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Comment by 2Gkashmiri | Hacker News Reader

Hey. You said you do off grid. I have a question. I own a 5 kW on grid setup for net metering. Works fine but now I want to use backups. I have two options, either lithium 100ah 48v which is expensive or 12v 200ah x 4 to make 48v. Cheaper but limited life because the home load would be on it entirely.

I dont know. The calculation of kWh checks out to be 100ah x 48v = 4800wh. And for lead acid 200ah x 12v x 2= 4800wh.

What do you suggest or is there something I am thinking incorrectly... space is not an issue BTW.

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google "DIY powerwall" if you're up for a bit of a technical challenge.

4 x 12V 200Ah isn't big enough for most houses, unless it's like a cabin in the woods somewhere. and you're right that the cycle life of those and cost is high.

but commercial li-ion based solutions are also expensive. However, if you can keep them at 80-85% state of charge, and rarely if ever discharge them below 30%, they should have lifespans 5-6x longer than the lead acid solution.

in either solution you can't really consider the full volts * Ah capacity of the battery to be usable. If you were to ever use all of the watt-hours in a 12V 200Ah battery you'd kill it, you never want to go below 20% state of charge... so to be conservative the actual capacity of a single battery is something like:

12 * 200 = 2400 Wh, then (2400 * 0.70) = 1680Wh.

Current LFP battery prices are around $120 per kWh delivered: I just ordered 14 kWh (48V 16S) for 1700 USD delivered.

This is cheaper than an Apple laptop.

A quarter of the price paid is shipping since it comes directly from China so source price is already under $100/kWh.

A recent test by Will Prowse:

https://www.youtube.com/watch?v=3U4ZfQ_IToI

$433 280Ah LiFePO4 Cells Tested: Cheaper than lead acid!

Many people have the same good experience with these kind of cells:

https://diysolarforum.com/

Added: 14 kWh will run a 20 Watt laptop for a month.

Where are you getting new LFP batteries for that price?

There is a fellow down in Port St. Lucie Florida that has launched a 100ah LiFePO4 with a 10-year warranty for ~ $550usd each. I saw his shop in person when I purchased some solar from him and was quite impressed with the work they've done and the research and testing that has gone into their products. I had already purchased my batteries by the time I discovered his product or I would have purchased from him: https://outlandsolar.com/

My partner and I live aboard a 41ft sailboat and both work remotely as programmers. I installed 1920w of solar on the boat and get approximately 3.5-4.5kwh per day in output, depending on which way we are oriented while at anchor. I installed 600ah of LiFePO4 batteries @ 12v and when combined with that amount of solar, we have pretty much all the power we need. We frequently cook with an air fryer and have a dishwasher on the boat that we run daily-- all off solar.

That price is really good. I'm curious what cells they used.

I'm pretty sure they're Sinopoly cells, but I'm no expert. The BMS they use is quite impressive too, from what I could tell. While I was there in the shop they had a battery cut open and I was able to see inside.

Where/how do you fit ~2000W of solar on your boat? That’s impressive.

I ended up mounting two ~39x70in panels on top of the davits and one on each side of the stern, mounted to the stanchions like 'wings' that can be propped up while at anchor. Strangely enough, I found I was getting so much power from the panels that I don't bother propping them up. Because of the way the panels are mounted (basically at least 1 panel facing the sun at all times), I'm able to get roughly 500w from 8:30am until just after 5pm. I was pleasantly surprised.

SOK Battery also has an 100ah battery for $570. They look a little more reputable then a DIY'er, but kudos to him for selling. a product.

Will prowse has a link to the vendor he chose in the youtube video description, if you search for "supplier" in the forum you'll get plenty of feedback on various known vendors.

I chose one (after sending a few private message to forum members) but I'm not recommanding it until I get and test my batteries.

My dad gave me Will Prowse's book as a resource when I was rigging my van with solar - it was a great read but not a necessary buy because all the info you need is on his website.[0] His YouTube channel is a great resource that I'd definitely recommend! Interested to hear what the brand is if your tests turn out satisfactory.

[0] https://www.mobile-solarpower.com/

Beware of suspiciously cheap lifepo4 batteries from China, they are by no means all the same.

If you look for known good chinese LFP vendors on forums it minimize the risk, lots of people are reporting to spec or very close to spec batteries. And when they are issues looks like most chinese vendors will just send a replacement if you document your problem well (compare this to local store return processes ...).

But LFP batteries have gotten really cheap, so the price for a mistake is no longer prohibitive.

The part in that youtube video where he's using uninsulated steel wrenches across the top bus bars to tighten down the terminals is extremely scary.

I don't think there is a huge risk when you're dealing with 12V batteries.

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