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12V 280Ah LiFePo4 battery Build and Test

Pi3.14 said:
not sure how many amps the 6m terminal screws can handle.
Click to expand...
My 280Ah cells are rated to a 1C discharge rate 280A and they have 6mm bolts.
Of course, it's not about the size of the bolt, but the cross section of the conductors in their entirety and the contact resistance.
Clean your terminals and bus bars well in a low humidity environment, lubricate them and immediately lay a thick bus bar down on the entire terminal. Secure with the recommended torque.
 
What is the width of the terminals on your batteries? That width should dictate how wide the bus bar needs to be. Thickness I don't know how to calculate, but I believe there are bus bar calculators or formulas out there.
 
HRTKD said:
What is the width of the terminals on your batteries? That width should dictate how wide the bus bar needs to be. Thickness I don't know how to calculate, but I believe there are bus bar calculators or formulas out there.
Click to expand...
Yes, there are a number of bus bar calculators out there. You need to measure the cross section of the bus bar, or transpose the formula to identify how big the cross section has to be for the current you desire.
For manual calculations, the rule of thumb for copper is 1.6A per square mm of cross section. Aluminum is 1.0A.
Example:
If you have a copper bus bar that is 3/4" (19.05mm) wide and 1/4" (6.35mm) thick, that is 121mm^2. 121 X 1.6 =193.6A capacity.
If you want to go the other way, start with your desired current rating, and divide by 1.6.
You can then play with the numbers to find the width and thickness that fits your application.
 
1.6 A/mm² is pretty low though, but it all depends on how much heat rise you can accept.
 
Any bus bar I buy or create has to at least cover the terminal. A busbar that doesn't cover the entire terminal would seem to leave some conductivity on the table. I can't find specs on the terminal width, only the terminal height. So how wide are the terminals?
 
The bolt is M6 and the drawing shows the terminal is a bit bigger than 3x M6 so it's around 20 mm ;)
 
I have found this source of information, seems to confirm to some extent the "rule of the thumb" going around, but more technical.
https://www.copper.org/applications/electrical/busbar/bus_table3.html

For my application - a max 400A current draw - I've decided to procure a 1" x 1/4" flat , as I don't trust the flimsy bars that come with the cells.
 

Attachments

  • Electrical_ Busbar - Table 3_ Quick Busbar Selector.pdf
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HRTKD said:
A bit smaller than expected. Seems a bit dainty to me. Bummer. A 3/4" wide bus bar will hang over just a tad. Better too wide than too narrow.
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The surface area of the terminal is 206mm^2. Being aluminum, that is 206A by the rule of thumb. That's not quite 1C, but it's far more than I'll ever ask from the cells. The BMS will smoke long before I get there.
 
Makes me wonder what EVE's intended application is for a 280AH cell with a 206A terminal?
 
JC777 said:
Makes me wonder what EVE's intended application is for a 280AH cell with a 206A terminal?
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I don't know of too many applications that require a 1C rate. Using all your power in an hour sounds like poor planning, or abuse.
What application do you have that needs more than a 0.74C rate?
 
I don't have anything close to a 1C load rate, but if I have to run a generator to recharge, I will want short run times.
Why is using a battery to it's specs poor planning or abuse?
Did you factor in the loss of the fastener hole in the surface area calculation?

When they arrive, I will test my 12 volt battery as close as I can to Section 5 of the attached LF280 spec sheet.
 
Sorry, forgot to attach.
 

Attachments

  • 280ah 3 2v lifepo4 battery.pdf
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JC777 said:
I don't have anything close to a 1C load rate, but if I have to run a generator to recharge, I will want short run times.
Why is using a battery to it's specs poor planning or abuse?
Did you factor in the loss of the fastener hole in the surface area calculation?

When they arrive, I will test my 12 volt battery as close as I can to Section 5 of the attached LF280 spec sheet.
Click to expand...
Standard design practice is to build margin into your design between the limits of any component of the system and the use case.
Do you replace the 15A breakers in your house with 30A because the specification for 14AWG wire says it will handle that much current? Do you drive your car at max RPMs because it's supposed to handle it?
I would also call those things abuse or poor planning.
Anything pushed to its limits will fail prematurely. All it takes is one small detail to slip and your batteries get puffy.
There are several threads on here from people who have done exactly that.
Just don't say that you were never warned.
 
Haugen said:
Standard design practice is to build margin into your design between the limits of any component of the system and the use case.
Do you replace the 15A breakers in your house with 30A because the specification for 14AWG wire says it will handle that much current? Do you drive your car at max RPMs because it's supposed to handle it?
I would also call those things abuse or poor planning.
Anything pushed to its limits will fail prematurely. All it takes is one small detail to slip and your batteries get puffy.
There are several threads on here from people who have done exactly that.
Just don't say that you were never warned.
Click to expand...

I think that the truth lies somewhere in the middle. I don't think its fair to call using the battery at or near its rated limits "abuse" but I do think that its not ideal and leaves little margin.

Correct me if I'm wrong but I don't believe there is a magic C-rate above which your batteries are quickly and irreparably damaged and below which you are safe. I believe bulging batteries come from poor cells, overcharging or a combination of the two.

That said, most manufacturers reccomended C-rate is 0.5c, 0.3c, or 0.2c and I agree with you that its best (and usually not difficult) to design for much lower C rates.

Furthermore, the max continuous C-rate is relative to temperature. Some datasheets will have a table showing max continuous charge rate at different temperatures. Battery life will be best at lower c-rates. Here is an example from CALB:

calbtemp.png

On the other hand, if EVE says their cells can handle 1C, the included busbars should be bale to support 1C (or greater). If anything is a design failure I would say its this (though its an easy one to fix).
 
Dzl said:
I don't think its fair to call using the battery at or near its rated limits "abuse" but I do think that its not ideal and leaves little margin.
Click to expand...
You are right.
I just don't trust the specs on the blue aluminum case prismatic cells. Glad to be wrong but I gotta call it like I see it.
 
smoothJoey said:
You are right.
I just don't trust the specs on the blue aluminum case prismatic cells. Glad to be wrong but I gotta call it like I see it.
Click to expand...

I agree, my own personal preference, is to try to be conservative and leave ample safety margin wherever I can. In the case of C-rate I think I would have to struggle very hard to get anywhere near 1C continuous charge (or even discharge).
 
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