Generator AC vs DC

[benherman1]

Hello,
I'm still in the demo phase of my conversion so how I do my electrical is entirely up to me for now. I've been thinking keeping as many things on 24v DC as possible would be the easiest for a battery bank with solar, alternator, and AC (rv plug) charging. Obviously I'll have to have some AC power available for appliances, and regular devices inside but that will be easily handles by any decent inverter. To actually accomplish this I have a few questions.

There are plenty of charge controllers out there for solar and of course any alternator has a regulator whether it is internal or external. Is there any good way to run a controller that handles both inputs without them interfering with each other? Am I overthinking this and I could run them both at once on the same battery bank?

Has anyone converted a generator for DC output? obviously this could be done by replacing the generator with a collection of alternators or one large one. From What I have seen it can be done with a rectifier. I assume after that I would need some form of charge controller to get the correct voltage. Would I be better off just attaching it to a charger that uses as close to its max output as possible? The generator I have is an old Coleman 4000 watt generator which is a bit smaller than most people here seem to have but I'm hoping it will only be for a bit of supplemental power when the solar can't keep up.

Has anyone done a similar setup?

Thanks,
Ben

[TomC]

Suggestion-Use your inverter just for light duty stuff. My inverter items are all plugs, microwave, toaster oven, one 10gal water heater (to maintain hot water driving down the road-no heat exchanger), bathroom electric heater. Direct electrical with 10kw Diesel generator is refrigerator and freezer (both have automatic switch over to 12v), 2nd 10gal water heater, 3 roof top A/C's, powering inverter/charger. Also highly suggest you switch all DC to 12v. All big rig trucks are 12v dc since high crank efficient 12v starters are made. I switched my MT41 gigantic 24v starter to MT39 12v geared starter-1/2 the weight, 1/2 the power, twice the cranking power. I used to have 4 starting batteries, now use 3. 12v items are just the standard of the industry in USA. 24v stuff is hard to get (easy in marine applications). Good Luck, TomC

[benherman1]

I definitely agree with the inverter only running the small stuff. I figured I'd need some 12v feeds since 24v appliances and such can be hard to come by on occasion. I'd like to stick with 24v as my main power though since it is more efficient over each run. running 24v LED lighting also is much easier as far as controllers and wiring options go. All my stock electrical is also 24v of course and I don't want to mess with what already works. For its age everything that was done from the factory looks and works great.

As far as starting batteries go my bus came with a pair of batteries that would fit better in a car than a bus but it starts on the first crank every time so it seems happy enough. I have no idea what the starter model is but it is definitely large.

I never thought about appliances that automatically switch to shore power when it is available. That makes perfect sense and I'll be sure to wire things up to allow it where it can be used. No reason to run through all my systems when there's free power direct.

For AC and heat I'm leaning towards some sort of mini split system with a propane furnace to help in extra cold areas. Not sure on that yet. I really don't like the look of the rooftop units and they block space to jam in more solar so I'm not going that route.

[TomC]

The popular thing now is just to run a normal household type refrigerator through the inverter. My house 22cu/ft Kenmore side by side refrigerator takes 7.2 amps to start and 5.5 amps to run. Converted to 12v-that's about 75 amps starting and 57amps running. As comparison, my NovaKook 9cu.ft refrigerator takes 5.5 amps a 12v. Good Luck, TomC

[Fred Mc]

Go to (https://diysolarforum.com/)
There is lots of info/help for what you are looking at.

[buswarrior]

In the past, a few hardy souls built DC generators, but it was usually price point driven and was focused on battery charging.

The question was, "why am i making AC, to make DC?" It eliminated the expensive generator, and eliminated the expensive battery charger.

Homebuilt ruled the day, a handy engine was available, an alternator of some description was available or sourced. Pullies and belts were rigged. Whether a fancy multi-stage external regulator was added depended on the vigour and finances of the pursuing busnut.

The price points and efficiencies of all thebusnut electrical options has changed rather dramatically over  the last 2 decades, solar panels and pure sine wave inverters being 2 huge areas that have changed.

Do some math, price some things, don't forget the various losses in efficiency with each strategy, and see if "traditional thinking" needs a swift kick in the rear?

If every bus out there gets built the same, there's not much "busnut" in 'em... ?

Happy coaching!
Buswarrior

[richard5933]

Here's a thought...

The HVAC system (and everything in the sleeper cab) in my company truck runs on a bank of eight 12vdc batteries.

The engine has an oversized alternator for charging them.

When I'm parked, I activate something called "Opti-Idle". It will run the inverter, HVAC, and anything else I need from the batteries, and when necessary due to voltage drop it will automatically start the engine and recharge them.

I believe that some new commercially made motor homes are utilizing a similar system.

With the growing advances in technology, is the time that far away when we in the bus world can employ the same technology? After all, many of us still have a tremendously-oversized alternator on the engine. Mate that to a decent solar array and battery bank and I wonder if the generator is needed. Even a/c can be run on inverter nowadays with installation of the newer high-efficiency units.

If a generator is needed, then a DC-output unit seems like it will fit the bill since all the AC we need can usually be delivered through the inverter.

[Jim Blackwood]

Even in something the size of a bus the cost of wiring can be substantial. Since power equals amps times voltage then the lower the voltage is, the heavier the wiring must be to carry adequate power, and undersized wiring means heat, losses, burned up motors due to undervoltage situations and potential fire. The point is that 12v requires 10 time more copper to carry the same load as 120v. And copper is not cheap.

This is the exact same argument that lead to the adoption of AC in the early power grid because with AC the voltages could be stepped up, current reduced, and power carried on much smaller and cheaper conductors. If you hadn't noticed, electric vehicles have been escalating their operating voltages. The reason has to be conductor size and amperage requirements.

It might be possible to create a main DC bus running the length of the bus (how many ways can you use the word bus?  ) and tap into it where needed but this upgrade is going to be more expensive than most of us would be willing to justify. Besides, there are good reasons to have a bus that makes it easy to use all available power sources.

Aside from that, the old transformer argument still applies. AC power can be stepped up or stepped down by using a simple passive device that is pretty efficient, called a transformer, after which it can be rectified into DC if needed. There is a small loss through the transformer, and another small loss through the diode bridge that creates the DC (another passive device) and then it may be filtered through a choke (another transformer but with only one winding) and smoothed with capacitors.  How do you make AC out of DC? Inverter is the simple answer, but how does an inverter work? It chops the DC into bite sized bits (an active device) and then may wave shape it to get a sine wave output, so a couple of losses there at least, and if you want a higher voltage than the input that generally means a transformer. We already know which method costs more. The second method is much more complex and therefore more prone to failure with the attendant costs.

It also means many more details in the system should be monitored for out of spec operation which can cause various failures and poor performance.

I'm not saying it's bad, just that it takes more money and more effort, neither of which I'm personally willing to commit. Regardless of which way you go, the bus will have redundant systems because you simply can't get away from the requirements for both power systems. It is as much a practical impossibility to have a 100% DC bus as it is to go 100% AC.

Jim

[TomC]

That "Opti-Idle" sounds good on paper. But who wants their big engine starting up automatically in the middle of a hot night in a campground? Our bus engines are not quiet. As compared to the Cummins or Mercedes truck and van engines that are just about as quiet as a gasoline engine. It never made sense to me to run a 500hp Diesel engine to charge batteries or run A/C when a 15hp Diesel generator would do the same thing. Good Luck, TomC

[richard5933]

That "Opti-Idle" sounds good on paper. But who wants their big engine starting up automatically in the middle of a hot night in a campground? Our bus engines are not quiet. As compared to the Cummins or Mercedes truck and van engines that are just about as quiet as a gasoline engine. It never made sense to me to run a 500hp Diesel engine to charge batteries or run A/C when a 15hp Diesel generator would do the same thing. Good Luck, TomC

Guess I was considering running the engine only as a last option. The advances in solar and batteries are making them nearly self-sufficient. Nearly, but not quite.

[benherman1]

It might be possible to create a main DC bus running the length of the bus (how many ways can you use the word bus?  ) and tap into it where needed but this upgrade is going to be more expensive than most of us would be willing to justify. Besides, there are good reasons to have a bus that makes it easy to use all available power sources.

I figured for the most part I won't need any crazy large wiring for most things. A single piece of welding cable running the length of the interior would be more than enough for anything I'd need. I'm not sure what the current state of the wire that runs from the battery to the starter is but I assume I could tap into that for anything that isn't too picky about a sudden voltage drop.

I figure I'll have to have an inverter anyway for most AC stuff. I'll want to run a computer or two at the minimum and they don't particularly like variations or drops in power changing over from a generator to shore power. With the age of my generator I assume the power isn't quite clean enough for a computer anyway. I could solve that issue with individual battery backups but then we're back to step one.

[freds]

Lot's of ways to skin the cat so to speak. I however do agree that 24V and as much stuff as possible running off of it with a small inverter for the AC loads that you can't convert are the way to go.

In my bus I am running a Node Red automation system and when I need AC power I say "Alexa Power On" (implemented) and if I forget it monitors power consumption and automatically turns the power off (working on).

[fortyniner]

This thing is kind of neat but dont see a 24v version:
victron Smart BMS CL 12/100

[freds]

Generally the BMS functionality is specific to the battery itself.

It looks like the device (Smart BMS CL 12/100) you are referencing is 12V only and is supposed to mediate between an engine driven alternator and a bank of 12v lithium batteries so you don't overload and burn out the alternator by pulling too much current which discharged lithium batteries can do.

Not sure why they use the word BMS for this device other than marketing.

If you are using the alternator to charge your home battery bank than it better to use an isolated DC to DC converter that has matching inputs and outputs.

Most people use solar or AC power input to charge the lithium house battery banks.

[windtrader]

The advances in solar and batteries are making them nearly self-sufficient. Nearly, but not quite.

Just now completing the switchover to lithium battery storage and large solar array or on-demand and battery charging, the next phase is developing viable options for removing the massive, diesel generator.


Over the next couple months, it'll become clearer how power is consumed for our personal use. Three aspects here: 1) lower usage through use of highly efficient appliances, LED lighting, etc. 2) increase power generator with more panels, 3) increase battery capacity for handing larger and longer peak draw.


So far, the 12kW battery bank has plenty of capacity. Last few days, overnight drawdown in about .5 to 1.0 volts, barely noticeable. Currently calculating drawdown using a new tech mini-split and feel it might be able to run all night if needed with the current system.


For periods of extended low solar generation, a small portable backup generator is being considered. A properly sized charger can fully charge the battery bank in a few hours. This is reasonable with the Lithium Ion batteries as charging with a 3kW generator charges the battery at .25C, well within spec, and can fully charge the battery at 25% SOC in three hours.