Building an unloader charger 500ma or 1A?

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compa

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Mar 29, 2021
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Hello everyone.

I am building an unloader charger, to evaluate 18650 cells. I have a question, I don't know whether to discharge at 500 mA or 1A.
At 500 mA, it takes a long time about 4 hours or more.

Does the capacity measurement vary if it is done at 500mA or 1A?

What do you recommend?

Thanks
 

floydR

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Aug 23, 2017
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I always discharge at 1A when discharging 18650's/26650's my results for the most part are in line with the cells's datasheet. on occasion I get a cell that is 200mAh over and under. But I have been testing the same brand cells, same model. My cells all have come from ninebot battery packs?cable modem 3 cell battery packs. So I say 1A, helps weed out problem cells.
Yes it does vary 500mAusually gives higher values.
Later
floyd
 

compa

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Mar 29, 2021
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Okay, I got it. You recommend that I download it in 1A. Then I will put a 3.9R resistor of 5W so that it has the discharge of 1A.

I'm making a prototype unloader charger controlled by an ESP8266, currently 12 18650 cells. The project will be 96 cells at a time.

Now I have a 7.5R ceramic resistor that causes it to discharge at 500mA.
 
Last edited:

floydR

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Aug 23, 2017
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Vale, entiendo. Recomiendas que lo descargue en 1A. Luego pondré una resistencia de 3.9R de 5W para que tenga la descarga de 1A
google translator
"Okay, I got it. You recommend that I download it in 1A. Then I will put a resistance of 3.9R of 5W so that it has the discharge of 1A"
Others may have different outlooks. but yes to the 1A discharge
Later floyd
 

Overmind

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Jan 16, 2019
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There is no reason to test under 1A if your resistors support it. Any cell in the last decade can do 1A+ on discharge by specs.
Values will differ but not extremely. Only if you exceed the continuous max current there will be big differences when measuring capacity.
 

compa

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Mar 29, 2021
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Thanks for answering. If I have set the download to 1A, I am already testing the prototype. I'll post photos ...
 

OffGridInTheCity

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Dec 15, 2018
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Another consideration that @daromer often points out - its recommended to test 'at least' as strongly as the max you plan to use the cells in your battery bank. Otherwise your battery may not perform as expected.
 

compa

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Mar 29, 2021
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On the powerwall, they will work below 200ma. But it was a generic thing to test cells. Now I have done it to 1A. I have the prototype almost ready, load, unload and reload 12 18650 cells. The data is sent to an MQTT server and saved in the database. Now I'm building the PCBs, to make the 96-cell one.
 

daromer

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Yes its always better to stress it a bit higher to rule out the less capable cells :) And most datasheets you will see that the nominal is tested in higher currents. So yes personally i would say 1A is better than 0.5A and minimum is to test at your max current :)
 

Redpacket

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Feb 28, 2018
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You might like to use a higher powered resistor. A 5W one will get very hot (burn skin). Maybe find a 10W or higher aluminium cased one & use a heatsink?
 

compa

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Mar 29, 2021
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In the end I use a 10W one and if I plan to put a heatsink and even forced ventilation.

I have already sent the PCBs to manufacturing, for my downloaded charger, I will surely make it public.
 
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