not healthy (overheating).

I appreciate your feedback and a couple of follow up questions if I may:

• Do you think that the Alt will be at 100% ‘duty cycle’ ie 100% output wherever it is on its RPM curve?
• I am unsure what “The ACR will isolate the house draw and from the starting bank” means. Is this the ACR opening on Low Voltage on the House side?

Thanks again.

@yahoo82 > just saw your post as I am replying to shrew, thanks and I will digest in a bit.

[shrew]

Originally Posted by ESGWheel

[*]I am unsure what “The ACR will isolate the house draw and from the starting bank” means. Is this the ACR opening on Low Voltage on the House side?[/LIST]

When you install an ACR, you connect the house bank and the starting bank.

1) When you start the engine, the ACR draws from the starting bank.

2) As long as the voltage is above 13.0V (such as when connected to shore power or the alternator is running), the ACR will combine the house and starting banks to charge both banks.

3) When the voltage drops below 12.75v (such as when there is no charging source (either shore power or alternator)) the ACR isolates the starting bank and only draws from the house bank.

The idea that you can't combine a charging source like a battery charger with an ACR is silly. Technically, I have one charging leg tied to my house bank directly and one charging leg connected to my starting bank directly.

The main benefit is that once I start the engine both house and starting banks are charging from the alternator. When I stop the engine, the ACR isolates the starting bank and only draws from the house bank.

Now I have a 40 foot trawler. I run multiple displays, a mid sized fridge and freezer (large by boat standards). The stereo is on constantly while I'm underway along with the fridge and freezer. Radar and autopilot are also constantly running.

My entire load is handled with an 80A alternator, and that still tops off the house bank (2 x 8D 12vdc batteries) while underway.

The DC loads being discussed theoretically here are simply not 'real world' unless you attempting to run the AC off of the inverter while underway.

If you are really worried about overheating the alternator, then get a Balmar External Regulator.

[ESGWheel]

@shrew. Thanks for the follow up. Roger on how the ACR works and it dropping out is what is what I thought you were referring to, so thanks.
As for “The idea that you can't combine a charging source like a battery charger with an ACR is silly” is NOT what I am suggesting. The whole idea of the ACR as we know is to take advantage of a single charging source for a Dual Battey Banks.

The scenario I am outlining above is different. Here is an analogy: You needed to charge up your car battery with a typical charger. It’s been a few hours and you go to start the car but you forgot to unhook the charger. What happens? Unless it’s a sophisticated charger with some overload protection, I think it will fry. That is what I am getting at: even though the loads I am suggesting are not realistic per se’, I think connecting the Alt to an I/C loaded in excesss of Alt. capacity will overload the Alt.

Understanding the limits of process or systems is how I think and thus it leads to a process or system design that I know will work. And after completing the above design I stepped back and thought of all the ‘worst case’ scenarios and said “Oh boy, what if the load on the I/C is much greater than the Alt capacity?”

Did not know about the Balmar products, so thanks for that. Seems like the Belt Load Management is a way to limit Alt. output.

@yahoo82. Your concepts of “process control” and “regulation” (Options A & B) is exactly how I am thinking albeit using different terms. I call the first “by policy” i.e., ‘follow the rules’ which is human dependent and can be violated, not the best way to protect. The other I call ‘by design’ i.e., does not matter if you try to break it, you cannot.

I strive to always create process or system protection by design (to a point) and thus remove the human element. “To a point” as you can design a foolproof system but at a cost of $$$$ where the ROI becomes negative.

I was not aware of the products you highlighted, so thank you. The DC to DC looked promising but is only one way, negating the ability to charge the Start Batt fm the I/C when on shore power.

I am now strongly leaning towards limiting the load on the I/C less than the Alt ‘by design’ using the AC Split Bus concept (post 3). And after evaluating the actual loads on the Alt I will suggest (if needed) a higher output Alt potentially coupled with a current limiting regulator.

Thanks all for your time and input.

[yahoo82]

I think there may be a flaw in your method to use the I/C to charge the start battery at the dock via ACR. The voltage profiles of the house vs start will be very different when you return to the dock. Ideally each should have a dedicated charger output to match the output voltage to the battery. With the ACR, the start battery will get a higher charging voltage than it needs while the house is charging. For lead acid, it will take about 15-20 hrs to replenish the house if it’s depleted to 50%. So instead of getting a float voltage during that time, it will get bulk charging voltages which could boil off water. If there is an existing AC charger, you could keep it in place to charger the starting battery.

[ESGWheel]

Originally Posted by yahoo82

I think there may be a flaw in your method to use the I/C to charge the start battery at the dock via ACR.

Good point. I did not consider this ‘worst case’ before so thanks. Plan is for lead acid and same battery across the board. I will look at putting some sort of ACR lock out when on SP (ACR has a method to disable it via an open ground connection & the use separate charger as suggested) or provide some sort of delay device (keep the ACR locked out until 24 hrs later or some voltage differential is met). Thanks!

[ESGWheel]

Wanted to close out this thread with the conclusion I have come to. Recall that I was looking to ‘stress the system’ in what perhaps is considered an unrealistic scenario but by using this sort of ‘thought experiment’ I can understand what the pitfalls are and put measures in place to minimize/prevent them. And in brief those measures fall into 2 categories: (1) a “policy” that requires human action (a do this before you do that) and (2) a “design” that will automatically turn off this when that is turned on.

My overarching concerns with coupling an Inverter / Charger with an ACR were twofold: overstressing the Alternator and causing a potentially detrimental cycling of the ACR.

I appreciated all the comments received and took the next step to contact The Guru himself: Nigel Calder of BoatHowTo.com and author of Boatowners Mechanical and Electrical Manual and others. If you are not familiar with their work, I encourage you to visit their site and YouTube channel. Nigel was kind enough to have a back-and-forth email exchange and in brief validated my concerns and provided suggestions.

His recommendations aligned with the comments made by @yahoo82 above which included (1) keep the load on the inverter / charger within the limits of the Alt or use a B2B type charger. And he also expressed, as @yahoo82 did, about the differential states of battery charge that could cause an issue when on Shore Power.

So, what does it all mean?

While you can do an I/C with an ACR need to limit the I/C loads while underway to less then the Alt output (while there was not a recommendation, I would put that at about 50% since output is a function of RPM). And then will need to have a relay set up to disable to the ACR when on Shore Power and have the I/C only working on the House and some other means to address the start batt if needed. Not an ideal situation.

Thus best to, as Nigel (and yahoo82) urged, use a programmable B2B charger. This is direction I am talking to my friend about.

[shrew]

Fantastic. What loads do you have on a 24 ft express which will equate to 50% load on the alternator??

I respect Nigel and I'm curious about his thoughts on an external regulator, which would dynamically limit the alternator?

[ESGWheel]

Nigel’s suggestion was a B2B charger that was fully programmable “or some other device that limits the current flow” from the alt. An ext regulator that dynamically limits the output would fit that criterion.

For the 50% loads > I do not yet have the intended loads he plans on using while underway… this was one of reasons I started asking myself ‘what is the limit?’ and what would it do if exceeded the alt output?

As a side note: I was pleased but not surprised that Nigel took up an email exchange. Not surprised because I have experienced over the years that by reaching out to various companies and either taking live or having an email exchange with their tech support has yielded mostly positive results (ie they engage and engage fully). For one company, regarding an inverter for 12 DC to 120 AC, but not in a typical application for my RV, the tech support was unable to provide an answer so the next thing I know I was having an exchange with the engineer who designed it! As a get older (I am 67 but still working FT) and have joined a few forums to both learn and help, I have found that there is a ton of misinformation out there that could be readily corrected by just asking the right folks. And frankly, as you well know, that is what these forums are all about: sharing, leaning and helping 😊