Hi, specific to the TK40 manual, there's no indication of using lithium primaries, only AA and NiMH. And I can only assume when 4sevens does not recommend lithiums was for safety reasons.
What I'll be saying here is based on research and have past experiences of leaky alkalines (AAA, AA, C and D) uncounted for. Mostly in serial configuration from 2 to 4 and above. When I started understanding how these batteries behave, my problems with them have long gone.
Primaries are very sensitive and has a lower resistance, although they are efficient with high current draw applications, they also cannot be drawn too rapidly or they'll get very hot. The reason why it gets hot very fast when putting in reverse polarization because they discharge rapidly when short circuited. It is not caused by the TK40 product.
I personally find lithium primaries are fine if used properly. Assuming the electronic tolerances of the TK40, when putting fresh L91 batteries, they are at 1.80v. Which equals to 7.2v with 4 primaries in serial. Looking at the current draw at 2.3-2.6A (with 1.2v Nimh) on turbo, this load should be perfectly fine with lithiums (the amps draw result should be lower because of higher voltage). For increased capacity, this is where the parallel circuit comes into the picture with owners having problems of leaking alkalines, over-heated lithiums... In general, when running on any chemistries (NiCAD, NiMH, Lithium, Li-ION, alkalines) in parallel, each side should be in the same state of charge. Being primaries, it is more sensitive and must be given attention when using 4 of them in serial, and parallel with another 4 in serial.
An example, during operation of the TK40, when one battery has a lower voltage than others, it depresses the group performance, and the end of discharge point will reach sooner than expected. that battery can be shorted, prolong operation will cause leakage or rapid heat generated by lithiums. But more to it, that group still contributes to the parallel circuit, while the other group of 4 lithiums still performing, rapid discharging occurs and the batteries get heated up rather quickly. And on the other hand, the TK40 has IPX-8 standard which also seals mostly what's inside, I can't assume what will happen... But not a very good end result.
If you have a group of primaries with different voltages, you can still operate your TK40 with 4 of the same voltages at least. When it depletes or when the light start dimming, replace them with the next 4, it appears to be safer. If you have a digital multimeter, it's good to check the voltages if you have been using the batteries over an hour continuously for example, so you'll keep in mind how much it has been drained and how long you can store with the small parisitic drain.
Do you know that your 18650 Li-ONs are serial, parallel design similar with the TK40 concept? It is stable because they are wired permanently and with other factors, some too technical. Most Li-ONs has protection circuit we know about, which monitors each cell connected in series and strings in parallel.
Well, there you go. I think why they didn't recommend was more related to safety. Corrections are welcome.
