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Have Flooded - want to build Lithium 1200 AmpHour

John Mitroff

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Jan 23, 2021
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Hello, I’m northern Ontario Canada, looking to build lithium battery for my off Grid house, to replace twelve - 2 Volt flooded Batteries at 1200 AmpHour, 24 volt system.

please suggest best lithium components and vendors?


Thanks
 
To size the bms we need to know your maximum draw.
Do you require low temperature disconnect?

Big blue prismatic cells are very popular on this forum.
The 2 main flavors are 272ah and 280ah

40 cells * 280ah * 3.2vdc = 35840wh
40 cells * 272ah * 3.2vdc = 34816wh

Contacts

@Amy Wan (Luyuan)
@Michael B Caro
 
4kw @ 24v nominal yields 167A continuous current. LFP nominal is closer to 26V nominal due to its nice flat curve. So4kw @ 26V yields 153A.

1200AH of lead is approximately 500-600AH of LFP.

What's your main charging source, Gen, PV, other?


Do you need redundancy such that each individual battery can carry the entire 4kw load?

The ~280AH aluminum cased prismatic cells are a good value currently. Here is my initial suggestion pending answers to the questions above.

Build 2 8S packs from 280AH cells. Each with its own 110-120A FET BMS. This is probably the cheapest route, and you have some redundancy, but you are limited to 2.4-2.8kW if one pack drops out.

This is only ~560AH nominal (504AH usable with conservative cycling), however with the higher charge efficiency, almost 1.0 peukert, and higher nominal voltage, you will have similar or greater performance than the 1200AH lead pack. 504*26V = 13.1kWhr, and all that is available, even at 150A rate.

Sourcing cells for best value typically requires you to buy cells direct from China via the grey market. These are typically ~B grade, which means they aren't perfect. Most of us have had good luck with these cells for C/2 rate off grid applications.
 
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If you want a single pack, you can go with 2-4 cells in parallel for 560-1,120AH. So 2p8S-4p8s. Then use a contactor based BMS. There are some options depending on your tech needs and skill level. The most basic is a FET BMS which drives the contactor instead of carrying a load. There is the Chargery, and Daly lineup as well. Orion etc. Take note you may need a separate balancer for big single packs like this, though 150A is low enough for a pack this size, it might be okay with a 1A passive balancer.
 
Much appreciate,

We have 2,400 watt of solar PV charging and two 13kw generac generators for backup.
As it’s becoming our year round residence the priority is longevity vs good price.
Thank you,
John
 
Assuming new cells not located in a very hot region, you can expect 7-10 years of service, probably longer if you use them conservatively. If you need additional assurance you can purchase a few spare cells, and keep them on hand.

Charging from the gen will be a lot more efficient with the LFP, they will take 4kw right until they are full. If you need higher charge rates, that would factor into your BMS choices.
 
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