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Fewer cells sounds better, but in practice I don't think that you will be able to get cells that large, at least at a reasonable price. I suggest just parallelling enough cells at the lowest level to reach your desired 400 Ah capacity, then stacking four of those sets in series to get a suitable voltage.

The cheapest source of lithium cells is probably salvaged EV battery packs; however, they may not be the desired chemistry. For RV applications LiFePO4 is often preferred, but it is not commonly used in EVs.

I have considered using Nissan Leaf modules for an RV or travel trailer. A challenge is that they are 2s2p in configuration (33 Ah per cell in the first generation, so 66 Ah and 7.5 volts per module), so they can't be combined in parallel at the cell level unless you also parallel the BMS terminals, and ensure the any current imbalance is not excessive for those smaller terminals. Only two modules would be needed for "12 volts", but a dozen modules would be needed for about the desired capacity - if they were all stacked together that would be a stack sized
11.9" (303 mm) x 8.8" (223 mm) x 16.5" (420 mm)
and 101 lbs (45.6 kgs)

Any of the pouch cell EV batteries using larger modules will be difficult to reconfigure for the desired voltage. The prismatic cells of the German brands should be easier to work with, especially the Sanyo 25 Ah cells originally used in the VW e-Golf (which has since changed to Samsung SDI) because those Samsung cells have threaded terminals (easy to build jumpers and cables). Of course with 25 Ah cells you would need (400/25)x4=64 of them.

Keep in mind that the normal operating voltage of a series set of four lithium cells - even LiFePO4 but especially automotive chemistries - is very high for the nominally 12 volt systems of an RV, and the charging voltage is even higher. You might consider a DC-to-DC regulated power supply (set for no more than 13.2 volts) for at least the more sensitive equipment. I find that furnaces make a lot of noise and may have problems even at common lead-acid charging voltage.
 

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I would not recommend EV packs or second-hand cylindricals etc, in fact any chemistry but LFP for House bank usage in a mobile living space.
Few RVs put the battery in the living space. they're usually on the tongue of a travel trailer, or under the floor of a motorhome. "Camper" could mean just about anything; if it means a slide-in unit carried by a pickup truck... then those have the battery within the coach body, but they should still be externally vented and sealed from the living space.

As I said, LiFePO4 (a.k.a. LFP) are commonly preferred for RVs. I think there are multiple reasons for this, including:
  • lack of any thermal management
  • low importance of power density
  • better voltage match to 6-cell lead-acid
 

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New quality large LFP prismatics go for $7-9 /AH before shipping, in the US market.
Are CALB cells of the quality that you're describing? They're well under $2/AH per cell, which would be less than $7-9 /AH for 4 cells in series.

That's a couple thousand dollars, before shipping, interconnections, BMS, and housing.

Cheapest quality deep cycle **FLA** is $1 / AH.
That would be at 12 volts, not single-cell, right? I assume that the expected $0.85/ AH is for a single cell (or parallel group of cells), not for a stack of them to reach 12 volts.
 

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Drop-in 12v lithium batteries (battleborn, etc,...) are still very expensive. I bet I can build a LFP pack much cheaper.
I agree. These off-the-shelf batteries incorporate an internal BMS, and combining four of them (100 Ah @ "12V" is typical) results in an excessively complex BMS and housings. Also, they are marketed to people who have no clue what is inside, just want a drop-in replacement for common 12 V lead-acid batteries, and are willing to pay a lot for increased performance.

Use will be a house battery for a campervan conversion. Sort of RV I guess. Think of a DIY Class B RV. I want power abailable and a 12v 400AH pack would do nicely. 600AH would be even better but it's even more money and cells.
Any RV conversion of a van is a Class B. 400 Ah (@ "12V") is huge capacity for Class B (similar to four "golf cart" batteries), but some people do it, especially if they are planning for refrigeration and cooking to be done electrically. RoadTrek offers even larger in their optional EcoTrek systems.

In a larger motorhome the battery can go under the floor, but in a van it will likely end up on top of the floor, under the bed. Please enclose and vent it appropriately.
 

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I am converting a small van, under 21 feet length. I still will probably install house battery under the floor. Really depends on the house battery I end up getting.
Great if you can do it, but good luck with that. The traditional rear-wheel-drive vans fill much of the underfloor space with propeller shaft, fuel tank, and exhaust system... and the remaining space is broken up by the frame and body structure. In what's left, you would ideally want to mount the fresh water tank, grey waste tank, possibly black waste tank, maybe generator, perhaps furnace and/or water heater... and the battery. The Ram ProMaster (Fiat Ducato) is front-wheel-drive and puts the fuel tank under the front seats, so it's a bit better for usable space underneath. What van are you planning to use?

It's easier to build fluid tanks (fresh and waste water) to conform to the available spaces under the floor than to package large rigid blocks of cells to fit.
 

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It’s only Class B of built by a RV manufacture and titled Class B. Self conversions are still just vans.
No, the classes are just descriptions of recreational vehicle configurations. Lots of people have built Class C motorhomes, and they're still Class C motorhomes, not just trucks. But it's yours, so I suppose that you can call it what you want. :)

By the way, the most impressive home-built Class C that I've seen is shachagra.
 

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Two strings give redundancy without buying a spare cell
Are you suggesting paired cells, four pairs in series, or two four-cell strings in parallel? Two strings would double the BMS complexity.

RVs rarely have battery redundancy, although lithium cells without widespread availability might make that wise. sportcoupe was explicitly looking for the minimum number of cells.
 

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You do not need a BMS for a 4S system. Would be complete waste of money as is using lithium in your application.
It's the same problem as with 96S, just in a smaller scale. Even a single lithium cell needs to be protected from undercharging and overcharging, and even in four cells one can get out of balance. Are you suggesting four voltmeters and manual monitoring? Keep in mind that in an RV there is no driver watching gauges while the battery is being discharged.

As for lithium being a "complete waste of money", the set of four to six GC2 lead-acid batteries which would be the alternative (depending on how much of the battery's capacity that you judge to be usable) is very heavy, and that is a concern for small RVs.
 

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A good higher end FLA like a Trojan Industrial or a Rolls 4000 or 5000 series has the exact same usable capacity of 80% capacity.
The general public's rule-of-thumb is to not discharge lead-acid batteries below 50% SOC. There's nothing magic about this level, but it looks reasonable given the shape of the cycle life versus discharge level curve, and even Trojan quotes cycle life at this level (3600 cycles at 50% discharge). What most people miss is that even if discharging to 80% depth of discharge (20% SOC), expected life is still substantial, if only half as long as at 50% (the same Trojan with 3600 cycles @ 50% is rated for 2000 cycles @ 80%). In an RV, how many years does it take to cycle the batteries two thousand times?

(from Trojan's Solar Industrial Line Flooded product page)

How many cycles are the LiFePO4 cells expected to last?

... set your Inverter to trip off-line at 11.5 volts.
I think some of the reason for caution in deeply discharging lead-acid batteries is that they are routinely used (particularly in RVs) with no system protecting against accidental excessive discharge. An automatic shutoff is typically considered necessary with lithium, and would be wise for anyone pushing the limits of even lead-acid batteries. However, this is not just about an inverter: an RV typically uses 12 V DC power for lighting, ventilation, the furnace fan, controls for all appliances (furnace, water heater, refrigerator), and pumping water... so the protective shutdown (or at least a warning) should be for all loads, not just the inverter and whatever it powers.
 

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You can't argue that discharging past 50% will shorten the life of a FLA.
It's as if we didn't already have this discussion...

There's nothing special about the 50% DoD (Depth of Discharge) or SoC (State of Charge) level. Did you look at that graph in post #34? The trade-off between life (in cycles) and depth of discharge is a continuous function - you can pick any DoD for which you are willing to live with the cycle life.
 

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Sounds like you might be stuck inside a 12 volt toy box. Last thing in the world you want to do is install Pb batteries in parallel. Guaranteed to cut cycle life in half. That is why battery manufactures suggest parallel installations. Smart money is if you need 500 AH, then buy 500 AH batteries. They will not be 12 volts, they will be 6 or 4 volt batteries wired in series.
Normal RV practice for capacities larger than is practical with a single battery is to use two 6V batteries in series, most commonly of the GC2 size simply because deep-cycle batteries in this size are readily and economically available. Yes, some parallel 12V two-battery installations are used (I have one in a trailer), usually to be able to use cheap "RV/marine dual purpose" 12V batteries.

For capacity beyond about 200 Ah @ 12V, the typical practice is to use four 6 V GC2 batteries. While just two large 6V batteries in series would be ideal, most builders, owners, and maintainers don't want to deal with the weight of a battery large enough to handle half of this total capacity in a single box; that Trojan SIND-06-610 weighs 100 kg. Three 4V batteries in series is a rational way to handle this, but it's hard to find a 4V battery as small as 500 Ah, and even if you find one it's going to be at least 50 kg.
 

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Personally I would go with a higher voltage battery running a DC to DC convertor so I could put a bunch of small cells in series.
This is one way to reduce problems of paralleling, but if you're building a pack from individual cells is that really a problem? Production EVs normally parallel at least two cells at the lowest level, and those using tiny 18650 cells parallel dozens of them at the lowest level.

The DC to DC converter would provide a nicely regulated output voltage, but it would also add conversion loss. A similar problem would exist on the charging side, with none of the usual RV charging sources able to charge the battery without the cost and efficiency loss of a voltage conversion step.

I have considered the idea of a higher-voltage RV battery, but specifically to make 120 V AC integration better in a motorhome: with a nominal battery voltage over the peak-to-peak voltage of 120 VRMS, the inverter could be very simple and efficient, the raw 3-phase output of a typical "inverter" type generator could be used by a rectifying charge controller without an unnecessary 60 Hz step, and a regenerative braking system could produce several kilowatts without excessively high current. All of this is probably viable only with substantial custom component construction.
 

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Well I am hear to tell you is it is common practice to run at 24 volts, and a few 48 volt systems. But be warned going to a higher voltage eliminates your prime power source, the 12 volt alternator to recharge your batteries whenever engine is running. Pretty much means you would need a generator.
In fixed solar installations, sure, but in an RV? There are lots of 24 V vehicles (mostly military), but I've never heard of anyone doing that in an RV - do you have links to any RV which comes this way, or a company converting them? I wonder what they use for conversion from battery voltage down to 12 V (nominal).

In a quick search I only found a single example of a 24 V RV system (DIY RV Solar), and that was a poor choice driven only by availability of a cheap inverter/charger.

I already identified the issue with charging from the engine, which could be done with a DC-to-DC charger.

I considered 48 V, but I don't think I would bother with 24 V or 48 V, which for an RV doesn't seem high enough to provide much benefit to offset the hassles of being at anything other than 12 V.
 

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The second application is perhaps something you have over looked. Class A Motor Homes/RV with diesel engines and 24 volt electrical systems. Like a Tour Bus Conversion with all the Bells and Whistles.
That's a good example of where it would make sense. Those are a tiny fraction of the RV world (I don't think even a normal diesel Class A would be 24 volt).

The deal breaker IMO is loosing the engine alternator as a primary power source. However people who use solar have no fear of loosing the alternator. They just use a genny or do without for times of clouds/rain.
Sure, but we're discussing an RV, and one that is unlikely (I would guess) to have a massive solar installation and will almost certainly have a 12 volt automotive charging system. Carrying a generator just because the system wasn't properly set up to charge from the vehicle would be silly, and doing without power is not reasonable.
 

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But for those with some good reason I can't imagine, 24V and even 48V alternators do exist, can be custom wound.

Maybe on a boat using electric propulsion or very big winches, bow thrusters etc
24 volt alternators are for vehicles that are 24 volt (military, maybe still some large commercial vehicles). 48 volt? Who knows... I don't know what is used on boats. I checked a random bow thruster manufacturer, and their smaller units are 12 volt while the largest are 24 volt.
 

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Well you would be surprised how many folks use Solar as their primary power source.
In fact, I wouldn't be surprised. Lots of people use solar panels rated under 200 watts to meet their needs in travel trailers (when camping in conditions in which they are happy without air conditioning)... typically with about 200 amp-hours (nominal) at 12 volts (nominal) of flooded lead-acid battery capacity. Individual energy requirements and travel conditions vary, of course, and sportcoupe has chosen a higher capacity.

As long as you keep panel wattages to less than 1000 watts which is the vast majority, 12 volts works just fine. Kicker is and this is the big kicker, if you have an Electronic Battery Isolator and drive at least every 3-days, then solar is just a wasted expense. Most setups I recommend and design only use Solar as a Supplement in the 200 to 400 watt range into a pair of 6-volt batteries wired for 12 volts...
That's what I was saying. So it seems that we agree that higher-voltage systems appear to be unlikely to be an improvement over 12 volts for the subject of this discussion.
 

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Brian I think you are overlooking some things. 24 and 48 volt super high efficiency air conditioners like Truckers, Off-Griders and Telecom use. I agree with your POV. If all you need is lighting, some entertainment like a TV/Radio/AV and low power devices 12 volts is just fine. But you have people who insist on electric cooking, refrigeration, and the impossible air conditioning requires a lot of power that a 12 volts is just not capable of doing safely, efficiently, and economically.
Some of the air conditioners for trucks run on 120 VAC anyway, and are installed with a dedicated inverter, so the only benefit of higher voltage is smaller cable. Of course in a 24 volt vehicle, 24 volt air conditioning would make sense. Similarly, in a facility where 48 volts is used for other purposes, 48 volts would make sense for air conditioning.

Lots of RV users cook electrically with 120 VAC appliances run by an inverter powered by a 12 volt battery bank.

I agree that if demand is huge, higher voltage is more appropriate. That demand would be unusual in an RV. If it isn't justified by huge demand it can still be used, but it's hard to see a net benefit. I would discuss what sportcoupe might need, but apparently that isn't allowed. :rolleyes:

Mobile compressor-based refrigerators normally operate on 12 VDC, which is no problem because their power demand is small compared to air conditioners and cooking appliances.
 

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I need a battery that supports my future camper van electrical needs/requirements as follows...
1. refrigerator (40 watts)
2. hot water [unknown power, possibly as much as 1800 W]
3. water pump [minimal power, typically less than 100 W]
4. LED lights [likely a few tens of watts]
5. device charging [as much as a couple hundred watts if charging a laptop plus multiple mobile devices]
6. small tv [likely less than 100 W]
7. gasoline heater (14 watts)
8. air conditioner (410 watts, occasionally)

None of those loads appear to be high enough to require currents that would make cabling unreasonable at 12 volts (although the water heater could be significant if large). The air conditioner startup load could be substantial, but still not unreasonably large among RV 12 volt cabling practices. I don't see a reason to consider higher-voltage configurations, whether lead-acid or lithium.

400 Ah @ 12 V is 4.8 kWh . All of those loads together do not appear to be high enough for the discharge rate to be an issue for a battery of this size (regardless of voltage), so this looks like a straightforward energy storage issue, without a power capacity issue. If this were for propulsion power (it's not), this would be equivalent to a battery-electric scenario (energy priority) rather than a hybrid (power priority). That makes nearly any battery chemistry viable.

The use of an electric water heater in an RV with a fueled space heater seems a little unusual, but the vehicle is apparently not diesel, and gasoline-fueled water heaters are rare, if available at all.

I note that there are no cooking appliances listed. Assuming that this is correct, it removes most of the large loads often seen on RV electrical systems; however, there's also this:
I still have to source an efficient microwave that will still pop popcorn. :cool:
Even the smallest available microwave oven has power demand comparable to the air conditioner. As long as there are not multiple large appliances, or they are not run in combination with the water heater or air conditioner, current and power demands should still be non-issues for this size of battery at 12 volts.
 
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