A new diy lifepo4 battery for van

Kokireg

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Apr 30, 2022
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Hello everyone,

I'm opening a post to share my set up and ask for advice too.

following the mishap of my first battery.

So I'm going to make a complete new battery from scratch.
Because I don't want to use the undamaged cells as well as the BMS of my first battery just in case if it wasn't a mistake on my part for the incident.

So I went looking for new cells and my choice is eve304 or 280k.

I contacted famous sellers on this forum on alibaba ( jenny at docan and amy at qishou).

They both propose me EVE 208K cells because there is no more than that in stock.

I will take these cells from one or the other depending on the price.

For the BMS I was thinking of taking a JK 2A8S20P 200A. (assembly in 4S 12V)
I don't want any more daly. Moreover the jk has the active balance 2A.

What do you think for the moment.

thanks
 
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What is the largest load in your system?
Hint, its usually the inverter.
In other words what is the continous watt rating of your system's inverter?
 

Kokireg

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It is a pure sinus edecoa inverter in 2000w continuous 4000 w peak.

For the solar load it's a victron mptt 30A.

For the load when the vehicle is running I use a victron dc/dc 30A.
 
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2000 ac watts / .85 conversion factor / 10 volts low cutoff = 235.294117647 service amps
235.294117647 service amps / .8 fuse headroom = 294.117647059 fault amps

235 amps is over 1.0c for your cells.
Typically we would should for less than .5c.
You BMS would also be under spec for that load.

The main circuit should be pure copper with insulation rated for 105C 2/0 awg(70mm2) minimum with a 300 amp fuse.
 

Kokireg

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at the maximum I use 1500 W during a short time.
During the tests on my old battery. At maximum I draw 150 A.
Most of the time when I looked at the DALY smart bms application I was running at 10A.
Only when using the coffee machine I was running at 120 A. It is my biggest machine in consumption with a very short use.


I don't understand:
"The main circuit should be pure copper with insulation rated for 105C 2/0 awg(70mm2) minimum with a 300 amp fuse."

The cable between the battery and the converter (12->220) is only 30 cms long and 25mm² in diameter. Protected by a 225A fuse.

I thought that for 2000 W I would have about 170 A in 12 V
so a 200 A BMS should be enough and a 225 A protection for the fuse too.
 
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I don't understand:
"The main circuit should be pure copper with insulation rated for 105C 2/0 awg(70mm2) minimum with a 300 amp fuse."
What part?
The cable between the battery and the converter (12->220) is only 30 cms long and 25mm² in diameter. Protected by a 225A fuse.
225 amps is too big for 25mm2(4 awg) wire.
4 awg should be fused no higher than 150 amps.
You wire is protecting the fuse.
It should be the other way around.
I thought that for 2000 W I would have about 170 A in 12 V
so a 200 A BMS should be enough and a 225 A protection for the fuse too.
I showed you the math.
 

Kokireg

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OK I understand.
so for you in my current configuration :
* the BMS is not good?
* the wire length of 30 cms between the battery and the converter is too small? (it takes 70mm²)
* the fuse is too small ? it takes 225 A

If I understand correctly.
 
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OK I understand.
so for you in my current configuration :
* the BMS is not good?
The bms ampacity rating is not high enough to supply your inverter.
* the wire length of 30 cms between the battery and the converter is too small? (it takes 70mm²)
I'm talking about wire thickness not length.
* the fuse is too small ? it takes 225 A
The fuse rating is too high for 25mm2 wire.
If I understand correctly.
It seems that you don't.
 
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I use this app and table for voltage drop and ampacity.
It assumes pure copper wire with insulation rated for 105 Celcius.

This resource will give a rough idea of the relationship between awg and mm2.
 

Kokireg

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I'm laughing out loud, it's my English that's not so good.
in fact on the link given, they are tables for the calculation of the sections of wire in 12 V according to their length (round trip) and of the admissible power (W or ampere).
 
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I'm laughing out loud, it's my English that's not so good.
in fact on the link given, they are tables for the calculation of the sections of wire in 12 V according to their length (round trip) and of the admissible power (W or ampere).
Watts is only meaningful in wire size if we know the voltage.
I don't see anything useful at the link you provided.
 
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Ignoring voltage drop here is what I use.

service current is 80% of fuse size.
6 awg, 100 amp fuse
4 awg, 150 amp fuse
2 awg, 200 amp fuse
0 awg, 250 amp fuse
00 awg, 300 amp fuse
0000 awg, 400 amp fuse
 
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I should mention that its my policy to size the rest of the system for the full continuous load of the inverter.
 

Kokireg

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I thought (stupidly):
1) that the battery provides 12 V (the voltage)
2) that the inverter takes the 12V to put it in 220V.
3) if I put a BMS of 200A max, I can protect the circuit with a 225A fuse because at max I will have 200A (if the BMS doesn't work anymore the fuse will be the protection)
4) knowing that I have only 30 cm in length of cable between the battery and the converter (we will say 1 meter return), according to the table that I put a link it recommends a cable of diameter of 10 mm² (I put 25 mm²). because for a length of 2 meters we can admit 2571 W or 214.3 A (that is to say above the 200 A of the BMS)


Translated with www.DeepL.com/Translator (free version)

section cable.jpg
 

Kokireg

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according to my links a 25 mm² cable "resists" to 214 A on a length of 2 meters so since I have less than 1 meter it will largely resist to 200 A.
Hence my choice of a 225 A fuse below the resistance of the cable.
 
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I thought (stupidly):
Hopefully I can help you.
1) that the battery provides 12 V (the voltage)
For LFP...
14.6 volts maximum
10.0 volts minimum
12.8 volts nominal
2) that the inverter takes the 12V to put it in 220V.
Between 14.6 and 10.0 volts.
As the voltage goes down the amp draw goes up.
3) if I put a BMS of 200A max, I can protect the circuit with a 225A fuse because at max I will have 200A (if the BMS doesn't work anymore the fuse will be the protection)
The fuse is sized to protect the wire and other passive components along the circuit.
The inverter should protect itself.
4) knowing that I have only 30 cm in length of cable between the battery and the converter (we will say 1 meter return), according to the table that I put a link it recommends a cable of diameter of 10 mm² (I put 25 mm²). because for a length of 2 meters we can admit 2571 W or 214.3 A (that is to say above the 200 A of the BMS)
Wire length is not a factor in the ampacity rating for our application.
Wire length is a factor in calculating voltage drop.
The metal, wire thickness and wire insulation rating determine the ampacity of the wire.
 

Kokireg

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that's it I'm getting less stupid.....(maybe)

so if I stay on a 200 A BMS (knowing that I will rarely pull more than 125 A on it)
I just have to change my cable (70mm²) and my fuse ?
I have good ?
 
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