How to wire 2 12v Lifepo4 batteries such that they stay 12v and not destroyed.

[dbhost]

So I make no secret about my desire to be able to air condition my camper, and run a fridge, and charge my laptop, easy as pie to do when the truck is going down the road off of the alternator, but solar on the other hand is a completely different much lower power ball of wax.

To accomplish this I have ended up with 2 300w Dokio portable panels, a 40 amp MPPT controller / dc to dc charger, and 2 200 amp hour Chins 12v Lifepo4 batteries. I'd have been a LOT of money ahead just sticking with a traditional generator, but most parks and festival camping grounds don't allow generator use after 10:00, and when it is 84 deg F and 90% RH overnight during festival season, I gotta have AC!

Anyway, so the big question is, How do I wire the batteries to the each other, the charger / charge controller and the output devices like the inverter?

I know the solar panels wire together in parallel via a Y cable, so the positives form a Y, and the negatives form a Y.

I am assuming, correctly or not, that I would do the same to and from the battery so for example using a set of buss bars, the battery lead connection on the positive would have 2 leads, 1 to each battery, and quite likely the charging source positive lead, and same thing but on the negative side, and then each output device, the 12V sockets, inverter, USB charger ports, fridge port get connected to the other buss lugs (and fused on their own circuit).

Does this sound correct, or even sane?

 

[Ozarker]

Get a battery switch, wire through the switch so you use one battery at a time. You can do this mann8ually but you must pay attention. Secondly, you can use a floating regulator that kicks the switch from one bank to another as one battery bank gets low, you'll need help with building this most likely, it might be on youtube and it will cost more.

You'll still have a problem with 200 W in storage, after the sun goes down. On a sunny day your AC might run off of solar alone with 2300W, but when that isn't there, you're soaking the batteries.

Wiring - to - and + to + doesn't change voltage.
If you just wire in L4 batteries in parallel, you'll drain one before the other depending on the condition of the cells. The switch keeps them balanced, that's also the job of the MPPT Controller or a fabricated regulator mentioned.

Do some math and and figure out what the AC draws X the hours required when the is no sun, at night your panels put out zip!

 

[dbhost]

Get a battery switch, wire through the switch so you use one battery at a time. You can do this mann8ually but you must pay attention. Secondly, you can use a floating regulator that kicks the switch from one bank to another as one battery bank gets low, you'll need help with building this most likely, it might be on youtube and it will cost more.

You'll still have a problem with 200 W in storage, after the sun goes down. On a sunny day your AC might run off of solar alone with 2300W, but when that isn't there, you're soaking the batteries.

Wiring - to - and + to + doesn't change voltage.
If you just wire in L4 batteries in parallel, you'll drain one before the other depending on the condition of the cells. The switch keeps them balanced, that's also the job of the MPPT Controller or a fabricated regulator mentioned.

Do some math and and figure out what the AC draws X the hours required when the is no sun, at night your panels put out zip!

Dometic recommends 180 amp hours for 12 hours of run time. Plus the fridge, laptop, CPAP, and charge my phone.

I'm not sure I understand the purpose of the switch. Are you talking about a battery isolator switch? The DC to DC charging circuit isolates the "house" battery bank from the "starter" battery. I am talking about connecting the 2 lifepo4 batteries as a large-ish battery bank.

So what this guy is showing here, positive to positive and negative to negative between batteries, then charging and load positive on positive on one battery, and charging and load negative on the other. At some point I assume this is going to need to go to chassis ground but should it need an addtional chassis ground separate from the one on the DC to DC charging circuit?

 

[vintageracer]

Hire a solar professional.

 

[dbhost]

Hire a solar professional.

Well that wouldn't let me learn anything new now would it?

 

[vintageracer]

Nope!

But your solar system would be fixed, finished, ready to go and all you had to learn how to do was "Write A Check".

That's the fashionable way to do things around here!

 

[Alloy]

Well that wouldn't let me learn anything new now would it?

Start (leaning) by doing a load calculation and then single line ( + wire only) wiring diagram.

FYI 600W solar and 400Ah isn't much for AC.

 

[jonyjoe101]

I would connect them in parallel, that way they both charge and discharge at the same time. My 220ah lifepo4 battery bank is 2x110ah batteries hooked up in parallel, been running them that way for over 3 years. The lifepo4 have bms so if one discharges before the other one it will shut down but the other battery will still be working. But you want to make sure not to drain them too low where the bms activates, because different lifepo4 have different ways to reactivate the bms.

I have a total of over 500 ah of lithium and I wouldn't consider running an AC. It can be done but you have to have a good and reliable way of charging up the batteries to full the next day. Solar by itself might be able to put back 200ah on a good sunny day.

For night time I would consider a swampcooler, I have used them here in california in the hottest humid days that I encountered, and it does work, running all night I might use less that 20 amps total. On humid days is the only time I run it all night. Also vent your fridge. The fridge compresser is like a heater (over 120 f) during summer, pumping massive amounts of heat in your van. I vent mine outside the van and it helps alot.

This is a post that shows someone trying to use a AC in there van then switched to a swampcooler.

Experimenting with portable AC unit

I bought a Hisense 5000 BTU portable air conditioner at Lowes. My current setup isn’t optimal, but I have some hope this could work somewhat well. At least well enough to have a few cool hours every other afternoon or maybe every afternoon if I don’t feel like being around people. Mornings I...

vanlivingforum.com

 

[crazysccrmd]

I don’t see an inverter listed. Is your air conditioner unit powered by 12v DC power? If it’s one of the Dometic RTX units those advertise a draw of 19amps in eco mode and a max of 50-58amps. Given those numbers you could expect a fully charged 400AH battery bank to last between 6-18 hours depending on how much the AC has to work to cool the air.

Your solar panels would provide about 33 amps of power in ideal conditions. On a full sun summer day in Texas that’s 400AH of potential recharge but likely closer to 200AH realistically.

For the batteries you will connect the positive terminals of each battery and the negative terminals of each battery. All trailer power gets connected to one battery and all grounds to the other battery. That keeps both batteries in full use and let’s them charge and equalize properly.

 

[dbhost]

I am looking at the Dometic RTX1000 and I am trying to cool a hi top fiberglass topper. The roof is already coated with Henry Tropicool. Dometic ad copy says run time of 12 hrs on 180 amp hours.

 

[dbhost]

Nope!

But your solar system would be fixed, finished, ready to go and all you had to learn how to do was "Write A Check".

That's the fashionable way to do things around here!

Screw that. I'd rather build my own.

 

[GlennA]

So I make no secret about my desire to be able to air condition my camper, and run a fridge, and charge my laptop, easy as pie to do when the truck is going down the road off of the alternator, but solar on the other hand is a completely different much lower power ball of wax.

To accomplish this I have ended up with 2 300w Dokio portable panels, a 40 amp MPPT controller / dc to dc charger, and 2 200 amp hour Chins 12v Lifepo4 batteries. I'd have been a LOT of money ahead just sticking with a traditional generator, but most parks and festival camping grounds don't allow generator use after 10:00, and when it is 84 deg F and 90% RH overnight during festival season, I gotta have AC!

Anyway, so the big question is, How do I wire the batteries to the each other, the charger / charge controller and the output devices like the inverter?

I know the solar panels wire together in parallel via a Y cable, so the positives form a Y, and the negatives form a Y.

I am assuming, correctly or not, that I would do the same to and from the battery so for example using a set of buss bars, the battery lead connection on the positive would have 2 leads, 1 to each battery, and quite likely the charging source positive lead, and same thing but on the negative side, and then each output device, the 12V sockets, inverter, USB charger ports, fridge port get connected to the other buss lugs (and fused on their own circuit).

Does this sound correct, or even sane?

You seem to be on the right path. Tie the two battery positives together and the two negatives together. That keeps them at 12V and adds up to 400ah you're aiming for. Tie the + to a buss bar and the - to a separate buss bar. Connect the solar controller "charge " wire + to your + bussbar and the - to your - bussbar. Connect your inverter/charger to the bussbars while observing polarity. Connect the + from your dc/dc charger to the + bussbar. If the dc/dc charger connects to chassis ground, you'll need to connect your negative bussbar to chassis ground as well.

Do yourself a favor and draw out a simple schematic using red for + and black for -. That will help keep things straight in your head.

Hope this helps.

 

[DirtWhiskey]

Keep your cable lengths all the same and don't skimp on gauge.

 

[Peter_n_Margaret]

I am looking at the Dometic RTX1000

Better to look at an inverter drive unit with some decent efficiency like the smallest Mitsubishi Heavy Industries split unit.
Depending on what else you want to run from the Li batteries, configure them as 24 or 48V (instead of 12V) with a matching inverter and slash the DC cable sizes.
Cheers,
Peter
OKA196 motorhome