lowest safe battery voltage

[Kobe1]

Hi folks,

I am running a dual 18650 unregulated series pwm mod with a built in voltmeter.

My question is what is the lowest voltage reading to safely run the batteries?

I have read that 3.5/3.6V is the lowest you should run a single sell. My assumption would be that for a dual series box this should be doubled, resulting in 7.0/7.2V. Are my assumptions correct?

Also, I only see the "true" voltage reading once my atty is not connected. I'm running a 0.4 ohm dual coil and when i set my potentiometer to 100% I get a 6.69V reading. When I disconnect the atty i get a 7.65V (+- 1V more than with atty connected) reading.

Do I need to disconnect the atty every time I want to get the true reading? Or can I just set the pot to 100% with atty and add 1V?

Any advice would be extremely helpful.

Thank you in advance

 

[K_Tech]

Hi folks,

I am running a dual 18650 unregulated series pwm mod with a built in voltmeter.

My question is what is the lowest voltage reading to safely run the batteries?

I have read that 3.5/3.6V is the lowest you should run a single sell. My assumption would be that for a dual series box this should be doubled, resulting in 7.0/7.2V. Are my assumptions correct?

Also, I only see the "true" voltage reading once my atty is not connected. I'm running a 0.4 ohm dual coil and when i set my potentiometer to 100% I get a 6.69V reading. When I disconnect the atty i get a 7.65V (+- 1V more than with atty connected) reading.

Do I need to disconnect the atty every time I want to get the true reading? Or can I just set the pot to 100% with atty and add 1V?

Any advice would be extremely helpful.

Thank you in advance

Hi @Kobe1. The lowest "safe" battery voltage depends on the battery, but it's usually under 3 volts. For example, the LG HE4 data sheet has the end voltage (under load) at 2.5 volts. "Resting" (unloaded) voltage will be higher, of course.
However, if you look at voltage vs. percent charge left, once you're at around 3.5 volts resting, that cell is at or very near 0% charge state (Again, depending on the cell).

For the tl/dr version, if you can, charge your cells when they get to 3.6/3.7 volts unloaded, they'll last longer. Repeated discharging until they're empty shortens battery life.

As to how you should check the voltage, if your setup consistently reads 1V lower when it's being discharged, you can use that as a reliable method to guesstimate the resting voltage, and I'd suggest charging when your reading drops to 6.2 - 6.3 volts.
That being said, and not to be "that guy", but do you mean your build is 0.4 ohms, or you have two 0.4 ohm coils in parallel? Because that would be 0.2 ohms, and with a series 18650 setup you're pushing over 30 amps, which may not be a great idea. There aren't many batteries capable of 30 amps CDR, and none that are rated for +30 amps, regardless of the hype the marketing department would like you to believe.

 

[Bunnykiller]

just to be on the "safe" side... 3.3 for single/parallel or 6.6 series(2) no need to drive them down to the minimum...

 

[Kobe1]

Hi @K_Tech thank you very much for your response. I am infact using the LG HE4's so your example was spot on

My build is reading 0.4 ohm so each coil will be 0.8 ohm. I do not intend building any lower than this for this setup.

Thanks @Bunnykiller I think I'll go with the safe option of 6.6 but in an emergency will know that I can go down to 6.3