DIY Powerwalls

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DIY Powerwalls

Postby iconicflux » Mon Feb 22, 2021 9:53 am

I've been looking at doing a DIY powerwall for my solar storage either out of scooter batteries or buying a battery tab welder and building banks of 18650's with BMS's in fireproof canisters (like 50cal ammo cans). Right now I'm leaning towards using scooter batteries because they're relatively cheap and it's pretty easy to overcome the small technical hurdle to use them. (The BMS's need a specific serial instruction every 5 seconds).

Anyone else done this? Would anyone else be interested?
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Re: DIY Powerwalls

Postby saltydawg » Mon Feb 22, 2021 11:23 am

Just buy some lifepo4 cells and a bms, safer, cheaper in the long run than new 18650s.

4 -260 amp hour cells are just are about 420 bucks.
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Re: DIY Powerwalls

Postby Socal Tom » Mon Feb 22, 2021 11:34 am

Are you sure those are "cheap" in terms of price per amp-hour small batteries tend to get very expensive.

Two of these would give you 230 amphours ( at 12V) and cost about $270, so a little over $1 per amphour.https://www.batteriesplus.com/productdetails/SLIGC115?storecode=758&locationofinterest=&locationphysical=9061192&utm_source=google&utm_medium=shopping&utm_campaign=ecommpla&gclid=CjwKCAiAyc2BBhAaEiwA44-wW7dovh93CApkydQvmwVOeT2NDZ27LkIRhab-yMvnQL5Cc_e5Hqh0MRoClg4QAvD_BwE

It would take 30 of these https://www.batterysharks.com/Razor-E20 ... IsQAvD_BwE
to get the same 230 AH, which would cost $675 so roughly $3 per AH
Is this for a trailer or for a house? If its for a house, and weight isn't an issue, you might even be better off with some 2V batteries designed for that purpose ( My brother has an off-grid cabin).
If its for a trailer, then you should really determine what your required power needs are and try and stay close to that ( with some extra buffer of course). Technically any of us that have solar panels and a battery have a "power wall" . Mine is an 85 AHr deep cycle marine battery that cost about 100$ at walmart. just a bit over $1 per AH.

You also have to pay attention to the discharge rate for the batteries you get. the cheaper batteries will probably have really low discharge rates, and may not support what you want to do. What size inverter are you planning to use?

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Re: DIY Powerwalls

Postby John61CT » Mon Feb 22, 2021 2:58 pm

Lithium chemistries other than LFP and LTO, are too fire risky for inside a home

forget about tiny mobile living spaces.

In the NA market good deep cycling FLA is $1/Ah @12V hard to beat.
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Re: DIY Powerwalls

Postby John61CT » Mon Feb 22, 2021 2:59 pm

SQL error messages from the server
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Re: DIY Powerwalls

Postby saltydawg » Mon Feb 22, 2021 5:58 pm

John61CT wrote:
In the NA market good deep cycling FLA is $1/Ah @12V hard to beat.

Until you see the cost doubles for actual usable amperage, then quadruple when it comes to life span.

With a simple lifepo4 battery build 280amp hours for under 500 bucks, usable out of that is all 280 ah for 3000 cycles, or down to 80% and get 225 amp hours for 5000 cycles.

In that time you would need 4 or 526 bucks of those 6 volts ones linked to have the same usable power and you would replace them at least once probably twice before the lifepo4 died. And the lifepo4 would weight around 40 lbs vs 200 lbs.
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Re: DIY Powerwalls

Postby saltydawg » Mon Feb 22, 2021 6:04 pm

post ed it again due to sql error stuff
Last edited by saltydawg on Mon Feb 22, 2021 6:05 pm, edited 1 time in total.
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Re: DIY Powerwalls

Postby saltydawg » Mon Feb 22, 2021 6:04 pm

sorry rapid fire posting
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Re: DIY Powerwalls

Postby bdosborn » Mon Feb 22, 2021 7:18 pm

I've built 3 batteries using LiFePo4 prismatic cells. I was thinking about playing with the 18650 cells until I read the analysis of the Arizona Power Service, McMicken Battery Energy Storage System failure. It scared me away from anything but LiFePo4.

https://liiontamer.com/wp-content/uploads/APS-DNV-GL-Report.pdf

This was a professional installation using top quality cells and they had a thermal runaway. The BMS worked, the fire suppression worked but it still blew up. They are changing the fire codes because of this incident, it was a real eye opener for the entire industry. I don't think anyone realized how hot lion cells burn and how hard it is to manage a cell in thermal runaway.

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Re: DIY Powerwalls

Postby MtnDon » Tue Feb 23, 2021 12:07 am

Yes, about the only batteries with worse fire risk would be the NaS (Liquid Sodium-Sulfer) research battery we have in Los Alamos. The fire department training is all about forming a perimeter and watching the fire burn out.
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Re: DIY Powerwalls

Postby iconicflux » Tue Feb 23, 2021 6:39 am

I was actually meaning 36v scooter packs like those from jehu but it turns out theyre only 10ah a pack. Next best option is the 280ah lifepo4 packs like the people from the diy solar group tend to use.

Lead acid sounds good on paper until you double it because youre only supposed to discharge to 50%. So you end up with a super heavy and large battery pack. Between the cost being not that much cheaper once doubled and the weight im just not sure they make as much sense.


Id love to hear people's experience with the 280ah packs.
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Re: DIY Powerwalls

Postby iconicflux » Tue Feb 23, 2021 6:54 am

Thanks for posting that. It should be noted that the gases made during the runaway appear incompatible with life. Aside from the explosion they had displaced the oxygen in the air.

Really a great read.



bdosborn wrote:I've built 3 batteries using LiFePo4 prismatic cells. I was thinking about playing with the 18650 cells until I read the analysis of the Arizona Power Service, McMicken Battery Energy Storage System failure. It scared me away from anything but LiFePo4.

https://liiontamer.com/wp-content/uploads/APS-DNV-GL-Report.pdf

This was a professional installation using top quality cells and they had a thermal runaway. The BMS worked, the fire suppression worked but it still blew up. They are changing the fire codes because of this incident, it was a real eye opener for the entire industry. I don't think anyone realized how hot lion cells burn and how hard it is to manage a cell in thermal runaway.

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Re: DIY Powerwalls

Postby tony.latham » Tue Feb 23, 2021 11:20 am

I've been looking at doing a DIY powerwall for my solar storage either out of...


I assume this is for your camper. How many amps do you need?

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Re: DIY Powerwalls

Postby iconicflux » Tue Feb 23, 2021 3:23 pm

I'm figuring a use of about 4.6kw/day if we wanted to boondock completely off-grid.

Given that usage I'd need about 383Ah at 12v or about a 400a system. If I went to using 2 of the 280ah in parallel for 580ah then I'd have some extra wiggle room and we could plan on pulling into an RV place every 3-4 days to charge overnight.

If I add another solar panel to give myself 1040watts solar (instead of 875watts) then the average sun of 3.83 hours in Maryland (and similar latitudes) would let me get pretty close to running everything off batteries. I'd probably have to shut things down to a minimum about every 3 days and use that to recharge the batteries. If we can figure out when those maximum sun hours are and cut off the loads that draw so much power (mostly computers/internet from my calculations) during those hours then we should be able to dump most of the power back into the batteries.

But it still leaves me figuring out how to do storage for about 4.6kw/day.

I'd do a 24v or 36v system but I think (not positive yet) that a 24v system at lower amps would take longer to charge the batteries. Still trying to work out calculating charge rate for given voltage/amps.


tony.latham wrote:
I've been looking at doing a DIY powerwall for my solar storage either out of...


I assume this is for your camper. How many amps do you need?

Tony
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Re: DIY Powerwalls

Postby bdosborn » Tue Feb 23, 2021 8:17 pm

iconicflux wrote:I'm figuring a use of about 4.6kw/day if we wanted to boondock completely off-grid.


Wow, that's a lot. We usually use about 1kW-hr max a day, and that's with two compressor fridges. Are you using electric cooking? Here's my usage while boondocking for a couple of days.

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I have 360 amp-hrs of CALB 180 cells, and 400 watts of PV. I've got a 12V system because I want to charge from the tow vehicle and I don't want 12-24v DC conversion losses. I never use our inverter, everything runs off 12V.

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