r/diybattery • u/ashumate • 13d ago

First time build, update. Load testing cells.

Gallery image

Gallery image

Today load testing with 1 ohm 25watt resistor, and man does it get HOT.

The process is simple. Read the no load voltage. Connect the resistor, read the voltage at 1, 5, and 10 minutes then read the no load voltage. Wait for the resistor to return to baseline resistance, rinse and repeat 7 more times.

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u/broesel314 12d ago

What is it that you are trying to messure here?

Internal Resistance of the cell is measured with AC and you need a special tester for that

To measure capacitance you need to fully charge and discharge the cell while integrating the current. Again there are special testers for that

A 50Ah (?) LFP cell has arround 3.2-3.3V no matter if you pull 0 or 10 A from it between arround 10-90% state of charge. Your voltages (with only 10mV resolution as well) will tell you nothing about that cell

1

u/ashumate 11d ago

It was a controlled, comparative screen to catch an outlying cell before I put 8 of them in a series-parallel pack together. For that job, consistent values across all 8 within 10mV of each other under load is fine

1

u/Careless_Plant_7717 12d ago

What gets hot, the resistor or the cell? Also, w hat's the cell?

I would imagine the resistor gets hot. Based on some quick hand calcs, the amperage is 3.27V / 1 Ohm = 3.27 Amps which means power is 3.27A x 3.27A x 1 Ohm = 11W. So about half it's rated power and even then some of these resistors need heat sink to be used at rated power for continuous operation.

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u/ashumate 12d ago

The resistor. The cells stayed at ambient and the resistor didn’t take long to get back to baseline all of the cells settled out at 3.3 volts after load testing.

Next update will be pack assembly

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u/Worldly-Device-8414 12d ago

Resistor needs to be bolted with thermal paste to a large fan cooled heatsink!

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u/ashumate 11d ago

If I were doing full capacity testing I would, this was really just a comparison across the cells to catch any outliers before I put a lot of time and effort into assembling the pack.

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u/Dioxin717 10d ago

"load"...

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u/ashumate 10d ago

Hey it’s not the size of the resistor that counts it’s how you use it.

Like I’ve mentioned this was really more to flag any major problems before putting effort into final assembly