First 24v Build with BMS

[John Frum]

10awg

Ok, In that case maybe it can provide ~20amps @ ~29.2 volts

 

[Positronium]

Ok, In that case maybe it can provide ~20amps @ ~29.2 volts

On most balance chargers the balance current is 1A to 2A. The balance leads themselves don't need to be as large of AWG.

 

[KnewB]

Ok, In that case maybe it can provide ~20amps @ ~29.2 volts

The power supply is only 10a so probably can’t get anymore than that.

On most balance chargers the balance current is 1A to 2A. The balance leads themselves don't need to be as large of AWG.

Def bulk charge first

 

[John Frum]

On most balance chargers the balance current is 1A to 2A. The balance leads themselves don't need to be as large of AWG.

Agree but I was talking about the main charge leads.

 

[KnewB]

Agree but I was talking about the main charge leads.

do you think 8s 24v 50a Daly BMS is the correct size for this battery?

 

[John Frum]

do you think 8s 24v 50a Daly BMS is the correct size for this battery?

Assuming the cells are rated for 1c continuous discharge rate which is 30 amps, yep I think its right in the sweet spot.
30 / .66 = ~45.

 

[KnewB]

Assuming the cells are rated for 1c continuous discharge rate which is 30 amps, yep I think its right in the sweet spot.
30 / .66 = ~45.

yes 1c

also, why is the end of discharge voltage so low at 2.0v? should these cells be 2.6v?

 

[John Frum]

yes 1c

also, why is the end of discharge voltage so low at 2.0v? should these cells be 2.6v?

I can't say with certainty but its not the first time I've seen this.
My best guess is that these are either old cells or cells of an older design.
The 5mohms is also atypical.
For the record typically the min voltage for a lifepo4 cell is 2.5 not 2.6 volts.

 

[KnewB]

I can't say with certainty but its not the first time I've seen this.
My best guess is that these are either old cells or cells of an older design.
The 5mohms is also atypical.
For the record typically the min voltage for a lifepo4 cell is 2.5 not 2.6 volts.

ok thank you!

 

[KnewB]

I can't say with certainty but its not the first time I've seen this.
My best guess is that these are either old cells or cells of an older design.
The 5mohms is also atypical.
For the record typically the min voltage for a lifepo4 cell is 2.5 not 2.6 volts.

can you explain the internal impedance is layman's terms?

 

[John Frum]

can you explain the internal impedance is layman's terms?

Not really.
I'm not sure I have a firm handle on that myself.

 

[John Frum]

@snoobler likely can though ^

 

[snoobler]

im·ped·ance


- the effective resistance of an electric circuit or component to alternating current, arising from the combined effects of ohmic resistance and reactance.

In the context of DIY Solar, it's easiest to think of it as simple resistance. When stated in the datasheet of a cell, it's the effective resistance in response to an impedance-type test.

Many impedance testers used for measuring cell/battery internal resistance (4 wires, 2 per probe) work by generating a ±1V sine wave at 1kHz. When this signal is applied to a battery, it calculates a Ω measurement based on how the signal is altered.

It's really only useful as a comparative measurement, i.e., if you're measuring 8 LFP cells, and they're all really close to the same mΩ reading on an impedance tester, they have highly similar internal resistance.

Impedance based resistance values applied to cells are consistently lower than a DC load test that follows Ohm's Law, V= I * R.

 

[snoobler]

Also worth noting that smaller cells often have higher resistance/impedance than the giant 280Ah cells. Pretty common for the low capacity cells to be in the whole numbers of mΩ vs. the .15mΩ common to 280Ah cells.

 

[KnewB]

Also worth noting that smaller cells often have higher resistance/impedance than the giant 280Ah cells. Pretty common for the low capacity cells to be in the whole numbers of mΩ vs. the .15mΩ common to 280Ah cells.

great thank you!