Estimate remaining Lithium primary capacity by resting voltage???

I've been using this table for estimating remaining capacity by measuring the unloaded (resting) voltage with LiOn and LiPo for a while and find it very useful.

4.2V – 100%
4.1V – 87%
4.0V – 75%
3.9V – 55%
3.8V – 30%
3.6V – 0%

"Remaining capacity" will vary based on how hard you're pushing the cell under discharge compared to its capabilities. At low loads, <.5 A on RCR123, for example, you'll have more useable capacity left without damaging the cell than at much higher loads since the safe cutoff is based on the voltage under load and voltage droops less under low load than high load.

Does anybody have similar information for Lithium primary cells? I'm just looking for a rough guide.

Mike

lithium primary doesn't work the same... you can have 3.19V open circuit, and under a load it'll drop to 0.2V and act totally dead. The only way to test a lithium primary for capacity is voltage under a load.... I've seen lithium primaries that test 2.9V open circuit, that would give 12 "flash amps" into a short, where others with higher open voltage readings that couldn't dump an amp into a short.

I like your li-ion chart there.. in my experience that is pretty close to accurate depending on what type of load you are dealing with (like you already mentioned)...

I have to agree with Mdocod. I found a similar chart for 1.5v alkaline cells. I check them with my multimeter, then I check them on a battery tester. The chart voltage levels of good, weak, and bad, pretty much match up to the analog reading of the battery tester.

But once the battery becomes critically drained the multimeter reading is meaningless. I've had basically dead cells at rest now seem to be weak cells when read on the multimeter.

A weak cell is just find for my Fenix lights, but that false reading without a load leads to the battery crapping out before my eyes. Multimeter readings are fine in the beginning, but become less dependable once you reach battery half-life.

Thanks for the reply guys. I'm well aware that open circuit voltage is a poor guide for remaining capacity on alkaline/NiMH. I've found that a decent NiMH generally has about 30-50% left in moderate drain apps at 1.28 V and 60%+ above 1.32 V but that's not something I depend on.

I was just hoping Lithium primaries behaved more like LiOn/LiPo since I'm using them more since getting an P1D CE. My other lights work well on RCR123/18650 and I've gotten spoiled by being able to top them off frequently and having a known capacity left.

Oh well, I'll just have to carry a spare CR123 until I pick out a "3.0 V RCR123" to feed it.

Mike