Help converting an RV house battery to lithium

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Hwy89   1 kW

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Help converting an RV house battery to lithium

Post by Hwy89 » Nov 02 2018 2:01am

It is time to replace the lead in my RV. I am heading out in a few weeks to spend the winter dry camping in the Sonoran Desert. I am currently in negotiations with a seller to get another Tesla module so I will have 444 18650 cells to play with. I plan on taking 2 of my 180 watt solar panels which put out 27 volts open circuit with their Mpp at around 17 volts.
I am interested in hearing from anyone who is using a lithium battery in their RV and finding out the specifics of their system. What configuration did they use to assemble the battery, ( 3s or 4 S) and what solar charge controller did they use? If they set it up for 4s (16.8 v full charge) were the RV components able to handle it and survive or was a buck converter needed to drop the voltage to ~13v.
I have, so far, disassembled 3 Tesla modules and put together a few bike batteries but this will be my first go at trying to emulate an old fashioned 12 v deep cycle battery so suggestions are welcome.

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Re: Help converting an RV house battery to lithium

Post by amberwolf » Nov 02 2018 2:21am

I don't have direct RV experience, but I use a 4s NMC lighting pack on my bike and trike, and have tested it out on a number of "12v" automotive items. Simple things like lighting (LED or not) work fine, and most of the electronic stuff works on it too, including a very old (1980s) car stereo, and some of the lighter-socket USB power adapters and other such devices (like an FM-radio-output "ipod" adapter). A "mini fridge" that just uses a Peltier and fan works ok, but runs very hot at the output heatsink (without making the inside any colder), despite the computer-case fan used on the heatsink running much faster at the higher voltage.

But some things either just shutdown to autoprotect, or blew up. One blown up bit is an older inverter to get 115VAC; I think it was Kensington, can't remember for sure. Blew up a few lighter-USB adapters. Another inverter autoprotects and won't even turn on, but still works when put on a pack discharged down to 15v or less.


Similarly, a number of non-simple electronics stop working on a 3s pack well before the pack is at LVC. (like the peltier-fridge and the inverter).


I don't know about other RV-specific systems, but I'd expect some to work fine and others to have issues of varying degrees.

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Alan B   100 GW

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Re: Help converting an RV house battery to lithium

Post by Alan B » Nov 02 2018 8:17am

3S Lithium Cobalt voltage is too low and 4S is too high for standard 12V loads.

There are two basic directions to go for an RV system.

One is 4S LiFePO4 and doing a 12V system with a max voltage of about 14.6 which is acceptable for 12V loads. 4S LiFePO4 is well matched to 12V. Various 4S RV LiFePO4 batteries have built in BMS systems, so these are the easiest but are expensive. See Battle Born, Bioenno Power and others.

The other is a 24V system and a 24:12V down converter. 24V systems have a wider range of voltage compliance and can be adjusted for voltages in the 7S range. Two or more panels in series can be used for charging.

With a 24V (or 36V or 48V) system the 12V loads see a nice regulated voltage, and very few items see the full voltage, basically the charge controller and the 12V output converter, and if you have one a 120VAC (or 240VAC) inverter. Inverters are more efficient at 24V or higher.

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Hwy89   1 kW

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Re: Help converting an RV house battery to lithium

Post by Hwy89 » Nov 02 2018 9:51am

Thanks Alan B, I hadn’t thought of the 24 volt system. I have several 7s packs already built that I use in series for my 14s bike.
I could make several spare bike packs and swap them in and out of house battery as needed. So I would need to find a 24/12 volt converter beefy enough to handle the load of the RV. Any suggestions on that?

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Re: Help converting an RV house battery to lithium

Post by dustNbone » Nov 02 2018 12:56pm

24VDC is most commonly found in large commercial vehicles and boats, so those are probably good places to start looking. Figuring out your maximum likely load (on the 12V side) will allow you to choose one appropriate size and spend as little as necessary.

My RV (26', no slideouts) has a 55A converter that easily handles everything 12V running except the leveling jacks, those need help from a battery.

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Re: Help converting an RV house battery to lithium

Post by Alan B » Nov 02 2018 12:57pm

Generally the 12V load current of the RV is not high, the exception can be high current motors like the landing gear on a fiver. Most loads are low current, and the high current jacks could be wired to operate from the vehicle 12V battery and separated from the RV house battery. RV/Solar places like Arizona Wind and Sun carry 24 to 12 volt converters, as an example. They have various sizes depending on the current required. 30 amps is generally adequate, the higher currents are for charging the 12V battery that is not needed here. But it needs to be sized for your rig.

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Re: Help converting an RV house battery to lithium

Post by kcuf » Nov 02 2018 3:03pm

12v house system may also crank genset if equipped

could be tied to chassis cranking batteries like any power gear stuff, jacks, slides, etc

contemplating 48v tesla modules here
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Re: Help converting an RV house battery to lithium

Post by Alan B » Nov 02 2018 10:27pm

If the 12V system has high peak demand, a small LilFePO4 battery could be floated across the 12V output converter for handling these peaks. But the bulk of the energy storage would be at 24/36/48 volts.

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Hwy89   1 kW

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Re: Help converting an RV house battery to lithium

Post by Hwy89 » Nov 03 2018 10:48am

The RV is a 25’ Jayco bumper pull trailer. The only high amp loads would be the tounge jack and the slide motor. I will measure the amp draw on those before purchasing the DC/DC converter.
I have been researching a solar charge controller that can be set up to output 29.4 volts to match the 7 s pack configuration but poorly written and incomplete product descriptions make that a frustrating chore. Any suggestions on a charge controller?

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