1986 TMC 102A3 - inverter design question

jraynor · June 27, 2018, 06:54:23 PM

Hello all, I am new to this forum but have been looking for such a forum with people who have been there and done everything in converting a bus. I am currently designing my electrical system and even though I would love to discuss it all (I love doing electrical work) I will keep it short to get to my questions. I was originally going to put a variable source camper fridge in the RV but I have decided to go with a regular small-medium fridge. With this decision I know I have to supply 120v power at all times during it's use, even as we travel down the road. My design includes a 12vdc to 120vac 4kW,8kW surge pure sine inverter. I have just decided to go with a 24vdc to 120vac to decrease the wire size.

My question is, can this inverter run off of the bus batteries while traveling down the road?

I have seen some posts about second alternators and some about battery banks. I am curious as I can't find any information online about buses in specific versus RVs.

Also, can someone provide a part number about a switching inverter that will charge the batteries when vac is supplied? I just heard of them tonight and that could eliminate a set of battery maintainers I was planning on installing while parked/on shore power.

Thank you,
Jonathan

Geoff · June 27, 2018, 07:24:02 PM

I have a nice stainless steel Sanyo apartment refer in my bus and it has rarely been shut off in fifteen years. 120v supplied by shore power, inverter, or genset. Works fine for me. Oh, I use a Trace SW2512MC to take care of my batteries.

eagle19952 · June 27, 2018, 09:22:55 PM

search here or google 24v inverter charger
Magnum is the current cat's meow.
My bus has had a single bank combined start house battery system.
my generator has a separate battery on an echo charge from my XANTREX Freedom Marine inverter/charger
i carry a spare inverter SW2012 Trace.

sledhead · June 28, 2018, 04:14:36 AM

this is what I had on the M C I . amazing

https://invertersrus.com/product/magnumenergy-msh4024m/

dave

bevans6 · June 28, 2018, 04:37:40 AM

Seeing as you have a TMC/MCI 102A, you should have a 24 volt bus system with a Delco 50DN alternator putting out roughtly 6500 watts at 28 volts full chat. That is ample to run the bus, all accessories and an inverter running down the road. I have a similar MCI MC-5C and I use a Magnum 4024 with a house battery section, bridged to the bus batteries and the alternator when the bus is running. I run a roof top air conditioner with mine, not the fridge, but it does fine and has ample capacity. I have a separate house bank so that when parked I can separate off the bus start batteries, so they don't run down, and I can jump-start with the house bank if needed on a very cold day. In short, what you want to do is well proven by many people, and will work great. Just don't save money on the inverter - the market is flooded with cheap import inverters with over-stated specifications and ratings that don't stand up to the test of time and load. I went that route first, threw that inverter out when it blew up, and went with the Magnum. It has a great charging system as well as a built in load transfer switch.

jraynor · June 28, 2018, 06:41:41 AM

Bevans6,

So the setup that I have designed should work fine without a battery bank and while driving down the road correct? I'm considering pulling the positive side right off of the battery disconnect for the bus using a 4/0 Cable. I know that should be sufficient but is that overkill?

Thank you,
Jonathan

bevans6 · June 28, 2018, 07:01:24 AM

There should be an existing 1/0 cable from the batteries to the copper bus bar inside the AC relay box mounted on the ceiling of the front luggage bay, driver's side, and that's what I used. I tied in to the bus power at that bus bar. For various reasons, I mounted my inverter there, I installed the house batteries in the AC condenser compartment, along with my portable generator. You can run the inverter directly off the bus batteries without a house bank but I don't recommend it, particularly in your case where you plan a constant draw to power your refrigerator. That is not a large draw but over 24 hours will drain your bus batteries significantly. I would use a 250 amp-hour 24 volt house bank, either 4 6 volt batteries or 2 12 volt batteries, and connect to the inverter with a combiner switch so you can power the inverter from just house, or house plus bus/alternator. That is what I did.

Brian

jraynor · June 28, 2018, 07:44:11 AM

Brain,

Will the 1/0 carry the load from a 4kW inverter?

The only times I will be running the fridge will be:
1 When we are packing to get ready to leave, which shore power will be connected for keeping the bus cool
2 While we are traveling, to keep the food cool
3 When parked, which shore power will be connected.

When connected to shore power, the inverter will be off and ill have either an inverter charging the batteries or battery maintainers keeping the batteries charged. The only other time that the fridge may be running without shore would be to get fuel. At that point, I'll cut the fridge off for the 30 minute stop.

Do you have a link or brand/model for the combiner switch?

Thank you,
Jonathan

bevans6 · June 28, 2018, 10:11:02 AM

The ampacity of 1/0 cable is 150 amps, times 28 volts (while the bus is running) is 4,200 watts. That is sufficient if you don't overload your inverter, since it's a continuous rating. The other thing is you can simply not load down the inverter all that far, my biggest load is the air conditioner, and that's under 2,000 watts. So for me, it was enough. The cables in the bus are 1/0 from the alternator to the starter motor stud in the engine bay, 4/0 from the starter motor to the starter motor stud and up to the batteries, and 1/0 from the battery disconnect switch to the AC breaker box, and 1/0 from the AC breaker box to the front electrical panel. If you get a Magnum, the transfer switch is built in. If you want an external transfer switch, you need to choose 30 amp or 50 amp service, if 50 amp how many legs, and so on. Iota is one brand, I have a couple of those.

Your usage plan sounds fine, you can probably run the fridge for several hours without doing anything odd to the start batteries. My bus usage is probably quite different to yours, I rarely park "on a pole" and like to be pretty self-sufficient with batteries and a small generator.

Oonrahnjay · June 28, 2018, 10:14:20 AM

Others have given good advice and described systems that have worked well for them. But let's go back to basics:

"Start batteries" - the ones made to start the engine and provide an electrical reservoir (like powering headlights in stop and go traffic when the engine turning over revs to turn the alternator enough to make lots of power) are good for what they do. They provide a big jolt of power for engine starting and recharge quickly. If you're 100% assured of external power to run things when the engine isn't running, you can depend on them to act as the reservoir basis of a power system.
"House batteries" - the ones made to provide long-lasting supplies of even, moderate power are called deep-cycle batteries by their manufacturers. Most of us have these to supply 12V or 24V power when the engine isn't running and external power isn't available. This kind of battery is not built to provide the big rush of amps needed to start an engine but is perfect for keeping our systems running.

Here's what most of us have --
1) The original bus system with alternator, "start batteries", and ordinary vehicle systems (lights, horn, etc.).
2) A bank of "house batteries" dedicated to feed power for camping needs.
3) And inverter set up to take the DC power from the house batteries and convert it to AC 120V. A useful inverter will have a high-tech charger set up to provide the charging profile for the house batteries.
4) A shore cord and switching system to provide "plug in" power to run the house system and charge batteries (note, this system will charge both house and start batteries when parked and plugged in, with the proper setup).
5) A generator, to provide sufficient power to run everything when parked for periods of time and plug in power isn't available.

Now, here's the first question that we're always asked -- "Isn't that EXPENSIVE, complicated, heavy, hard to maintain, time-consuming?" The only honest answer to that is yes. We do it because we've found that we need it.

Once you have the full system, you cover all the bases. You have power for all your needs (and the little things, a water heater - even a small one, for showers, dish washing, etc. uses lots of power, a TV, a coffee maker, an on-board water pump, a hair dryer, charging cell phones and laptops -- and you'll notice we haven't even begun to talk about air conditioning yet).
You should begin to think in watts. Systems are designed in volts, components are designed in amps, but the big picture is watts. Look at what you want to power in a day and total up the watts used and the time you'll need it. If you don't design for those loads from the start, you'll always be playing catchup and you'll be miserable until you finally do.

Yes, the start batteries on a bus are made to start a big engine with a heavy starter that needs lots of power. A number of people have calculated how long you can run a small air conditioner off a bus's start battery combo when the engine isn't running (I say combo because there are usually 2 or more large batteries tied together as the start battery system). The calculated answer comes out to being 25 - 40 minutes before the batteries are drained so much either they're damaged and won't take a full recharge again, or, they're so flat they won't start the engine again -- or BOTH. So, a simple question "can I run an air conditioner off the alternator and batteries on my bus engine?" isn't so simple. One answer is "Yes, you can ... if you don't use any power for reading lights, cooking, pumping and heating water, heating or air conditioning, etc. while the bus engine isn't running." Another shorter answer is "Not really, in a practical sense".

OK, blah blah blah ... I'll shut up now about "big picture" design. But I feel I have to state an agreement with the good details people have given you about inverters. Look on the Web and you'll see ads "3000 Watts, 7000 Surge Watts, Pure sine wave -- $250". Look VERY closely at such items. If you want a practical view, take a few minutes to look at this YouTube, from an experienced electronics designer and industrial electrician:

https://www.youtube.com/watch?v=jQafeM6vnFQ&t=886s

This is an inverter of the type made in China; they're the same inside and sold under the AIMS, PowerBright, Cobra and similar brand names. It's cheap, but it doesn't do surge, it doesn't do 5000 watts - in fact, it will only do about 1000 watts continuously and even then, the quality of the output is not very good. An inverter like this is a perfect example of what you should NOT spend your money on. It also does not have a built in charger. Don't waste your money on a poor quality, poor output inverter. Just say no and walk away -- or run if you have to.

Many people have mentioned Magnum inverters, for good reason. Great quality, genuine output (although I wouldn't put more than 80 - 85% of the rated watts through any inverter for a continuous period of time). A Magnum will also give you reasonable start-up-surge power for an air conditioner and that's a big thing; air conditioners are notorious about needing lots of power to start and they're picky on the power quality.

A final thing, people sometimes think "batteries are easy, you use them and charge them up". That usually means that the batteries die in short order. Batteries that are used for large loads need a very specific and tightly controlled charging regimen; if they're not charged to the right profile (controlled amps at moderate voltage for the big charge, a higher voltage with lower amps for "finishing", and a float to maintain charge), they'll rapidly fail. A 150 pound, $300 paperweight is nobody's idea of a good thing, especially if you have to replace 4 (or even

of them. It *is* possible to get the proper charge profile from an engine alternator output (by adding a separate voltage regulator system), but part of the important part of this system is that it needs to be continuous and carried on until the perfect final charge is completed. That's hard to do with an engine-driven alternator.

Sorry to throw a lot of things at you, but when you get into the real-life aspects of electricity on a converted bus, there are lots of things to cover and consider. Keep working through what you'll need and stay in touch.

Hope this helps, BH

richard5933 · June 28, 2018, 11:47:09 AM

Quote from: bevans6 on June 28, 2018, 10:11:02 AM
The ampacity of 1/0 cable is 150 amps, times 28 volts (while the bus is running) is 4,200 watts. That is sufficient if you don't overload your inverter, since it's a continuous rating.

Necessary to also calculate the voltage loss incurred with the cable running the distance from the front bay the battery compartment towards the rear of the bus. Using 20 feet as the distance (may be more) there would be 2.11% loss in voltage resulting in 27.41 volts at the inverter capable of supporting approx. 4111 watts.

Still might work, but it certainly doesn't allow for much head room for surge capacity or a/c start up.

Generally speaking, it's recommended to keep cables between the inverter and the batteries as short as possible.

Branderson · June 28, 2018, 12:23:52 PM

Quote from: Oonrahnjay on June 28, 2018, 10:14:20 AM
Others have given good advice and described systems that have worked well for them. But let's go back to basics:

"Start batteries" - the ones made to start the engine and provide an electrical reservoir (like powering headlights in stop and go traffic when the engine turning over revs to turn the alternator enough to make lots of power) are good for what they do. They provide a big jolt of power for engine starting and recharge quickly. If you're 100% assured of external power to run things when the engine isn't running, you can depend on them to act as the reservoir basis of a power system.
"House batteries" - the ones made to provide long-lasting supplies of even, moderate power are called deep-cycle batteries by their manufacturers. Most of us have these to supply 12V or 24V power when the engine isn't running and external power isn't available. This kind of battery is not built to provide the big rush of amps needed to start an engine but is perfect for keeping our systems running.

Here's what most of us have --
1) The original bus system with alternator, "start batteries", and ordinary vehicle systems (lights, horn, etc.).
2) A bank of "house batteries" dedicated to feed power for camping needs.
3) And inverter set up to take the DC power from the house batteries and convert it to AC 120V. A useful inverter will have a high-tech charger set up to provide the charging profile for the house batteries.
4) A shore cord and switching system to provide "plug in" power to run the house system and charge batteries (note, this system will charge both house and start batteries when parked and plugged in, with the proper setup).
5) A generator, to provide sufficient power to run everything when parked for periods of time and plug in power isn't available.

Now, here's the first question that we're always asked -- "Isn't that EXPENSIVE, complicated, heavy, hard to maintain, time-consuming?" The only honest answer to that is yes. We do it because we've found that we need it.

Once you have the full system, you cover all the bases. You have power for all your needs (and the little things, a water heater - even a small one, for showers, dish washing, etc. uses lots of power, a TV, a coffee maker, an on-board water pump, a hair dryer, charging cell phones and laptops -- and you'll notice we haven't even begun to talk about air conditioning yet).
You should begin to think in watts. Systems are designed in volts, components are designed in amps, but the big picture is watts. Look at what you want to power in a day and total up the watts used and the time you'll need it. If you don't design for those loads from the start, you'll always be playing catchup and you'll be miserable until you finally do.

Yes, the start batteries on a bus are made to start a big engine with a heavy starter that needs lots of power. A number of people have calculated how long you can run a small air conditioner off a bus's start battery combo when the engine isn't running (I say combo because there are usually 2 or more large batteries tied together as the start battery system). The calculated answer comes out to being 25 - 40 minutes before the batteries are drained so much either they're damaged and won't take a full recharge again, or, they're so flat they won't start the engine again -- or BOTH. So, a simple question "can I run an air conditioner off the alternator and batteries on my bus engine?" isn't so simple. One answer is "Yes, you can ... if you don't use any power for reading lights, cooking, pumping and heating water, heating or air conditioning, etc. while the bus engine isn't running." Another shorter answer is "Not really, in a practical sense".

OK, blah blah blah ... I'll shut up now about "big picture" design. But I feel I have to state an agreement with the good details people have given you about inverters. Look on the Web and you'll see ads "3000 Watts, 7000 Surge Watts, Pure sine wave -- $250". Look VERY closely at such items. If you want a practical view, take a few minutes to look at this YouTube, from an experienced electronics designer and industrial electrician:

https://www.youtube.com/watch?v=jQafeM6vnFQ&t=886s

This is an inverter of the type made in China; they're the same inside and sold under the AIMS, PowerBright, Cobra and similar brand names. It's cheap, but it doesn't do surge, it doesn't do 5000 watts - in fact, it will only do about 1000 watts continuously and even then, the quality of the output is not very good. An inverter like this is a perfect example of what you should NOT spend your money on. It also does not have a built in charger. Don't waste your money on a poor quality, poor output inverter. Just say no and walk away -- or run if you have to.

Many people have mentioned Magnum inverters, for good reason. Great quality, genuine output (although I wouldn't put more than 80 - 85% of the rated watts through any inverter for a continuous period of time). A Magnum will also give you reasonable start-up-surge power for an air conditioner and that's a big thing; air conditioners are notorious about needing lots of power to start and they're picky on the power quality.

A final thing, people sometimes think "batteries are easy, you use them and charge them up". That usually means that the batteries die in short order. Batteries that are used for large loads need a very specific and tightly controlled charging regimen; if they're not charged to the right profile (controlled amps at moderate voltage for the big charge, a higher voltage with lower amps for "finishing", and a float to maintain charge), they'll rapidly fail. A 150 pound, $300 paperweight is nobody's idea of a good thing, especially if you have to replace 4 (or even of them. It *is* possible to get the proper charge profile from an engine alternator output (by adding a separate voltage regulator system), but part of the important part of this system is that it needs to be continuous and carried on until the perfect final charge is completed. That's hard to do with an engine-driven alternator.

Sorry to throw a lot of things at you, but when you get into the real-life aspects of electricity on a converted bus, there are lots of things to cover and consider. Keep working through what you'll need and stay in touch.

Hope this helps, BH

Well said! This sounds just like my set up. I have a question about the OP (original poster) He said he would turn the inverter off when connected to shore power. I don't do that because I was under the impression that the inverter is helping recharge/maintain my house batteries. Am I wrong in thinking that or is that just a specific set up I have.

bevans6 · June 28, 2018, 12:59:13 PM

That is the setup you have. An inverter does not charge batteries, it converts DC to AC volts. The OP is contemplating an inverter that does not charge in addition to the inverting (why they named in "inverting" I have no idea, it always sound stupid to me when I get into these explanations), so since it does nothing but supply AC, if there is another source of AC you disconnect it (manually or with a switch) and turn it off. The OP contemplates a battery maintainer or charger in addition to the inverter, which he would turn on when parked and on a pole. We are suggesting a combined unit, of course, that is both an inverter and a charger - the Magnum 4024 is the example, and even it turns off the inverter when the charger is turned on, and vice versa.

There is third type - the hybrid inverter/charger. If it's on a pole and is so configured, it keeps the inverter on while the pole is supplying power and the charger is operating. If it senses the need for power that exceeds the pole supply, it automatically stops charging and starts inverting essentially instantly, so that the inverter supplies the excess load required for hopefully a short duration. It draws current from the batteries to supply that load, so all the rules about battery life apply.

Brian

jraynor · June 28, 2018, 03:59:43 PM

BH,

Thank you for the thorough response. I calculated 4kW which was rounding up from a calculation of running a few items. My calculations we're based on the max each item would draw so that worst case, the inverter would still support it all. And these would only be during over the road. Otherwise it will be shore power or Genny.

Branderson, exactly as Brian answered. I wasn't aware of inverters that changed to charge mode when shore power is connected so I would have just cut it off since I have the AC shore power.

How it seems right now, I plan to continue with my design and later I can add the house batteries. I'll have to replace my start batteries soon so that cost will be enough at that time so I'm trying to maximize progress with minimum costs without sacrificing quality. The house batteries I can plan where to add them now and but them when money is better flowing.

I don't plan to run any A/C units with the inverter so the only large load is the fridge.

Richard5933,

I plan to setup my electrical in the front bay to limit the length of the cables.

1 more question, what type cable have y'all used? Battery cable type or THHN like you can get from Lowe's or HD

richard5933 · June 28, 2018, 04:20:40 PM

Quote from: jraynor on June 28, 2018, 03:59:43 PM
Richard5933,

I plan to setup my electrical in the front bay to limit the length of the cables.

1 more question, what type cable have y'all used? Battery cable type or THHN like you can get from Lowe's or HD

Not sure that we're talking about the same things, but maybe we are. Your chassis battery bank's main function is starting the engine, so it is located near the engine and connected to the starter/alternator with some pretty beefy cables.

The inverter pulls quite a bit of current from the batteries, so it needs to be located near the batteries as well.

It sounds like you're talking about running the inverter off your chassis batteries/alternator. If you are putting your electrical bay in the front, then will you move your chassis batteries there as well? If so, then you'll have quite the voltage loss getting the power back and forth between the engine compartment and the batteries. Better to leave the chassis batteries near the engine.

If you are putting your electrical bay in the front but leaving the chassis batteries in the rear, then you'll have a long run of battery cable between the inverter up front and the batteries in the rear.

The 120v output from the inverter, however, can run on rather minimal wire sizes, just like the wiring in a house. Put the inverter near the batteries, connected with heavy cables, and then run the 120v to the electrical bay with the proper 120v wiring.

Now, if you are also installing a set of house batteries and putting them up front, that makes sense. Then you can leave your inverter up front near the house batteries. Sounds like you might be there down the road, but for now it sounds like you'll be powering the inverter from the chassis batteries.

Regarding wire type...I have battery cables for everything in the engine bay and the chassis battery box. For the house batteries, inverter, etc. I used welding cable. It's easier to work with, but apparently doesn't hold up in dirty/greasy/hot conditions like the engine bay. For the smaller 12v circuits I used multi-strand twisted wire rated appropriately for the voltage/amperage.