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Help supplement my Tesla batteries
#1
I've got two Tesla Model S modules (~237Ah each, still testing) powered by 1040 watts of solar. My batts run my 24v 2400w inverter to power some critical loads in my house 24/7. My inverter draws between 7-20 amps during a day, about 10-50% of it's rating, spiking to 60 amps for fridges starting. I want to dabble in rolling my own batteries and need some help.

If I build a 100Ah battery (or a couple of 12v 40Ah) and parallel it with my two Teslas, I'm effectively limiting their capacity to 100Ah (40Ah) each, right?

If left connected my inverter will kill my 100Ah pack while still drawing happily (and unaware) from my Tesla batteries, right?

Ultimately I want to build a 24v xxxAh battery to compliment my two Tesla batteries. I want to start small and take my time. I think that I would like to build 4-6 12v packs that occupy the space next to my batteries (not up above, that's for later...). I want build 12v packs because I envision being able to pull one or two offline in an emergency and use them for whatever.

Here's a pic of what I have. I would appreciate your suggestions for getting some packs built, getting them incorporated or ready to use and any other considerations I missed. Thanks!
Scott

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#2
>If I build a 100Ah battery and parallel it with my two Teslas, I'm effectively limiting their capacity to 100Ah each, right?
Assuming the 100ah is 6s 18650 (same chemistry / same voltage) you can do this - especially at the low amps / cell of DIY powerwall.   You will have 237 + 237 + 100 = 574ah.    The current will move around between the 3 batteries because they are in parallel and the voltage will remain the same across all 3.       It will be easier (increase the life span) on the 100ah battery if you bring it up to the 237ah range so that less current flows.   At the level you are describing its low ma/cell so its not big a deal compared to say 10a/cell. 

>If left connected my inverter will kill my 100Ah pack while still drawing happily (and unaware) from my Tesla batteries, right?
Since all 3 batteries in parallel, they share the same voltage.   The inverter will draw until low voltage cutt-off - e.g. until the overall power is drained down from the sum of all 3.

If you think about it - an 80p pack will have 80 cells in parallel with different ah(s)...   and because they are in parallel they 'equalize' among each other to deliver amps from the pack.  Its no different with 3 bigger batteries in parallel of the same voltage range / chemistry.  

There are various articles on google about this - here's one example  https://batteryguy.com/kb/knowledge-base/connecting-batteries-in-parallel/
Sed6 likes this post
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#3
Makes sense, thanks. It's putting cells in series where capacity matters if I Googled that correctly.

So then my plan for 4-6 12v 20-40Ah batteries, built 1-2 at a time will work. Anybody want to chime in on design or form factor? Probably keep positive and negative on one end (left side, facing the Tesla's, away from the door), or bottom. Definitely no wires on top or the right side, want them clean and smooth.

I'm about to research staggering my cells versus straight lines. My driving decision is design and function. Especially considering my low amp draw. Any opinions?

Thanks for looking!
Scott
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