What is the lowest voltage you can safely drain an 18650's to ?


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B8rez 2g4

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Jun 21, 2021
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I'm programming a warning alarm to alert when the voltage is too low.

Would 3.2V be the most commonly accepted low voltage reading before it needs to be recharged?
 

OffGridInTheCity

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Dec 15, 2018
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I'm programming a warning alarm to alert when the voltage is too low.

Would 3.2V be the most commonly accepted low voltage reading before it needs to be recharged?
Lithium-ion can go down to 2.8'ish (typically). You can check the specs of your specific cells to double-check.
However....
1) It's hard on them to drain them 100%
2) There isn't much power below 3.4-3.5v because of the discharge knee

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If you look at the chart above, using 0.5C as an example, the discharge curve is pretty flat from 3.7v -> 3.6v and you get 4 times the power as you do from 3.6v -> 3.5v and 8 times the power from 3.5v to 3.0v. You're cells will vary a bit but the chart is representative of all lithium-ion chemistries.

This is why folks shoot for 4.0v to 3.5v operating range for lithium-ion if you want the batteries to last for a long time - e.g. better to get 75% 600+ times that 90% 300 times and perhaps make the battery larger to maintain the 75% DOD.
 

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paddy72

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Nov 17, 2021
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It always depends on the discharge current rate and your application or capacity you need. For an ebike, where you want as much capacity as possible (in some situations) you would go a little less with the min. voltage (3.4 ... 3.0) but when you have enough capacity and discharge current is rather low you can stop at 3.5... 3.3V.
The less Delta SoC - the more cycles you get out of your cells and the less current you need the more capacity you can use.
 
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