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Old 12-04-2020, 11:07 AM   #1
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18650 house battery?

hoping that there are some DC electronic gurus here that can help me with my question:

I have built a lot of large 18650 batteries in ebikes. I am considering building one for my RV to replace the SLA. they do not have the life cycles of a Lifepo4 but they are substantially cheaper for a home build (dont do this with out experience), and they still have far more life cycles than SLA with much better performance and far less weight.

so anyway, I can build a 18650 pack with comparable capacity to a house battery SLA for about $235. (600 whr of useable capacity). I would probably triple the capacity to about 1800 whr for the build but its substantially cheaper then Lifepo4.

so my concern is the voltage.

My SLA house battery Tops out at around 12.9v. the closest that I could get a 18650 to is either 12.6 for a 3s config or 16.8 for a 4s build. It gets a little more complicated then that as well because some cells like to retain more of their capacity in different voltage ranges so if running on a 3s setup on my current choice cell, I get %50 of my capacity between 10.8v and 9.9V.

I would obviously have to add a charger in line with the converter for charging because these batteries do not like to be overvolted when charged.


My question is the 12v system of the RV. how much voltage variance do these appliances typically work right with? water pump, slides, furnace, jacks, lights water heater, fridge and such? there are ways to deal with these voltage dips if some of these appliances/systems do not like lower or higher voltage however, they come with trade offs and there is a point where its just not worth it.
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Old 12-04-2020, 01:51 PM   #2
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I don't think you're going to find much in the way of a standard for operating voltage range. Some 12V devices are automotive and may operate at that spec. Some devices will list their operating range in their spec sheets, for items that have spec sheets (furnaces, water heaters, pumps). Others are just '12V' and you get what you get. Since there's such a wide and varied level of equipment and sources I think it pretty much comes down to empirical testing.

I've built some 18650 packs (small) and for 12V applications I've used 3 cells. Regulating or converting a nominal 16V pack down to 14 or less at RV power levels would probably negate any cost or efficiency benefit.

Mark B.
Albuquerque, NM
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Old 12-04-2020, 02:16 PM   #3
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My thought would be to run a 4S config and use an efficient dc to dc converter to bring the voltage down to 12v levels, then you could use the full capacity of the cross.

The real question will be how much would the loses in the dc to dc converter affect you. Although it's the same thing those running high voltage solar and 24, 36, or 48 volt battery banks have to do to run 12 volt items in the RV.
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Old 12-04-2020, 03:26 PM   #4
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Quote:
Originally Posted by jrollf View Post
My thought would be to run a 4S config and use an efficient dc to dc converter to bring the voltage down to 12v levels, then you could use the full capacity of the cross.

The real question will be how much would the loses in the dc to dc converter affect you. Although it's the same thing those running high voltage solar and 24, 36, or 48 volt battery banks have to do to run 12 volt items in the RV.
The loss of step up or step down is one of the tradeoffs that concerns me. I al going to have some capacity loss there, i just don't know how much yet.
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Old 12-05-2020, 06:20 PM   #5
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I would worry about the runaway fire risk. Not a big deal if a cell phone catches fire or a bicycle but certainly a problem inside a house or RV.
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Old 12-06-2020, 01:41 PM   #6
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I would worry about the runaway fire risk. Not a big deal if a cell phone catches fire or a bicycle but certainly a problem inside a house or RV.

They are really only problems when built wrong or using fake Chinese cells (not being derogatory here its just a well known fact that fake 18650s are especially unsafe and come from china).


They have been in most laptops and other battery powered devices for years. Even the old teslas used them. You just have to have the knowlede to spec it right, use quality cells and build it correctly. The old Chinese hoverboards and such typically used fake cells and also did not use good build techniques.
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