Switching house batteries to lithium

[Laura & Charles]

We’ve been full timing since 2016, worked full time up until last May (2024). That lifestyle didn’t allow for much boondocking or need to have much need to be concerned with battery capacity since we’ve almost always been on shore power. We’re retired now and our house batteries need replaced. We expect to be off the grid considerably more going forward and have decided to up our available power resource. So here’s what we’re dealing with:

~We have a (Magnum 2812) 2800 watt sine wave inverter. (Customizable charge profile option)

~We have a 10kw generator, and not afraid to use it daytime hours (so no immediate plans to explore solar)

~Planning on installing four 12-volt 230 ah (920 total ah) LiFePo batteries. (LiTime 12V 230Ah Plus Deep Cycle LiFePO4 Battery with Low-Temp Protection (not heated)

~Planning on installing a dc/dc converter to protect the coach engine’s alternator.

~We have three rooftop 15K btu A/C/heat pump units, soft start caps on all three.

When newer, our four 6volt GC2 batteries would allow us two nights and one day without needing the generator. (Inverted 120volt: Some TV with dish network satellite receiver, a couple cups of coffee, residential fridge, one CPAP X two nights (heated/humidified). Plus 12volt lights, water pump, control boards, etc.

We don’t anticipate any deep woods or desert boondocking or anything long term. We just want to become more carefree nomads and be able to travel at will with out worrying about reservations.
~~~~~~~~~
Questions/concerns:

~Will my inverter be able to run an A/C or two should we need it?

~Will 920 ah’s be able to handle an A/C or two? For several hours?

~Currently, of course, the inverter isn’t wired to power the A/Cs. Would it make more sense to simply wire the A/C breakers to receive inverted 120? Or better to add another inverter dedicated to the A/Cs?

~Anything I’m not considering and should be?

 

[TonyL]

I can only advise by looking at what happened to hueypilotvn's set up.
He installed a system to run AC from the 12volt system and it nearly caught fire.
If you have access to a clip on ammeter, try and get a current draw whilst running an AC unit on mains power, probably the easiest place is the wire from the circuit breaker.
Let's assume the AC draws 10 amps running at 120 volts, that's 1200 watts.
Now take that 1200 watts and divide by 12.7 volts (fully charged battery)
and you're near 100 amps per hour. That's without figuring losses through the inverter.
Just my thoughts, but you have a perfectly sound generator so why not use that when you want to run air conditioning? You're going to have to run it to recharge the batteries anyway if you don't have solar.
At night you could use desk fans to move air, much quieter and they'll easily run through a 300 watt inverter.
We also use rechargeable fans by our bedside with built in run timers.

 

[Kirk]

A 120V, 15,000 BTU air conditioner/heat pump unit typically draws around 12 to 16 amps. Consult the unit's data plate for accurate amperage information.

 

[No cats]

Each of the batteries mentioned come with 200A BMS, that gives you a possible 800 Amps at 12.8 VDC. That is 10,240 Watts. Going to the AC side that equals 85A at 120 VAC. As others pointed out, that DC side of things will get hot quickly running high Amps.

Figure out your loads and make sure you have the right size fuses and wire.

 

[John From Detroit]

One thing many forget is you do not wire per what the battery is capable of (If you did that youd' need 00 ga throught your house or bigger) you wire according to the fuse or breaker.

IF you can draw say a maximum of oh 50 amps. You need wire for oh sa 60 amps (And breaker the same) with a breaker within 12" of the battery. (Recommended) I forget the size of the "T" fuse in my inverter. My XC-2000 says "250 amps" So I fused and wired for 300 on the initial install (Smaller now) and a 2nd 80 amp line to the rest of the RV. (Common only at the battery) that last was the factory wire (80 amp)

 

[No cats]

Going by Kirk's numbers, one 15k BTU AC would need 16 running Amps at 120 VAC. That is 1920 Watts on the output of the inverter. On the DC side, the input cables have to carry 150 Amps at 12.8 VDC. Since the inverter is maybe 97% efficient and caution says to add some margin (10% ?), the 150 Amps grows to near 170 Amps.

That's a lot of juice! besides that, a short before protection (CB or fuse) could expose cabling to the full capacity of the battery bank - 800 Amps. Need to be real careful... just saying. Especially if things get loose with the RV on the road.

 

[HueyPilotVN]

Use your generator for the air conditioners. Trust me. You do not want a fire.

 

[RV_Lee1]

X2 on just use the generator for AC. It can be done off lithium but you'll have to cover the entire roof in solar panels to not just drain the battery within hours. 920AH would take several days, possibly a week plugged in to recharge at an RV park otherwise. I have 460AH of lithium, its a ton of power for everything besides AC and even just 300 watts of solar keeps it topped off if there is sun.

 

[Laura & Charles]

Thanks to all for the replies/advice. I’ll heed the wisdom y’all have given and not attempt using inverted 120 for the A/Cs.
I appreciate the sound knowledge.

 

[Primerump]

Going by Kirk's numbers, one 15k BTU AC would need 16 running Amps at 120 VAC. That is 1920 Watts on the output of the inverter. On the DC side, the input cables have to carry 150 Amps at 12.8 VDC. Since the inverter is maybe 97% efficient and caution says to add some margin (10% ?), the 150 Amps grows to near 170 Amps.

That's a lot of juice! besides that, a short before protection (CB or fuse) could expose cabling to the full capacity of the battery bank - 800 Amps. Need to be real careful... just saying. Especially if things get loose with the RV on the road.

quite correct.. that reason alone is why one should NOT run high wattage loads from low voltage ( 12 V )
if you need high wattage then 24 or 48V is a much better design choice

 

[Primerump]

Use your generator for the air conditioners. Trust me. You do not want a fire.

yes, you learned the hard way that high currents and low voltage are not a good combination...

 

[DonTom]

yes, you learned the hard way that high currents and low voltage are not a good combination...

When currents are that high, from a bunch of 12-volt batteries in parallel, I would want to put a fuse on each battery. IOW, if six 12-volt batteries are in parallel, each battery as well as the battery wires can be a sixth of the total current value. These six fused outputs go to a common point where a much thicker wire is used to feed the inverter. If there is a serious short, which should be very unlikely, all of the six fuses will blow and do the job of preventing a fire from the super-high current.

While such a configuration will be a bit unusual, IMO, is there anything wrong with such an idea?

-Don- Reno, NV

 

[Primerump]

When currents are that high, from a bunch of 12-volt batteries in parallel, I would want to put a fuse on each battery. IOW, if six 12-volt batteries are in parallel, each battery as well as the battery wires can be a sixth of the total current value. These six fused outputs go to a common point where a much thicker wire is used to feed the inverter. If there is a serious short, which should be very unlikely, all of the six fuses will blow and do the job of preventing a fire from the super-high current.

While such a configuration will be a bit unusual, IMO, is there anything wrong with such an idea?

-Don- Reno, NV

if a system is designed correctly, there will be no excessive high currents.. period. !

your suggestion of fusing each parallel battery is standard practice when using FLA, lithium is a different beast but would still benefit from the same practice... ( and I'll wager most diy implementations do not have fuses.. )

In all the years I've been implementing ( RV ) systems, particularly solar, almost NONE of them have had any high power consumption at 12 Volts. ALL the high wattage draw has been from 120V.. therefore it's a simple solution to design the battery voltage and A/h to the loads required.
My recommendations:

for loads up to 1500W then use 12V
for loads up to 3000W then use 24V
anything > 3000 then 48 V is required.

 

[DonTom]

if a system is designed correctly, there will be no excessive high currents.. period. !

your suggestion of fusing each parallel battery is standard practice when using FLA, lithium is a different beast but would still benefit from the same practice... ( and I'll wager most diy implementations do not have fuses.. )

In all the years I've been implementing ( RV ) systems, particularly solar, almost NONE of them have had any high power consumption at 12 Volts. ALL the high wattage draw has been from 120V.. therefore it's a simple solution to design the battery voltage and A/h to the loads required.
My recommendations:

for loads up to 1500W then use 12V
for loads up to 3000W then use 24V
anything > 3000 then 48 V is required.

While there is no doubt using a higher voltage is usually a better idea, as you can get the same wattage (power) with half the current (amps) by having double the voltage. The problem is everything DC in every RV I have seen runs from 12 volts. Means the entire rig has to be redesigned or else have a have a lot of DC2DCCs to get back down to 12 volts for almost everything else.

With the half dozen fuses for six 12-volt batteries, can you explain why it is a better idea with L-A batteries than with Lifep04's? I would expect the same benefit either way.

-Don- Reno, NV

 

[Primerump]

While there is no doubt using a higher voltage is usually a better idea, as you can get the same wattage (power) with half the current (amps) by having double the voltage. The problem is everything DC in every RV I have seen runs from 12 volts.

your not seeing the picture here...

yes, RV systems are usually 12V and generally low power and run off the "house" battery, or the tongue battery on a TT.. the rest of the system for the inverter and additionally the solar can be any other voltage.
12 V is usually the lowest power requirement in a TT, especially for a boondocker..
powering 120V appliances is a different story..

Means the entire rig has to be redesigned or else have a have a lot of DC2DCCs to get back down to 12 volts for almost everything else.

you don't need to get "back" down to 12V. the RV already has a 12V charger/psu built in to maintain the battery. to be clear here, my design philosophy for high voltage implies the RV still keeps a 12 V battery in place as a buffer for that system.. and the inverter/solar system uses any other voltage as required.
these systems are independant of each other.

With the half dozen fuses for six 12-volt batteries, can you explain why it is a better idea with L-A batteries than with Lifep04's? I would expect the same benefit either way.

-Don- Reno, NV

fusing each battery in parallel strings is to account for possible cell shorts and the subsequent fault current. something that is rare these days, but still desirable. most FLA failures according to a Trojan engineer I spoke with are sulfation and high impedance due to under charging. never the less, chite happens and a fuse will save the day..

generally speaking, Lithium with a BMS has intrinsic fault management and will most likely fail open circuit, however, if the BMS output device(s) fail from overheating and/or component failure, perhaps due to excessive current draw ? then it's possible to have a faulty output device and no protection ( such as a MOSFET device acting as a switch that's failed in a short condition ). under this condition the battery will behave as a FLA. short out this puppy without a fuse and we get smoke/fire/screaming... LOL
of course, in practice, I'll bet 99% of installs will have no issue for their lifespan.. however, the 1% will be the RV fire that will make the news... that's just statistical probability working.

 

[DonTom]

your not seeing the picture here...

I guess not. Are you saying the higher voltage batteries are going to only be running an inverter for the A/C? All else stays at 12 VDC?

-Don- Reno, NV

 

[Primerump]

I guess not. Are you saying the higher voltage batteries are going to only be running an inverter for the A/C? All else stays at 12 VDC?

-Don- Reno, NV

we have a winner...!!

yes, most installs treat the 12V and solar/inverter system as one. this is fine for small wattage systems, but as wattage demands increase, people soon discover shortcomings with low voltage high current systems. ( Huey is one example ). My design mantra is to minimize currents, high current always incurs voltage drop due to cable and contact resistance and increased cost from heavy conductors and components.

My solution is to retain the "house" battery for small 12V loads, lights etc.. and treat the inverter/solar system as a separate system. that now gives one the freedom to use any voltage and therefore an optimized solution. This is particularly advantageous with high wattage solar.

for example, in my own system I have a 12V 100A/h self heated lithium battery on the tongue to power the RV system, my power requirements are very small @12V.. a few led lights, the water pump etc..
my high wattage appliances do however need a significant power source as you know, air conditioning, microwave, coffee maker etc.. all require upwards of 1000W.
In my case, the design was required to power the a/c system, microwave and/or the coffee maker concurrently, that required a 3000W inverter. 3000W @ 12V with an 85% efficient inverter will draw just over 290 Amps, that requires 450 A fuses, heavy cables > 4/0 and substantial copper lugs. with the addition of 12V mppt controllers, the entire project will be expensive and not very well optimized. Increasing the voltage to 24V will half those currents, reduce voltage losses by a factor of 4 and be much more cost effective. The batteries I chose for this design are four 12V 400A/h lithium connected in a 2S2P configuration. they were chosen mainly for their physical size as I have some physical constraints in the RV. the capacity of 19200W/hr is required to power the a/c overnight.
also, I had some gear left over from various jobs and that included several 100/50 mppt's and a multiplus 24/3000 unit. otherwise If buying new, I would have chosen 48V. that said, 24V does have an advantage in this case. if one battery should fail, I can remove the offending bank and still operate. if this were a 48V install then one battery failure would render the system unusable unless I fitted multiple 48V batteries..
my solar panels just fyi are 50V residential units.

 

[Mark_K5LXP]

Parallel connections do not scale perfectly. Even miniscule differences in cables and connections can cause a large disparity in current flow. Not just in cabling, that's actually a pretty stable constant. Connections and components become a large variable that can easily get away from you in the form of both lost power, voltage drop and long term reliability. A basic crimped cable terminal, copper nut and bolt, and another terminal (as you'd find on a bus bar) can easily have a few milliohms resistance. Run a hundred amps through that and you're dropping a quarter volt and dissipating 25 watts. 25 watts is a lot of power in a small volume like that - it's gonna get toasty. Run that load frequently and that junction is getting cycled hot and cold, expanding and contracting which can eventually loosen the connection. Causing even greater drop, a hotter connection until eventually it either opens up or melts something. Fuseholders and fuses can have even more resistance between their terminations, end/blade contacts and the fuses themselves. No two batteries will have the same exact internal impedance and connection resistances. Getting them to share identical currents into large loads is difficult. Throw in a number of connections, cables and components and the variables only increase. High(er) voltage can have challenges but low voltage/high current has it's own set of subtleties to deal with so there's no universal solution. The components and configuration are dependent on the end application.

Mark B.
Albuquerque, NM

 

[Sunnyfalls]

Just because it is possible to do something, doesn't mean it is the best approach or that you should do it...

At our remote southern Oregon homestead, we backup our 144kW grid power with a 5kW generator and a 5kW hydro power cross flow turbine. We can directly use these 5kW 120VAC 60Hz power sources or we can use our self-constructed full wave bridge rectifier with a capacitor filter to first convert these backup power sources to low ripple 48VDC power that can be either used to charge 48VDC LiFePO4 batteries or can be used to directly feed a single-phase 48VDC input pure sine wave power inverter to produce single-phase 5kW 120/240VAC power. For more power, the batteries can be used to feed a three-phase 48VDC input pure sine wave power inverter to produce three-phase 36kW 120/208VAC power which can then be used directly in our shop or which can then undergo secondary power conversion to produce single-phase 24kW 120/240VAC power for our residential power needs using a Scott T Transformer. We only need this 24kW power for our three 240VAC tankless water heaters that can each use as much as 24kW for the short duration that they operate. Otherwise, 5kW satisfies all of our long duration power needs. We don't have or need air conditioning at all because our buildings are super energy efficient (i.e., well-insulated and well-sealed ICF construction), are of a passive solar cooling/heating design and we have a low humidity, diurnal climate.

So while one can reduce DC currents by using 48VDC batteries and inverters, one still needs huge AH battery capacity to operate higher power appliances for long durations (i.e., air conditioners, heaters and refrigerators). And then one also needs huge solar panel capacity to charge the batteries...and perhaps a backup generator to do so when there isn't sufficient sun... Accomplishing this huge footprint within our small 17 foot trailer which we expressly designed for traveling far, fast and economically wasn't practical if even possible.

All of our RV 120VAC dry glamping short power duration appliances each use less than 800 Watts and are powered from our 3000 Watt Inverter and a single 100AH LIFEPO4 battery. These appliances include our coffee maker, hair dryer, toaster oven, microwave oven, toilet heated bidet seat and vacuum cleaner. We use our propane modified Honda EU2200i generator to operate our air conditioner and we use propane to operate our cooktops, furnace, tankless hot water heater and refrigerator when dry glamping.

Gayle & Bob
"Los Gatos Casita"

 

[DonTom]

my high wattage appliances do however need a significant power source as you know, air conditioning, microwave, coffee maker etc.. all require upwards of 1000W.

I will stick with the genny for A/C. Most of my RVing is in the middle of the winter anyway, often to southern AZ.

But when extra hot, I have these issues in my 2022 Class A. Perhaps a battery ran A/C would take care of that that issue, and also could run the refrigerator on 120 VAC. Perhaps I should consider a 48-volt DC system for this stuff. Come to think of it, I could have run my refrigerator on 120 VAC then, I just need to plug it into a different outlet and run the inverter. Or perhaps it will run on the stock inverter, I never checked for that, I didn't think of it at the time. I don't know if the stock 1200-watt inverter feds the refrigerator outlet ot not. It does not feed any of the outlets in the kitchen area for obvious reasons, but I don't know about the refrigerator. I will have to check that. But I assume it doesn't.

For all my other stuff, as is, I use a 300 AH 12-Volt Lifep04 battery to 4KW pure sinewave inverter--in each of my RVs. Runs the MW, hair drier, toaster and my Keurig and other stuff.

I have a couple of inverters in each RV that I use for different stuff.

I could consider wiring in a 48-volt system, I think I can find the room.

This is what I did a while back. I could put a 48-volt system under the other kitchen seat. Or consider changing this to a 48-volt system and use a DC2DCC for my ham radio and other 12-volt stuff. I could do this in both of my motorhomes.

-Don- Reno, NV