Multiple cell matching considerations in Fenix TK61 or TK75 or similar flashlights

Smurflight

Newly Enlightened
Joined
Feb 10, 2014
Messages
3
I am jumping headlong into Li-Ion cell flashlights (from NiMH) but have some questions regarding multi-cell lights like the Fenix TK61 /TK75. Despite working in computer software I do have a strong background in electronics. It seems that the Li-Ion batteries simply require the same caution and common sense as all other batteries, when it comes to handling and use, with the key difference that ignoring said rules comes with more significant consequences due to the energy densities involved. I reviewed a lot of safety posts and multi-cell flashlight reviews and am left with some questions.
It appears that most people are content to measure cell voltages after charge/rest and after use and use those criteria alone to "match" cells. That seemed OK enough until I read one TK75 review that noted irregularities in runtime expansion when trying out the extension kit. That led to an investigation of battery mAh capacities and subsequently uncovering a cell with something like 200mAh less capacity. I do not recall all of the details but I think that the voltage only method of matching would have missed that issue completely.
From this I draw a conclusion that matching multiple cells for a single flashlight should involve both voltage and mAh capacity matching to be properly safe.
Would this be an overkill position to take or just prudent safety with the materials?
Is voltage matching alone sufficient?
Is ignoring that current capacity mismatch asking for trouble? To this I am thinking yes as the cells age and the difference increases.
Opinions from more experienced forum members are most welcome.
 

SilverFox

Flashaholic
Joined
Jan 19, 2003
Messages
12,449
Location
Bellingham WA
Hello Smurflight,

Welcome to CPF.

In an ideal world, everyone would have access to the equipment needed to run capacity and voltage under load tests. That is the best way to match cells for multicell batteries.

Since many people don't have the equipment or the time to do such matching, the next step is to use protected cells. These cells have a protection circuit added to each cell. This means that the imbalance that can cause problems if one cell empties before the other is handled by the protection circuit shutting down. This is not "ideal," but it works in many cases.

Another approach is to put your cells in the light and run the light until it show signs of dimming. At this point you can pull the cells and measure voltage to see how well the cells are matched after use. Note the run time and use your light less than the full run time and that will build in a buffer.

With Li-Ion cells a voltage reading gives some approximation of remaining capacity. The problem is that each mix of chemistry used in the cells gives a little different reading so you can't transfer the results from one brand of cells to another. If you have the test equipment you can set up and test the cells you plan to use and come up with voltage targets.

Once again we are back to having the means and time to run tests...

Tom
 

Smurflight

Newly Enlightened
Joined
Feb 10, 2014
Messages
3
Those are all good points. I wonder if the accidents for most people involve those who did not go past the voltmeter stage of effort or possibly not even that far?

I am one of those who believes in either getting the right tools or not accepting the task/responsibility. I already have several good multimeters so I will at least invest in a ZTS MBT-1 load tester for the short term. I am going to have to give serious thought to some sort of multi-channel hobby charger down the road.
 

Roger Ranger

Enlightened
Joined
Jul 13, 2013
Messages
208
Hey Smurflight.
I think that part of the problem with "exploding lithiums" is that some of the batteries that are offered as new are actually recycled laptop batteries. Yes, you need a Multi-meter, etc. to check your batteries out, but the best advise would be to buy good batteries. I like good 2600's, because they offer 3/4 the capacity at half the price. (Or, the 3400's offer 1/3 more capacity at twice the price.) Be sure to buy a set of 4 junk-fire batteries ($7.99/set) to see WHY you need a multi-meter. Otherwise, if you buy good batteries and keep multi-battery "sets" for your multi-battery lights, you should be ok.
 

Smurflight

Newly Enlightened
Joined
Feb 10, 2014
Messages
3
Hey Smurflight.
...but the best advise would be to buy good batteries. I like good 2600's, because they offer 3/4 the capacity at half the price. (Or, the 3400's offer 1/3 more capacity at twice the price.)

Funny you should mention the price thing. I sat down with a calculator and worked out the cost per watt-hour of the various battery choices. Quality 2600 cells came out the cheapest. So far I got a handful of EagleTac 3400 and KeepPower 3400 for some single cell lights where I valued runtime over cost. I also bought four Tenergy (I know, some dislike these) 2600 and a dozen Tenergy 1500mAh LiFePO4 as an experiment. I got the LiFePO4 cells at a cost per watt-hour that matched the 2600 cells. I am not buying any name-your-fire batteries. When the TK75 arrives Friday I will try out the LiFePO4 cells to see what happens.

Funny thing is although I bought a TK75 I also could not resist a cheap Sky Ray Kung M4. It is sort of like with knives, you buy Zero Tolerance but you still enjoy a cheap Rough Riders. Go figure. :thinking:

Thank you for your thoughts.
 

blah9

Flashlight Enthusiast
Joined
Mar 10, 2011
Messages
2,105
I do the same thing. I get 2600 maH batteries for multi-cell lights and go with the 3400 maH ones for single-cell lights due to the economics of the situation. I have also had good luck with protected Tenergy 18650's over the last year and a half or so.
 

ven

Flashaholic
Joined
Oct 17, 2013
Messages
22,533
Location
Manchester UK
Regarding cells,i found some KeepPower 3100mah X4 protected cells for £22 delivered,or just under $40 which i think is good value compared to some.Its a good happy medium imo.

On a personal level 3100 or 3400 make no real world difference as i top off my cells after use,my use is not 2hrs on solid for example,so as you say go for best value to suit needs.
 
Top