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First project planning - need help
#1
Hello again,

I want start to build my battery project. I decide to make an easy pack at first.

My goal: 12 V / ~ 50 Ah (need it for a cooling box (40 W) in my car - should work for 1-2 days).

So I think I would connect 4 cells in "S" and 14 cells in "P" (so need 56 cells - each cell with 3.7 V / 3.500 mAh).

What do you recommend? 3 cells in S (11.1 V an then StepUp) or 4 cells in S (14.8 V an then StepDown)?

Which BMS do you recommend for this system?
And last question: Is it better to use less batteries with more power (3000 or 3500 mAh) or more batteries with less power 1000 or 1500 mAh)?


Thanks for your advice!

Best regards

MrRoyal007
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#2
Always go higher and step down. Uses less amps to run and runtime is longer. Also, bucks are usually a bit cheaper than boosters.
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#3
Thanks for this info!

So for my pack I should use for example 100 x 2000 mAh (12 V / 50 Ah) instead of 56 x 3500 mAh (12 V / 49 Ah), right?

Is it right that I only use the following parts?

- Batteries incl. assembly parts (bus bars etc.)
- BMS for 4S14P (3500mAh) or 4S25P (2000mAh)
- Buck-Controller
- On/Off Powerbutton
- connection part for 12V cigarette lighter

Everything forgotten?

Thanks
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#4
Yes, use the 100x cells if you have them, or planning on getting them. You could even do 100 x 3500mAh, unless it's a price range thing.

For the BMS, it's not going to care if it's 2000mAh or 3500mAh cells. It's only caring about how many are in series. Most low series BMS units can do 3s and 4s. Some even do 5s on the same board.
For the Buck Converter, get one that has two trim pots (these are blue rectangle cube like parts with a flat head type screw on top; or a flat disk with a flat head slot and two legs to one side) as this will allow you to control the upper amperage limit. So, the same unit can possible do 3A, but can be turned down to only deliver 500mA, for example. Some Bucks can do 5A, 10A, and even can do 15A for decent prices. Going higher than that and they start to get a little pricey and might be better off building one yourself.

For the "cig lighter", aka auxiliary, port, make sure to use about 16awg wire if possible. You don't want the wire getting hot when the cooler is running and eventually melt the solder connections. 40w @ 12V ~= 4-4.5A, accounting for resistance and inefficiencies in the cooler
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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#5
Thanks for this great information. That helps a lot.
Now I want get my cells ;-)

Last question: what is a 16 awg wire. Where will I get this?

Thanks
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#6
If I will use a BMS, is it important that I choose one which will support only 4S or can I use a higher BMS ex. 5S or 6S and let the unnecessary not connected?

Thanks
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#7
You must use a bms that is rated for a certain series. Most bms' can do 3s-5s, 5s-10s, 10s-16s or there abouts. Check the documentation on the bms and if it's not clear, ask the seller/manufacturer to verify.
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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