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18650 battery pack for solar
#1
Hello to all!

I intend to make battery pack for mu off grid solar system. 
I harvest 700-800 batterys and testing is in progress. Most of them are 2000-2300 mAh. I use Lito kala tester and charger. 
My batter need to be 12 or 24V. What is better for build, and what serial I should do? I see somewhere that for 24V I need 7s. 

7x4.2 is 29V. My solar panels give me 30V so I think that I will not get full charge? 
On another hand for 12V pack will give me in 4s 16.8V.. I am confused. Maybe best way to make 3s? 

What I have is that I make good and safe battery pack as my loog cabin is from wood. I dont want fire there.

Just to add, all my consumers are on 12V and my solar system is 24V where I use step down 24 to 12V.
I have 4x5 trays for batteries.
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#2
I think you need to review the FAQ, located on the main page. It would answer half of what you asked here and then answer some you haven't asked yet.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

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#3
I been using 3s 18650 packs for years. It runs all my 12 volt devices and also inverters. It easily charges with any controller. 4s 18650 pack would be harder to charge with solar, you need a controller that you can set the bulk setting to close to 17 volts to fully charge, most controllers might max out at 16 volts.

With a 4s or 7s you would need buck converters to reduce the voltage, with a 3s you might only need a boost converter to run a 12 volt fridge.

I like 3s because of the lower voltage gives me higher amp charging with my 240 watt panel. I seen up to 15 amps of charging power. With 4s lifepo4 or lead acid, my panel maxes out at about 12 amps.

As far as bms for your system, I recommend the chargery bms8t, it can handle up to an 8s system, I been using it for almost 2 years and its been rock solid. It uses a contactor/relay to disconnect the panel from the controller if the battery gets full. It also has audio/visual alarms and temp sensors that will pull the relay if temp gets to high. I use it with lifepo4 but I would definitely use it with 18650. You can program the max voltage for the cells to whatever you want. Other people use the baltrium bms but thats a 500 dollar bms, the chargery is in the 100 dollar range.

I also have an overvoltage protection relay(6 dollars) that will disconnect the panel if the bms fails. Always have a backup, or many backups, to prevent overcharging the batteries. In the picture you see my backup, in the actual system there are 2 of the 30a automotive relays, one is for the chargery bms.

With the chargery and the overvoltage relay I can leave the system unattended and not worry it will get overcharged. You can't always be there while charging so you need to make sure the system is as close to as 100 percent reliable as possible.

I never trusted the cheaper mosfet bms and wouldn't leave them unattended. I had one overcharge a battery and decided to find another bms that was safer to use with solar.

overvoltage protection relay
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#4
4v/cell * 4s = 16V. I don't see the issue here. 4V instead of 4.2V drastically increases the life span of the cells.
Going with 3s you can easily overcharge a cell. 4.2 * 3 = 12.9V. Standard Lead Acid charging goes to at least 13.5V.


Using a buck converter from 14-16 to 12V would be sufficient, and much safer. You don't want to see an overcharged lithium cell go rogue. Not a pretty site.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
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