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Old 03-03-2014, 04:36 PM   #1
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Default Debate about whether to DIY, hire an electrician or find an electrical engineer

On an ancillary note, not quite off topic but nearly so...

I added a 2500 watt inverter to an older camper that I own, a 1985 Chevy driven class C camper with a V8 engine.

It has a very large house battery(nearly amidships, just in front of the rear axles, and a truck battery under the hood.

I believed the inverter was well over sized for the load, and that the camper construction and wiring was adequate. I did use just a couple feet of very heavy cable for the run from the house battery to the inverter, both positive and negative because the camper box included wood and aluminium framing, and I did not want to involve the truck frame.

The output of the truck alternator, and the AC shore system that charges the house battery when plugged into 120 vac or 240 vac could all handle the charging of the battery.

I also knew that at max draw, the current pull from the battery would be greater than the recharge rate, but thought it would give the required time...

Then I took some teenagers and loaded this system up...

I drove home one evening and I swear that every light and component in that camper was on. I felt a bit awkward with this traveling light house, with the multiple televisions and video games running...

And of course, I overheated and shut down the alternator...

The inverter could indeed pull more current than could be returned to that battery, but at the same time, the charging system did try.

I stopped mid trip, shut down their 120 volt entertainment, and put them back on a 12 volt limit. (only 12 volts were available to them for the rest of the trip)

I have since tripled the size of that alternator, or at least the output. I did replace the belt, as I thought it prudent, but even though this is a camper and not a minivan, the process to purchase a high output alternator and confirm the vehicle's capacity to use it, was pretty easy to do with some internet help.

The pulleys, the mounting procedures, the wiring and the rest were all pretty easy with a bit of research. I generally agree with most parties here in that most of the perspectives are a bit single visioned, but most are also saying mostly the same things. Seems all the major points are close enough so most of the arguing is over the smaller points.

We do not really know the depth of knowledge of the OP. There have been many suggestions and potentials offered up, and the only one that can decide if the optimum plan for the OP's needs has been presented is the OP themselves.

Good luck to all. I eventually figured it out, and if I could do it, I am sure that most others people with any moderate evaluative and technical skills can do it too.

Proper precaution need to be taken, but this is not quite as hard as bringing people to the moon...
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Old 03-04-2014, 08:52 AM   #2
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The best advice I can give Segwaythedoor is to get a qualified electrical engineer to design and build the system for you.

Although you are getting a lot of advice from this site, you need an engineer to work through the details, to ensure that the system not only functions properly, but is safe for you and your workers. Providing any other advice to a person who is obviously has little electrical background is inappropriate, in my opinion. An on board charging system requires not only high-current DC, and 110 AC. There are a lot of people who tinker with electricity, but you are running a business, in Wisconsin, with USA liability laws.

Seriously, get a qualified engineer before you either burn your van/trailer to the ground or kill someone.

Perhaps I missed it, but isn't there still the question of whether any charging system can fulfill his requirements? Based on his statement, the worst case is 1 bar remaining, 30 minutes to charge, then a 6 mile tour. We don't know anything about the characteristics of the tour. Hilly uses more power than flat. Fast uses more power than slow. Nor do we know anything about the quality of the batteries.

The rate of charging is dependent on the Segway on-board charger, and, as I recall, the state of the batteries, including temperature. It would be really inconvenient for Segwaythedoor to spend a boatload of money and STILL not be able to meet the need.

Some experimental testing might be in order to determine whether the 30 minute charge is capable of doing what Segwaythedoor wants. Again, get an electrical engineer.
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Old 03-05-2014, 09:37 PM   #3
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I certainly agree in principle that having a competent engineer design the system is the best advice.

However, I disagree with the following:

Quote:
Originally Posted by Civicsman View Post
... Providing any other advice to a person who is obviously has little electrical background is inappropriate,...
I think that it is unlikely that answering Segwaythedoor's initial question with "That won't work, and go pay an engineer" would be viewed as a friendly answer.

What was done here (along with a lot of other stuff) was to point out various alternatives, and various tradeoffs, and unanswered factors. In other words - why it isn't simple.

But apparently Segwaythedoor may not be coming back to tell us anything more.
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Old 03-06-2014, 12:43 AM   #4
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Quote:
Originally Posted by Civicsman View Post
The best advice I can give Segwaythedoor is to get a qualified electrical engineer to design and build the system for you.

Although you are getting a lot of advice from this site, you need an engineer to work through the details, to ensure that the system not only functions properly, but is safe for you and your workers. Providing any other advice to a person who is obviously has little electrical background is inappropriate, in my opinion. An on board charging system requires not only high-current DC, and 110 AC. There are a lot of people who tinker with electricity, but you are running a business, in Wisconsin, with USA liability laws.

Seriously, get a qualified engineer before you either burn your van/trailer to the ground or kill someone.

Perhaps I missed it, but isn't there still the question of whether any charging system can fulfill his requirements? Based on his statement, the worst case is 1 bar remaining, 30 minutes to charge, then a 6 mile tour. We don't know anything about the characteristics of the tour. Hilly uses more power than flat. Fast uses more power than slow. Nor do we know anything about the quality of the batteries.

The rate of charging is dependent on the Segway on-board charger, and, as I recall, the state of the batteries, including temperature. It would be really inconvenient for Segwaythedoor to spend a boatload of money and STILL not be able to meet the need.

Some experimental testing might be in order to determine whether the 30 minute charge is capable of doing what Segwaythedoor wants. Again, get an electrical engineer.
You really don't need to be an electrical engineer. There's a world of difference between an electrical engineer, and an electrician, and this is well within the realm of an electrician.

In fact, I wouldn't trust a lot of electrical engineers with this. In fact, I recall one exam question in particular, to this day, which I got a zero on, because I was presented with the choice of a solution that would not operate the equipment, or one that would work, but set the building on fire even faster than the non-solution.

The prof obviously had never read the electrical code. I had, and knew exactly how stupid the question was. If he wanted to test my ability to handle an impedance matching problem, he should have just asked a sane impedance matching question, instead of trying to apply impedance matching to power distribution systems, where it is entirely inappropriate.

So that's an example of an MIT electrical engineering professor I would not trust to wire a light switch. (Which reminds me of another story, but...)

But it's really not that hard a design problem, technically, if you're willing to be careful and thorough. You need to figure out how much power you need, for how much time. Wiring sizes you get from tables. That's a matter of established safety standards, not an electrical engineering problem. (The standards are based on the engineering, but you don't repeat that process on your own. Follow the standards).

Alternators have spec sheets. Anybody capable of even contemplating such a project is capable of calculating pulley ratios, and designing a ratio that keeps the alternator within RPM limits when the engine is at its RPM limit.

Battery capacity is amperes * time. Power is voltage * current. If you're following the standards, you don't even need Ohm's law.

While such a system clearly needs some careful design work, and you have to deal with tradeoffs, uncertainties, safety standards that may be difficult to locate, etc....

I firmly believe that basic engineering is within the reach of pretty much anyone willing to put in the time and effort.

You do make some good points, however. In particular, liability exposure (which is one reason I harp on standards rather than what you can make work).

And the importance of ensuring the solution will meet the operational need, before going to all that trouble! Excellent point, that.
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Obviously, we can't have infinite voltage, or the universe would tear itself to shreds, and we wouldn't be discussing Segways.
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Old 03-07-2014, 08:50 AM   #5
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I understand the difference between an engineer and an electrician.

I too have worked with engineers who are more academic than real-world, but I've also known many electricians who are not capable of designing a system to do this.

In general, an electrical engineer is necessary to translate the customer's needs to an electrical design. An electrician takes the design and implements it.

The quality of both engineers and electricians varies. Some electricians are more knowledgeable than others and could surely do this.

From the liability perspective, you'll need an engineer to do the design work.
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Old 03-08-2014, 02:48 AM   #6
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Quote:
Originally Posted by Civicsman View Post
I understand the difference between an engineer and an electrician.

I too have worked with engineers who are more academic than real-world, but I've also known many electricians who are not capable of designing a system to do this.

In general, an electrical engineer is necessary to translate the customer's needs to an electrical design. An electrician takes the design and implements it.

The quality of both engineers and electricians varies. Some electricians are more knowledgeable than others and could surely do this.

From the liability perspective, you'll need an engineer to do the design work.
I will agree, that some electricians couldn't handle this. But they SHOULD be able to.

The electrical issues here aren't covered in an electrical engineering program. They're covered in your freshman physics class, second semester.

If you take things to much higher power distribution levels, you get into power distribution systems, which is a EE specialty. Or if you want to design the equipment -- connectors and such, that's something you'll need to be a EE to do, with a strong background in materials.

And yes, the guy who designed the original vehicle electrical system was no doubt a EE as well.

But an electrician applies standards and off-the-shelf components to solve real-world problems. This falls somewhere between that, and Electrical Designer, at the low end of the spectrum, at that.

I think that if you take it in the direction of NEEDING an electrical engineer, you've already lost the liability issue. What's needed is NOT an electrical engineer, but a solid knowledge of the applicable STANDARDS, and designing the system to stay well within those standards.

That's what the National Electrical Code is all about. The NEMA standards.

Treating this as a general electrical engineering problem is a mistake, IMHO, from a liability standpoint.

That's why I've tried to draw a clear line in areas where I do not know the applicable standards, as the transportation aspect adds factors I have no specific knowledge of.
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Obviously, we can't have infinite voltage, or the universe would tear itself to shreds, and we wouldn't be discussing Segways.
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