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Old 02-18-2019, 04:15 PM   #1
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Default UHG - why a separate alternator?

Why are manufacturers installing a second alternator into vans as part of the “under hood generator”? Why not use the feed off the existing alternator?

Prior to our current class b, I built my own van conversion based on a Ford Transit. When ordering the transit I opted for the heavy duty alternator for the extra amperage - I think it was 250 amp output. This is more than sufficient to charge the chassis and house batteries and any other loads.

Then there are 5-stage charge managers that will isolate the house batteries from the chassis when the voltage is below 13.5v (meaning the engine isn’t running).

Anyway - seems easier than adding a second alternator.
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Old 02-18-2019, 04:40 PM   #2
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Several reasons are commonly stated.


A lot of modern vehicles have the alternator controlled completely by the vehicle computers and do thing like charging while coasting, shutting down other times, and who know what else. They also measure amps going to the starting battery and probably compare it to the alternator output after figuring in what else is on, so could set codes or give other issues like turning way down and hot charging well.


Factory alternators are often not considered reliable at long, high output, amperages. Most that I have seen will get too hot after about running at 40% output for a while.


Mismatched battery types can mess up the coach charging profile, so a standalone can have a regulator of it's own to take better care of them and charge them more efficiently. Starting batteries are often hit hard for a time at the beginning of the charge and then the charging backs off a bunch.



You can get higher output from the standalone at 280 amps, if you need that much (although you won't net that much over time).


If you do cook the alternator with the heavy charging, at least you can keep driving.


A lot depends on how much battery bank you have and how fast it will charge. If you have bank that will only accept 80 amps, a good sized stock alternator will likely be fine for a long time. If you have a bank of lithiums that max out the stock alternator for hours at a time, I think you would be having durability issues fairly soon. In between could go either way depending on a lot of things.
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Old 02-18-2019, 04:47 PM   #3
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Not to mention, you can go with a 48 volt system at a much lower amperage, eliminating the need for cooling, as well as being much smaller and saving weight and engine load. This is what the Volta system is doing.
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Old 02-18-2019, 05:05 PM   #4
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For the reasons Booster stated, the OEMs place severe restrictions on the currents that an upfitter is allowed to draw from the vehicle power system. For example, the Sprinter BodyBuilder's manual states that current used for charging additional batteries must be actively limited to 40 amps. You as an individual owner may choose to ignore such guidelines (although this is increasingly a bad idea). But, Mercedes certified upfitters are contractually required to follow the guidelines.
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Old 02-20-2019, 05:33 AM   #5
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Originally Posted by wincrasher View Post
Not to mention, you can go with a 48 volt system at a much lower amperage, eliminating the need for cooling, as well as being much smaller and saving weight and engine load. This is what the Volta system is doing.
That's one of the reasons I like the Volta system... use a 56V alternator and you need much less current. Using 12V alternators for lithium systems baffles me... you'd need giant wires and there would be a lot of heat.
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Old 02-20-2019, 10:49 AM   #6
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Thanks - makes sense.

As far as the 48V or 56V setups - also makes a lot of sense, but the whole system needs to be spdesigned for that voltage. The lithium cells need to be built/grouped to 48 or 56 V, the inverter/charger, and any DC loads (fridge, a/c unit, etc) or just run everything off the inverter 110 (so the inverter runs all the time) or maybe a DC/DC converter to get 12Vdc for all those 12V loads like leds lighting, maxxfans, etc
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Old 02-20-2019, 01:06 PM   #7
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Thanks - makes sense.

As far as the 48V or 56V setups - also makes a lot of sense, but the whole system needs to be spdesigned for that voltage. The lithium cells need to be built/grouped to 48 or 56 V, the inverter/charger, and any DC loads (fridge, a/c unit, etc) or just run everything off the inverter 110 (so the inverter runs all the time) or maybe a DC/DC converter to get 12Vdc for all those 12V loads like leds lighting, maxxfans, etc
Yes, the Volta system uses a DC to DC converted to handle 12v loads..
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Old 02-24-2019, 04:24 PM   #8
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That's one of the reasons I like the Volta system... use a 56V alternator and you need much less current. Using 12V alternators for lithium systems baffles me... you'd need giant wires and there would be a lot of heat.
Yes, we do the exact same thing for our larger auxiliary power units as well.

Many advantages.

For smaller systems (1 kW class) , we still use 24 volts internally.
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Old 02-26-2019, 04:51 PM   #9
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If I limit this discussion to Promaster based vans I'd say I've seen more problems with the UGH than benefits. Not worth the trouble.

I recently added a 240Ah LiFePO4 battery to my Promaster van in parallel with the camper's 105Ah AGM battery and so far the upgraded 220A stock alternator has been fine. My Lithium battery charges to about 90% in about 30 minutes of driving. So for my electrical needs (24hour Alde furnace, fridge, lighting, microwave for meals and electric kettle for coffee in the morning) this battery setup is fine.

If you are trying to run an A/C off the grid for hours at a time I could see the value of a UGH. Oh, by the way the stock upgraded Promaster alternator had no problem running the rooftop A/C via the 2000W inverter so I don't think my Li battery is putting any more load on the alternator than that was.

As an aside, I just took the A/C unit off this week - I almost never used it and prefer a Maxxfan anyway.
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Old 02-26-2019, 05:06 PM   #10
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If I limit this discussion to Promaster based vans I'd say I've seen more problems with the UGH than benefits. Not worth the trouble.

I recently added a 240Ah LiFePO4 battery to my Promaster van in parallel with the camper's 105Ah AGM battery and so far the upgraded 220A stock alternator has been fine. My Lithium battery charges to about 90% in about 30 minutes of driving. So for my electrical needs (24hour Alde furnace, fridge, lighting, microwave for meals and electric kettle for coffee in the morning) this battery setup is fine.

If you are trying to run an A/C off the grid for hours at a time I could see the value of a UGH. Oh, by the way the stock upgraded Promaster alternator had no problem running the rooftop A/C via the 2000W inverter so I don't think my Li battery is putting any more load on the alternator than that was.

As an aside, I just took the A/C unit off this week - I almost never used it and prefer a Maxxfan anyway.



You must be starting at something like 50-60% to get to 90% in 1/2 hour?

If you have a battery monitor in the system, it would be interesting to see what the charge amps really are, or put a clamp on meter at the alternator output. Same with using a an infrared temp gun on the alternator case to see where it is running. Balmars, when used turn down at about 220*F for reference. I can't say for a Promaster alternator, but the stock alternators I have had experience with struggled to get above about 50% output for more than maybe 15 minutes without getting too hot.

I think any kind of information and data we can get is good for others who may be thinking of doing the same or similar, so they know what to look for. Did you increase the size of the cable to batteries when you added the lithium?
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