AC battery requirements, and 120v vs. 12v systems

[rruff]

I don't know much about it, but these aren't rooftop units and they use inverter compressors: https://en.wikipedia.org/wiki/Inverter_compressor

 

[Mfitz]

I did the math on this, and even the lowest consumption Coleman Mach AC runs for about 1/4 of the time per amp hour that a good 12v AC does.

Still hoping for some guidance on this. My camper already has a Coleman Mach 8, and I am trying to see if I can run it off my alternator and a DC-DC charger while driving and drawing juice from the alternator (not while the truck is turned off). The battery bank is two group 31 12v batteries. I would love to switch over to lithium but it isn't in the budget right now, nor is swapping the Coleman for a 12v AC unit. Can it be done safely, and if so, how big would the DC-DC charger have to be? My truck ('22 F350) has a dual alternator system so it will churn out fairly big amps for a vehicle.

 

[Peter_n_Margaret]

how big would the DC-DC charger have to be?

A DC-DC charger is a great devise to boost the charge voltage if the cable between the alternator is under size (or so long that voltage drop becomes critical).
If the cable is large enough you will get much higher charge rates by charging direct from the alternator without a DC-DC charger. The alternator output voltage is ideal and the battery chemistry will determine the actual charge rate.
I charge my 400Ah x 12V AGM house batteries direct from the alternator. I get up to 70A of charge from an 85A alternator at 14.4V output. As the SOC increases, the charge rate will decrease.
This is what happens with your crank battery. It needs no DC-DC charger. Spend the cost of the DC-DC charger on the copper wire and get a win-win.
Cheers,
Peter
OKA196 motorhome

 

[rruff]

I charge my 400Ah x 12V AGM house batteries direct from the alternator.

LiFePO4 is a different story. In some conditions (low battery charge) you can fry the alternator. Although there is some controversy about how much of a concern that is, and alternator specs are important. Lots of noise in this thread, but some signal too: https://diysolarforum.com/threads/alternator-charging-lifepo4.33521/

 

[Alloy]

A DC-DC charger is a great devise to boost the charge voltage if the cable between the alternator is under size (or so long that voltage drop becomes critical).
If the cable is large enough you will get much higher charge rates by charging direct from the alternator without a DC-DC charger. The alternator output voltage is ideal and the battery chemistry will determine the actual charge rate.
I charge my 400Ah x 12V AGM house batteries direct from the alternator. I get up to 70A of charge from an 85A alternator at 14.4V output. As the SOC increases, the charge rate will decrease.
This is what happens with your crank battery. It needs no DC-DC charger. Spend the cost of the DC-DC charger on the copper wire and get a win-win.
Cheers,
Peter
OKA196 motorhome

I agree but I would add a caution on voltage drop. I've seen it when wiring is sized for a 10-15% (not 1% or less) voltage drop. Under load there's 0.5-1.0V drop so the most the bank gets is an absorption charge.

 

[Peter_n_Margaret]

The right solution is to choose an appropriate alternator and match it with appropriate wiring. That will give the best result. Why strangle an otherwise great ability to charge at high rates? That is throwing away one of the great benefits of LiFePO4. Other solutions are bandaids.
Cheers,
Peter
OKA196 motorhome

 

[ReluctantTraveler]

It's worth noting that the 55amp draw of the Mabru is at max operation. Catalog (not tested :-/) draw for the low fan setting on Eco cool is 22.5amps and a low fan setting on Normal (think medium) cooling is 40amps. I have a very small camper in the design stages, like 8'x6'x6' so will not need much fan action and can use computer fans much more efficiently than the rooftop unit. My fervent hope is to get 9 hours a night at 40 amps which would take something like 370ah out of my batteries. If it's less than 400ah I'll be happy.

I would have liked to make a split unit work and I'm still keeping an eye on them. I just had a purchasing/shipping deadline to make and the Mabru was the most affordable option that would actually cool a camper enough to sleep when the humidity is 95% and the temps are well over 100deg even at night.

Curious how the Mabru worked out for you. Any updates?

 

[KiwiMtnClmbr]

Curious how the Mabru worked out for you. Any updates?

It only arrived via a sea shipment 2 weeks ago. My entire build is 9-12 mos behind schedule...

 

[ReluctantTraveler]

It only arrived via a sea shipment 2 weeks ago. My entire build is 9-12 mos behind schedule...

?

good to know as I plan mine. That sucks!

 

[ReluctantTraveler]

It only arrived via a sea shipment 2 weeks ago. My entire build is 9-12 mos behind schedule...

I’m realizing though that you might be in NZ? Did it ship from the states?

 

[andy_b]

Any progress or update on the mini split Andy?

It sounds like you got a different unit but finally got this one installed and running. Hopefully this update helps someone else.

It hasn't been super hot yet so the info is limited. At around 93* ambient, setting it to 63* (the lowest it would go) drew about 590W at 12 volts; this wasn't continuous obviously. My 800w array was able to keep up with this demand pretty easily (other loads were a 90L Dometic at 36*; water pump, Maxxair, and Wrappon were on standby but not running).

Overnight, set between 68*-78* (ambient unknown), we drew about 210ah from our bank. This was recharged via solar by 2pm IIRC. During that time, nothing apart from the fridge (and parasitic draws as above) was running. No alternator charging.

Indoors, the unit is quiet and pretty effective, although the fan on high has some trouble getting decent airflow to the forward-most corner of the cabover. This is an issue in our camper because our enclosed wetbath creates a little alcove that blocks airflow. It is also probably 15' between the internal unit and the forward wall. There are also no windows in the cabover yet which I'm sure would help with subjective comfort. When the compressor is off, the internal fan still runs but it is quiet, around a Maxxair at 20% power.

Outdoors, the unit is surprisingly loud. It is white noise but still, it will make you "that guy/gal" if you're around others.

The instructions are limited and the hoses are particularly crappy - would highly recommend getting new hoses made.

The heating function works but haven't tested how effective it is in cold temps.