How would I check battery health if only charging to 4.1V?

Apart from the obvious such as overheating cells, damage wrappers, age etc, how would I determine if my battery is healthy if I only charge them to 4.1V which is what I am doing lately? Was thinking about this last night as normally if a battery doesn't charge to 4.2V there is cause for concern, but setting voltage cutoff at 4.1v, what other ways could I use to determine overall health?

Maybe one way is to do a discharge/capacity test every so often. Set up a schedule, on a certain day of the month or every 3 months or whatever. Run a capacity test and internal resistance test and keep a log of it. I have also noticed in times past that when I have either a old cell at the end of its life or I just bought a bad cell, they would charge much quicker than a normal healthy battery. So maybe log the amount of time from depleted to your cut off at 4.1 volts.

Just a few ideas.

mah consumed from full charge to min voltage.

Charge to 4.2 cutoff. Discharge to min voltage and recharge to 4.2 with a charger that tells how many mah's are put back in. Or vise versa.

I would say that any battery that still puts out 80% or more of its rated mah is healthy.

Some good ideas, thank you toddcshoe and RotorHead. So a capacity test is still the best way then. I'm going to keep notes to do it every couple of months then.

There’s nothing magic about 4.2v, other than it being a good compromise between capacity and cycle life. You could charge to a termination of 4.3v and you’d gain about 10% capacity, however, you’d lose at least half your cycle life.

You have chosen to charge to 4.1v, which drops your capacity by about 10%, but doubles your cycle life for your battery. Just do whatever tests you would normally do at 4.2v, but do them at 4.1v. As long as you’re consistent, it should be a fair indicator of the health of your cell.

The only problem I see is it might be difficult to do a proper CC-CV charge to 4.1v, using a normal lithium-ion charger. If you terminate at 4.1v as part of the CC charge phase, your cell will probably drop a few dozen millivolts, maybe closer to 4.05v. A slow charge will help prevent that.

Thanks for the help. I can do it on my skyrc charger. When I set it to 4.10V target voltage with 0.10a termination, my two dmm reads the cells after a little rest at 4.103 to 4.104V.

I noticed today on my fairly new protected Keeppower 18650 batteries (sanyo ncr18650ga) that I use in my Astrolux MF02, that one of the batteries has taken 90 minutes longer to reach it's target voltage of 4.1V that I set for it. All four batteries were pulled from the flashlight with the same voltage of 3.687. Isn't 90 minutes longer to reach same voltage a sign that something is not right, or is that fairly normal?

Most probably that specific cell has a higher IR or needs to be ’properly ‘ seated for a lower IR.

You're right tatasal, thanks man! Just wiped the cell and and battery contacts and IR dropped from 357 to 160. Going to give a quick wipe to every battery from now on.