How do you tell when your Li-ion batteries need to be recharged?

[Marco Polo]

I searched LPF for this but did not find a satisfactory answer. I'm posting this question as one who has previously used only primary batteries in his lasers. I am now getting into higher-powered lasers which use lithium ion batteries, and my general "rule" is that I would rather ask a question and look dumb than make a mistake and look even dumber.

The way I know when my primary batteries need replacing is when the laser dims out. It's pretty obvious. But from reading about lithium ion batteries I get the impression that this method just won't cut it when it comes to using them - I do not want to ruin my Li-ions through over-discharge, and I really don't want to take safety risks when it comes to recharging them.

So, how will I know when it's time to recharge my Li-ions? Is it too late for the battery if the laser is dimming out? Should I just recharge the battery as a matter of course after I've finished using the laser?

Any advice in this matter would be appreciated and I thank you for your time.

Edit: The batteries in question are 16340 (from Orbtronic) and 18650 (from LG) lithium ion batteries as those are what my lasers will require, though I suppose the question is relevant to Li-ion batteries of any type.

 

[Flaminpyro]

This is how I do it, after using any laser with a Lithium cell I put the batteries in the charger if the red light comes on I leave em in there and if the light is green then I put them back in the laser...

Merry Christmas :san:

 

[Blord]

You make life so simple.

Li-ion batteries don't have memory effect. Charged them whenever you like.

 

[Laser Smith]

I usually check mine daily with a Multimeter and charge them if they even get near 3.5V

@FP And a happy new year :san:

 

[Hiemal]

Yep, only NiCd and NiMH batteries have memory effects. Li-ion batteries need be charged when they get around 3 volts if I remember right. If they get below 2.2-2.3 volts you may need to get new cells... Li-ions are sensitive to overdischarge.

 

[Cyparagon]

Like others, I use a volt meter. Anything below 3.5V or so is low on charge.

 

[AUTO XX]

Lithium batteries discharged below 2.8v will be permanently damaged but anything under about 3v is not good for them.
The voltage drops off fast after ~3.5v so there isn't much point using them below that.
You could always just get protected cells, the PCB will shut down before the battery drains TOO far if you forget about checking (I rarely check my batteries).

 

[LarryDFW]

Here is a good graph from Texas Instruments showing "State of Charge" vs Voltage:

You should let the cell rest for 30 min. first, for best accuracy.

LarryDFW