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Old 01-11-2020, 12:13 AM   #1
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Default Under hood Generator

I am wondering what advantages/disadvantage there are with under hood generators? We had a R-Pod so not an issue with trailers. How long can your frig run? a/c? tv? Without running your van? We are in process of buying camper van and not sure if the new underhood generator is the way to go? Thank you.
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Old 01-11-2020, 12:45 AM   #2
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I am wondering what advantages/disadvantage there are with under hood generators? We had a R-Pod so not an issue with trailers. How long can your frig run? a/c? tv? Without running your van? We are in process of buying camper van and not sure if the new underhood generator is the way to go? Thank you.
Sounds like you need to go on youtube and watch some reviews of class b rv's with under-hood generators (UHG's). Then you'll have a better understanding of their benefits and disadvantages (everything is a compromise in the rv world).

I'm no expert, nor do I own an rv with an under-hood generator (UHG), but let me try to lay out a few basics:

1) UHG's are needed to charge large lithium battery banks that would fry a standard engine generator at such high charge rates.

2) UHG's can produce equivalent power output to a typical Onan rv generator. But . . .

3) UHG's do not produce any power unless your chassis engine is running, so there is no "without running your van" in this equation.

4) UHG's typically do not exist and are not needed for most rv's with the conventional two lead batteries which charge fine from a standard engine generator.

So, UHG's are just another type of generator that runs off the van's engine and only while the van engine is running. But, UHG's are a relatively new idea that work well in certain applications (See #1 above).
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Old 01-11-2020, 01:03 AM   #3
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Mercedes certified upfitters are only permitted to take a max of 40 amps from the chassis electrical system, which isn't enough even for a medium-sized lead-acid coach battery. This fact is one of the main drivers for the introduction of UHGs. With modern vehicle electrical systems, it is a VERY good idea to keep your coach system completely isolated from the chassis electrics. A UHG permits this.

I have one and would NEVER own a van without one, no matter what size my coach battery. However, a large battery (Li or lead-acid) combined with a UHG and a good inverter/charger and you can really start living the dream.
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Old 01-11-2020, 04:31 AM   #4
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I also will never own a Class B RV without a second alternator (ie. underwood generator) and I don't even have lithium batteries. I can simply drive 20-30 min per day and completely recharge my AGM batteries and never need to bother with a loud and obnoxious generator. This allows me to run my refrigerator all day, use my lights, TV, computer, and all the other electrical components of my RV on battery power. I also can carry a spare tire where the worthless generator would have to be installed.

The second alternator is probably the best feature of my RV.
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Old 01-11-2020, 05:56 AM   #5
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I love my UHG. No lithiums for me... just my two Group 31 AGMs. Also have 270w of solar. And I can pretty much boondock for as long as I want. (the tanks would be the limiting factor) Of course if I want to run the AC, I just turn on the engine... or I could if I hadn't pulled off the AC and added a 2nd fan. It charges up my batteries pretty quickly if needed. Or if I want to use the microwave, I just turn on the engine.
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Old 01-11-2020, 07:54 AM   #6
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If you have your refrigerator on high how long would you guess it would run without starting your rig? Can you have the frig running at the same time you have tv on? thanks for your thoughts.
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Old 01-11-2020, 12:57 PM   #7
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If you have your refrigerator on high how long would you guess it would run without starting your rig? Can you have the frig running at the same time you have tv on? thanks for your thoughts.

Most of the frigs they use currently will use in the 20-50 amp hours of batteries a day, depending on weather. The rest of the power use is very variable between users as there are lots of ways to use power like lights, TV, recharging or using laptops and phones, even cooking. Most of us probably fall in 20-40 amp hours a day of other use, but it can be more or less than that.



Most units come with at least 200 amp hours of battery capacity so at 80% use you get 160 usable. That would make it so most folks would be able to 2-3 days without running the engine or driving, but again could be more or less than that.


Best if probably to make a list of what you would be needing/wanting power for to calculate what you would be using per day.


A modern compressor frig will only use 3-5 amps when running so will not really affect anything else like lights or TV at the same time.



If you are in the mid to lower power use ranges, adding solar can make it so you no driving periods can be much longer unless the sun conditions are horrible. We have 300 watts of solar and use in the 40-60 AH per day most of the time so if we get good sun even 50% of the time we would almost never have to run the engine. Of course, you will need to go dump tanks and get water regularly anyway, so drives do happen. We need to dump every 5-7 days most of the time.


I will add one point to jrobe's comment on driving 20-30 min per day to recharge his AGM batteries. Driving the length of time can easily generate enough power to cover a day or two use as he mentions, but it will not totally fill AGM batteries because of the very long (4-6 hour) time it takes to do the last 15% of the charging to get totally full. For most of us, it is not a concern in common use as all it does is reduce or usable capacity by about 20% if we don't get full batteries every drive. If you are close on use, you can lose a day, though and need to allow for that. You do need to get a good, long, full charger every 7-10 charge cycles to take good care of the batteries. Solar can be a big benefit in all this as the UHG will get you to 85% full batteries quickly, and then the solar can finish the rest as it is slower and longer anyway. Lithium batteries will get full quickly on the UHG as they don't have as long an end charge time.


There are lots of discussions on the forum here about power use and generation, so reading a few of them would certainly be beneficial and you will find many different opinions and use styles from the various members.
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Old 01-11-2020, 03:16 PM   #8
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There is a lot of good information on all the systems in a Class B RV on Sportsmobile's website. This is a great place to start educating yourself. For example, this webpage has a nice chart listing the power requirements of RV components including refrigerators.

https://sportsmobile.com/solar-panels/
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Old 01-11-2020, 04:35 PM   #9
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If you have your refrigerator on high how long would you guess it would run without starting your rig? Can you have the frig running at the same time you have tv on? thanks for your thoughts.
With decent sun... I can run the fridge indefinitely based on my usage... and I have a good sized 12v compressor fridge. I don't run it on high, but on normal required temps.

I don't have a TV since I never watch it. That would depend on whether you have a 12v TV or whether you have to run it on the inverter, which is a power hog. Many Roadtrekkers with a 110v TV add a smaller inverter to run it.

As an example for me... I tend to visit friends and relatives and driveway camp in nearby WI. I will have one of the fans running all day with the fridge on. In the afternoon, I will turn on the inverter for an hour or so and charge up my laptop and phone. Then in the evening, I will be using a few lights. If it is still chilly at night, I might plug in my 12v mattress pad. With that usage, my reading at the gauge during my middle of the night trip has never been below 12.3. So, I have never had to start the engine because I normally move on to the next person on my list in 3 or 4 days and the drive tops everything up. Thus I have never tested it beyond that length of time.
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Old 01-13-2020, 06:17 PM   #10
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We join the 'wouldn't own a Class B without a 2nd alternator' group. But this 'need' is based on our decision to emulate home-life while traveling. Translated . . . this means that we use lots of energy, not infrequently 3kwh/day. And while solar can supply our needs on a perfect day - - there are too many imperfect days. And while, further, there's shore power - - there are economic and other objectives that render this option 'spotty'.

A 2nd alternator is a powerful energy source (e.g. 3kw) that fills inevitable energy 'voids' and, when coupled with a good sine wave inverter, eliminates the oft-maligned Onan (etc) generator.
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Old 01-17-2020, 12:22 AM   #11
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Not sure I understand. I have a 2011 PW Excel and my alternator provides 80 to 90 amps to my house battery during a charge cycle as indicated by a Victron Battery monitor. Wonder why Mercedes would limit the alternator to 40 amps.
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Old 01-17-2020, 12:46 AM   #12
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Just a guess, but my Sprinter seems to do about five minutes of diesel exhaust treatment work after I shut it down. I would imagine that it takes a bit of battery power to make that work and my thought is that they have average run time/recharge time graphs that say they need to limit external amperage usage so the battery generally maintains a certain level. Again, just a guess.
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Old 01-17-2020, 12:49 AM   #13
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Avanti is probably up on it more than the rest of us, but it has to do with the energy saving charging protocol and the control of the alternator by the MB systems. If there is power being used to run the alternator, that they don't know about because it isn't going anywhere it recognizes, the system doesn't like it.
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Old 01-17-2020, 12:50 AM   #14
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Not sure I understand. I have a 2011 PW Excel and my alternator provides 80 to 90 amps to my house battery during a charge cycle as indicated by a Victron Battery monitor. Wonder why Mercedes would limit the alternator to 40 amps.
Because these are highly-engineered systems, and just because something happens to work in a given situation does not mean that it will work in all situations or that it is safe. Moreover, restrictions like this are often imposed on upfitters in anticipation of future engineering modifications.

Your MY2010 or 2011 Sprinter has an OM642 V6 engine. Although it has a LIN-controlled alternator, the LINbus is not used for much in this engine. It is mostly a "dumb" charging system. As a practical matter, experience has shown that there seems to be little harm in fudging this spec on this engine.

However, starting in MY2014 and the OM651 I4 engine, Mercedes introduced a "smart" charging system that includes a shunt and other sensors on the battery. These sensors are used to drive proprietary algorithms intended to improve performance and save fuel. For example, the LINbus is used to throttle back charging during acceleration in order to off-load the engine. Such systems also often "save" some battery space in order to harvest power during down-hill coasting, and so on. These algorithms necessarily make assumptions about external loads, both in number of amps and where in the system those amps are taken from. The "40 amp" limit is simply a constraint applied as part of the engineering design process.

I tried charging my 440AH AGM battery on my I4. The results were not good. That is when I installed my second alternator.
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Old 01-17-2020, 01:32 AM   #15
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So a question about your second alternator...I owned a trawler that, when I bought it, had a second alternator installed but not connected, and my understanding was something to do with balancing the pull on the crankshaft. It seems that after the previous owner installed the second alternator they heard that Cummins said it could cause excessive wear on the crank bearings. So are these UHG units balanced on the crankshaft and blessed by Mercedes or is it that nobody worries about it. I have no dog in this race, just trying to relate to my previous experience.
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Old 01-17-2020, 02:00 AM   #16
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So a question about your second alternator...I owned a trawler that, when I bought it, had a second alternator installed but not connected, and my understanding was something to do with balancing the pull on the crankshaft. It seems that after the previous owner installed the second alternator they heard that Cummins said it could cause excessive wear on the crank bearings. So are these UHG units balanced on the crankshaft and blessed by Mercedes or is it that nobody worries about it. I have no dog in this race, just trying to relate to my previous experience.
Sprinter engines are engineered for power takeoff. You can buy the bracket from MB.

Everybody buys their setups from:

https://www.nationsstarteralternator.com

Contact them for details for your rig. They are very good.
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Old 01-17-2020, 02:12 AM   #17
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Sprinter engines are engineered for power takeoff. You can buy the bracket from MB.

Everybody buys their setups from:

https://www.nationsstarteralternator.com

Contact them for details for your rig. They are very good.

I think they do have a horsepower limit on the take off, though, but it is higher than most of big alternators use.
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Old 01-17-2020, 02:19 AM   #18
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Good information, thanks.
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Old 01-17-2020, 01:32 PM   #19
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Not sure I understand. I have a 2011 PW Excel and my alternator provides 80 to 90 amps to my house battery during a charge cycle as indicated by a Victron Battery monitor. Wonder why Mercedes would limit the alternator to 40 amps.
Second alternators are nearly mandatory as a practical measure when the engine alternator charges at the rate of 40 amps, or even 80-90 amps as yours if you have lithium ion battery banks greater than 400ah or 800ah such as I have. Do the math in how much driving you desire to do in a day. If you desire to stick to the popular 330 or drive 3.5 hours per day or stop at 3:30 PM 40 amps can replace only 140ah. Solar on a B can provide a smaller negligible percentage of recharging per day, you can't run an Onan generator long enough for all practical purposes (by CG regulation, location or how much you can stand the noise) to recharge a battery bank. You thus are tethered to shore power campgrounds thus negating the need for high amp lithium battery banks. Shore power limits you to where and when you can camp.

High amp battery banks allow you to use your B electrically everywhere such as boondocking in a Walmart parking lot as transparently as if you were plugged into shore power. Every 120a outlet is hot with an inverter turned on, you can use your microwave or convection oven all the time, my Keurig coffee pot is at the ready anywhere, anytime and makes two cups of coffee while taking the time needed filling up fuel at a service station or a rest stop break, large compressor refrigerators are the norm, electric induction cooktops and instant pots can be used freely. What the heck, I have articulating beds that convert to lazyboy loungers at the ready with a vibrator mode to relax after a long hike.

With the second alternator coupled with high amp battery banks, of course you tend to take advantage of your batteries and use them profligately with no need to conserve or monitor closely. Some can reach the holy grail of the luxury of boondocking with air conditioning. It frees you up in completely eliminating propane, Onan generators and solar panels on your roof. With a small B eliminating those items can free you up in weight and space for other things. The roof trends are more active use such as rooftop decks, more skylights, and toys like kayaks instead of a deck of solar panels needing unobstructed sunlight.

As long as you drive at least every three days as most Class B's do you can live like this all the time. I use my B almost every day anyway for driving to a trail head, touring, going some place or just stocking up at a store. Not being tethered frees you up.

Everyone talks about this. The engine idling aspects and fears are overblown and over sold. I just checked, in over 85,000 miles and four years of travel, I have 17.5 hours idling on my van (it's monitored) and I can guarantee you most of that is testing it or demonstrating the capability to others.

That is what I have had for 4 years in an extended body 24 ft. Sprinter. Now I want that same capability in a 144 WB five feet shorter Sprinter (19'-5") on a 2500 chassis. You can't achieve that capability without a second alternator and lithium ion batteries with a 2500 chassis.
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Old 01-17-2020, 02:08 PM   #20
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Now I want that same capability in a 144 WB five feet shorter Sprinter (19'-5") on a 2500 chassis. You can't achieve that capability without a second alternator and lithium ion batteries with a 2500 chassis.
Sorry - slightly off topic: Davidd - I enjoyed following your last build online. It sounds like you are starting on a new van. Are you documenting this online? I'd love to see what you're up to !
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