Alternator to power roof AC while in motion

[Brad Hoffman]

Hi All,

I'm working on converting a Sprinter van into a luxury road cruiser and occasional camper.  I have no plans for boondocking so no solar.  I have installed a 110v Dometic roof AC unit.  My plan is to power it with a secondary alternator which I've already installed.  I installed a Nations 280 amp alternator to power my Victron Multiplus 3000 watt inverter.  This will power the AC unit.  Since we won't be doing off-grid camping I just have 2 100amp AGM batteries for LED lights and such.  

The question is how to wire this.  Can I run the alternator directly to the inverter?  Do I run the alternator to the batteries and then to the inverter?  If I do that will my alternator be limited to limited voltage because of the batteries?  The Nations alternator was specifically designed to charge the AGM batteries but I'm not sure if it will know to send additional power for the inverter draw.

HELP!!!

And thanks from chilly MN

[Star Dreamer]

Typically you wire the inverter to the batteries and wire the alternator to the batteries to recharge the batteries while the engine is running. Sizing of the wiring between the various items is critical and should be sized to handle the load. 

The alternator typically will only put out the maximum voltage it is designed for and what the batteries can handle, typically around 13.8 to 14 volts DC. You do not mention what the startup amps are on the AC unit. That is usually the limiting factor on if an inverter can power it. Installation of a soft start on the AC unit will help that. The amount of amp hours on the batteries will determine how long the AC will run which of course will be extended by the alternator recharging the batteries. But if the wire size between the alternator and the batteries is too small, it won't recharge them fast enough and eventually the inverter will shut down due to low voltage.

[Chad Heiser]

You cannot wire the Multiplus direct to the alternator.  A 3KVA Multiplus should be fused at 400 amps and requires very large wire to power the DC side of it as a result (typically 4/0).  

Your two batteries are not really enough to fully power a 3KVA Multiplus either.  The recommendation is 400 amp hours of available power, but it can be done with less if the AC loads are not excessive.  Additionally AGM batteries (especially with only two in the system) are not good for powering high draw continuous loads.  Their discharge curve will result in excessive voltage drop as the battery discharges and will cause the inverter to shutdown (due to low voltage) before the batteries are actually discharged (to 50% state of charge).  Most people running high draw continuous loads like an RV air conditioner do it from lithium batteries.  Their discharge curve is more of a straight line and they don’t suffer from the same (as much of a) voltage drop that AGM batteries experience as they discharge.

I don’t know what the continuous power rating of your alternator is, but I think it will be hard pressed to keep up with the DC draw of a 3KVA Multiplus at full load (or even partial load).  This means your batteries will most likely continue to discharge (even with the engine/alternator running) while the inverter is powering a high draw appliance.  I would need more information on the alternator’s continuous output rating, the wire size and distance from the alternator to the batteries and the actual draw of the air conditioner while running to give a more d definitive answer on this though.

Your air conditioner will require an easy start on it to prevent the initial start up surge from overloading the Multiplus also (assuming it is a typical RV air conditioner).

[lappir]

Not trying to derail the post and question, but maybe another way at looking at what you are wanting to do is instead of adding a complete separate HVAC unit, consider upgrading your current with an additional evaporator in the area you wish to add cooling to and maybe changing the compressor if required. 

I often wonder how the HVAC systems in the total electric vehicles operate, I have looked "under the hood" and asked at a couple of display vehicles in the past, but that was maybe 10 to 12 years ago. An auto A/C unit seems to be pretty robust and provides good cooling of all the vehicles I have owned including the full size "Minivan" I had once. I did opt for the additional cooling of the rear with the additional evaporator placed back there. 

The other option is getting a generator to power the rooftop unit when you need the additional cooling. 

It's all about choices, needs, vs wants and of course the funds to do it, maybe more than once. 

 

Rod 

[oldjohnt]

  In order to power a 120 VAC Compressor AC unit DIRECT from an alternator (Iffffffffff thats what one even wants to do ?? just saying) would require its voltage regulation and control circuitry to produce adequate current at a fixed 120 VAC REGARDLESS of RPM. While I'm NOT any AC expert, as an electrical engineer and while NOT saying it can't be done, I'M NOT A HUGE FAN OF SUCH....Sure engine driven Inverter Generators use rotational RPM to produce a steady 120 VAC at low or high RPM, but an engine belt driven alternator IS NOT AN INVERTER GENERATOR !!!!!!! 

 It would take a good sized alternator to supply enough energy REGARDLESS of RPM to supply X watts. Still 280 Amps at 14 volts does = 3900 Watts !!!!!!!!!!!!!!!! Iffffffffff thats continuous even at low RPM ??????

 My non expert preference would be to use a battery bank (I dont see your 200 Amp Hour AGM's as adequate) to run a PSW Inverter and let an engine powered alternator supply charge to the batteries... Still, of course, it would have to be rated to supply enough energy to maintain the batteries which is similar to my concern immediately above. However, if the alternator can deliver 280 amps at 14 volts (at low RPM??) that's 3900 Watts

NOTE make sure any engine driven alternator used to charge house batteries CAN SUPPLY THE CORRECT CHARGING VOLTGE AND PARAMETERS SUITABLE FOR YOUR CHOSEN BATTERY CHEMISTRY   They do make DC to DC Converter/Chargers that take raw battery and alternator engine energy and in turn supply well regulated proper charging for AGM or Lithium house batteries.  

 Again as a non expert I DO NOT like the idea of powering an Inverter DIRECT using an Alternator, but instead the steady DC power a battery bank can supply. Absent any specs I still doubt it would even work very well since the alternators output may be un filtered pulsing/rippling DC versus pure battery DC ??? Ask Vicron, but as a non expert I just dont see that working, I doubt it was designed to work powered by a rippling pulsing alternator waveform output especially regardless of RPM  ????

 A big enough Amp Hour battery bank (preferably Lithium and more then 200 AH AGM) ,,,,,,,,,,A big enough Inverter (say 3 KW),,,,,,,,,,A big enough Alternator to maintain the batteries SOC versus the  Inverters AC draw regardless of RPM ,,,,,,,,,,could to the trick......

 I prefer to wire the 280 amp alternator (IF adequate for proper charging voltages and parameters suitable for your house battery chemistry ??) to charge your house batteries and let your 3KW Inverter power the AC   BUT THATS JUST ME   

 NO WARRANTY, SEE WHAT THE EXPERTS HAVE TO SAY LISTEN TO THEM VERSUS ME

 John T

        

 

Edited January 12, 2022 by oldjohnt

[Wrknrvr]

   Through the last 15 years of doing mobile RV repair. Sometimes on Prevost bus conversations, I have seen up to 5 alternators on the engine. Belt driven, and the busses would have 6, 8D batteries. This was before lithium batteries of my knowledge.

   Also if alternators were run through a isolator, then the voltage on the alternators were adjusted high enough to compensate for the voltage lost through a isolator. That adjustment was built in at the alternator place of manufacturing.

  So investigating how new Prevos buss conversations are configured could give some worthwhile info.

 That is my 4 cents worth of info today.

 

 Just Vern in a T-shirt