Lithium system and A/C

[K9Krazy]

Looking for some feedback on how long your lithium battery systems hold a charge when using your A/C. In a 90 degree day, how many hours can you run your A/C on the lithium system assuming you are keeping your interior temp at 75 degrees?

[HarryStone]

It depends on how much current your AC uses per hour, and how big the lithium system is. A 15 amp/hour/120 VAC AC will consume approx. 150 amps of 12 volt power. So, if you have 1 AC, and run it continuously, it will deplete a standard 100 amp lithium battery in about 40 minutes, with absolutely nothing else running, and nothing recharging the battery. Realistically, running AC on a DC battery, lithium or otherwise, is not a good idea.

[TravelSolo]

The system I built in my small trailer with a 13.5k unit can run about 4 hours total time. If you assume 50% duty then that's about 8 hours. If you can keep the run time down to 25% duty cycle, that's 16 hours. If course assuming no other power use.

Also using a MicroAir easy start

[twinboat]

Running your AC on battery during the day will deplete your battery. No getting around that unless you have enough solar to completely cover the load.

Solar will not recharge the batteries overnight, no matter how much you have, so your going to have to run a generator to do that.

If running the AC on battery is not repeatable, you may as well run a generator.

[Danny969]

Depends on the size of your battery pack and how well you're insulated. I can go about 11 hours before the AC depletes my 12 KW pack. I have a 13.5K BTU AC. I think if I got a more efficient AC unit I could go a while longer.

[amosnandy]

I figured if I put 3600 watts of solar and 4 120ah batteries running 2 3000 watt victory inverters, I would draw 2280 watts just running the ac.
Let’s say at peak they are 80% efficient, that’s 2880 watts, leaving 600 watts to charge the batteries that means if I could get 9.6 hours at 80% I could run the ac and fully charge 400ah of batteries.

Pretty optimistic efficiency and charge rate.

That leaves 2.18 hours of running the ac full time.

[amosnandy]

Use this calculator to determine how long you can run what on lithium batteries

https://www.powerstream.com/battery-...calculator.htm

[Danny969]

Originally Posted by amosnandy

I figured if I put 3600 watts of solar and 4 120ah batteries running 2 3000 watt victory inverters, I would draw 2280 watts just running the ac.
Let’s say at peak they are 80% efficient, that’s 2880 watts, leaving 600 watts to charge the batteries that means if I could get 9.6 hours at 80% I could run the ac and fully charge 400ah of batteries.

Pretty optimistic efficiency and charge rate.

That leaves 2.18 hours of running the ac full time.

Not sure how you'll get 3600w of solar on a Class B. Most I've ever heard of is 1000w which leaves no room on the roof for an AC unit. You'd have to go with an undermount unit. Even with 3600w solar, you'll probably only get 2500w for 4hrs per day and much less during morning and late afternoon hours. Not sure why you'd need two inverters. Better to go with a 12vdc air conditioner (or 24v or 48v depending on your battery pack).

[twinboat]

Originally Posted by Danny969

Depends on the size of your battery pack and how well you're insulated. I can go about 11 hours before the AC depletes my 12 KW pack. I have a 13.5K BTU AC. I think if I got a more efficient AC unit I could go a while longer.

And then what ?

Now you need to recharge, using more energy for that then just powering the AC .

[Rlum]

Trying to keep the internal van temps around 75 in high heat uses a lot of electricity. The faster one runs their battery bank down, the total capacity is reduced. My neighbor got new Tesla with 330-50 mile battery bank. He could only get 193 miles before requiring a full charge. Why? Tesla battery capacity is based on 60-65 mph driving speed. My neighbor was driving 75-80. Tesla told him he was at that speed, he would get the full range capacity of his batteries.


Another major issue is how well is your van insulated. Most commercial vans are very poorly insulated. If your van is insulated like Embassy RV, then you will probably much better range running AC with battery bank.

[Rocinante440]

Don't forget another variable: Anytime you hit your batteries that hard, they and the inverter that powers your AC unit (assuming it's a typical AC-powered AC unit) are both gonna generate heat. Are your batteries and inverter inside your van?

[KanzKran]

I can get about 3-1/2 hours continuous run time on my 600Ah LiFePO4 battery, but it will run overnight with no problem once it cools outside a bit and it can cycle. It's also only an 11k roof unit.

I run it off the battery mainly when stopped for lunch or bathroom breaks (us and the pooch). The second alternator charges it back up as I drive. The alternator outputs about 150A or so normally, and the A/C draws about 150A, so it's a wash while it's running. The dash air keeps the whole van cool enough that the roof air doesn't kick on (set to 78F while driving), so the battery does recharge. The engine has an adjustable high idle control, but I don't use it as I never run the engine just to charge the battery.

300W of solar doesn't make a dent even at its 15A max output. It'll run the ~6A compressor fridge forever at 30-40% duty-cycle, which is really all I need solar for anyway. But no way can we boondock off the battery and solar with the A/C running. We tend to hop from RV park to RV park (with shore power), but sometimes have to overnight in a welcome center or truck stop, but again, the alternator brings the battery back up when driving the next day.

It's what we wanted, and it works extremely well for us. May not work well for someone else, though.

[MarkSaxton]

@KanzKran



"The engine has an adjustable high idle control, but I don't use it as I never run the engine just to charge the battery."


I don't either, but I do run the engine to run the A/C when I'm off-grid.


If one is running the alternator at idle, higher engine idle helps keep a longer duty cycle going on the alternator, for the below reason.



The alternator power output is proportional to the product of RPM x Field Current, so with 2k RPM one needs half the field current in the alternator for the same output power as at 1k RPM. This matters when its hot out (which it will often be when one wants A/C) as more field current means a hotter alternator, and at least with my setup the regulator will shut the alternator off when it gets above a programmed max operating temp, and leave it shut off until it cools to some number of degrees below that cutoff temp. With my system, the regulator adjusts the field current to keep the voltage at the programmed level, and at a lower RPM the field current needed will be higher, which means a hotter alternator in general. Also the higher engine RPM's blow more engine fan air over the alternator, not sure how much this matters.



If one doesn't know this, I think its natural to assume that higher RPM's are making the alternator work harder, which isn't really the case. Its making the van engine work harder and using more gas, but the alternator will be running in a potentially cooler condition.


The limiting thermal factor for me is often the inverter, though - it also shuts down at a pre-programmed temp when running the A/C and it can take a while to cool off. Its not in a very well ventilated spot - inside the rear passenger side bench, right in front of the wheel well, inside a pretty tight (its a bench) enclosure.

[Danny969]

Originally Posted by Rocinante440

Don't forget another variable: Anytime you hit your batteries that hard, they and the inverter that powers your AC unit (assuming it's a typical AC-powered AC unit) are both gonna generate heat. Are your batteries and inverter inside your van?

I have a 48vdc battery pack and a 48vdc AC unit so no inverter involved. The battery is inside the van, but doesn't generate much heat. I've never seen the pack temp higher than 80 and the casing it's in feels room temp.