Fenix TK75 Review including Extended Runtime Kits (2/4/6/8/10/12x18650 or 4/8xCR123)

[gigahertz]

You have to use the same capacity batteries and the same model/manufacturer. It is also strongly recommended that all batteries must be balanced (the same batch, the same number of charging/discharging cycles). These are condition for using batteries in multiple configuration.

I know it stings, but as AbbyY said, you really should use batteries from same batch/capacity/manufacturer. Better to order a whole new set of 3400s when you order the extension tube (eight of them if you buy one tube). You can either keep the original four 2600s as spares, or sell them on CPFMP, or use them in other lights. Mixing cells is a bad idea - those warnings are there for a reason, and people have found out the hard way that lithium cells can fail spectacularly if they are mismatched. Learn from the mistakes of others, and stay safe!

Thanks for your replies. The horror stories I read here about these cells going boom had stopped me from getting anything with lithium rechargeable.

That was until the TK75 was introduced, I love this light

 

[gigahertz]

I'm going to be controversial here and contradict the previous two posts.

The TK75's battery carrier uses 2S2P configuration which gives the 8.4V working voltage. If you add a second you still have 8.4V as the second carrier is also 2S2P, and is connected in parallel by the TK75's contact design.

Now the effect of this is that combined the cells are get 2S4P (remember that the total number of cells for XSYP = X x Y, so 2S2P has 2x2 or 4 cells and 2S4P has 2x4 or 8 cells).

The reason for talking about this is to reinforce the most important principles when combining cells.

If using cells in series, it is very important that they are as well matched as possible in capacity, charge level and that they are protected.

However, when using cells in parallel, this capacity matching is far far less important. In fact the only really important factor when using cells in parallel is matching the charge level. (Note that cell age can affect its ability to hold full charge, so cells should be similar in age - and age refers not just to calendar age, but to cycles as well).

When you have cells working in parallel (or even batteries - being a collection of cells), each individual cell will work together with the others, and if one cell is weaker, the others will simply work a bit harder. All the cells are linked in voltage, so it will be the strongest cell that works hardest. If you have one 2600mAh cell with one 3400mAh cell connected in parallel, during use, the 3400mAh cell would normally maintain its voltage better than the 2600mAh cell, but when working in parallel, as the load causes the voltage of the two cells to drop, the stronger cell will take the load off the weaker one but the cell voltages will stay the same.

So take this to the TK75's configuration...

The TK75 can run on 2 cells as long as you install these in 2S1P configuration (one cell with +ve facing towards the LED and one towards the tailcap). These cells are one 2S1P 'battery' and need to be closely matched.

Theoretically, you could now use 2 matched cells of a different capacity to make up the second 'P' part (to make it 2S2P), however it is far easier to avoid mistakes, by using a set of 4 matched cells in the TK75's battery carrier.

That is one battery carrier, so when moving onto a second one (which will be connected in parallel), the theory of using cells in parallel comes into effect. So if you have a second set of 4 matched cells that you can keep together in the second battery carrier, this will then be working in parallel with the first battery carrier so the actual cell capacity is not important, only the level of charge matters.


So despite the overstated paranoia regarding li-ions on CPF, given a bit of logical thought, I personally would be happy to use the cells as you described with 4x2600mAh in one carrier and 4x3400mAh cells in the second. I am happy to do this ONLY because they are used in parallel, if the combination of battery carriers was in series, I would not consider this safe.

Before doing this I would ensure all cells are charged to the same state of charge (measure the resting voltage of the fully charge cells), and each set is kept together and never mixed between the carriers.


Yes, ideally, in an ideal world, and one in which money is no object then get 8 new matching cells, but in the real world where money is a consideration, as long as you think about what you are doing, you don't need to buy 8 new cells, the four you are thinking of will be fine if used as I have described.

Thanks for taking the time to post such a detailed well explained reply. You reasoning make sense as that's how I understood in general how batteries in parallel arrangements work.

I will take your advice and order up some new batteries for my TK75 ext tube.

Cheers!

 

[tedscossie]

Hi there,
very nice and interesting review with nice pics!
I am very interesting in buying this torch because it has a good price concerning his lumens and abilities!
I look forward to have one in my hands!

Ted

 

[Goredoth]

Thanks for the review really detailed I was really interested in the extender kits so appreciate you going the extra mile and doing that for us!

 

[Richwouldnt]

I just ordered the TK75 Cree XM-L2 version and one extender. One thing that amazed me when reading the review of the light here is that a light weighing only a bit over one pound w/o batteries, and putting out 2900 Lumens, can run continuously in Turbo mode without suffering damage or melt down. LEDs must be getting considerably more efficient than several years ago or Fenix has done an outstanding job of heat sinking the LED to dissipate the heat and prevent heat buildup. To me the reasonable weight combined with the extremely high light output and the extended run time at maximum output allowed by the battery extenders makes this light the current champion of combined output and practicality for a high output torch.

I remember when the SR90 came out and the excitement at the time. It looks to me like this is about 1/2 the weight, has almost 50% more output and handles the heat of maximum output continuous operation better than the much more massive Olight ever did. Isn't progress great!

 

[1uke]

I just received this torch and it's a total beast! I now have to get a good holster as with the battery extender and extra batteries it weighs quite alot. I had it on turbo mode for around 30 minutes and it certainly heated up but not to any extent that I couldn't touch any part of the light. I'm very happy and I didn't realise the range until I got into the open.

 

[subwoofer]

I just received this torch and it's a total beast! I now have to get a good holster as with the battery extender and extra batteries it weighs quite alot. I had it on turbo mode for around 30 minutes and it certainly heated up but not to any extent that I couldn't touch any part of the light. I'm very happy and I didn't realise the range until I got into the open.

You are going to love this light.

I'm constantly torn between using the extenders and not. Without it is very carryable. With one it becomes more of a burden, but you get all that runtime. With two extenders...well it has to be a special occasion.

I've fallen into a routine of when it is in the house it has one extender on it. If I'm going out with it, frequently the extender comes off (but the battery carrier it taken loaded with cells ready to swap out).

 

[theOS]

Hi!
I must say i am kind of disappointed by Fenix ARB-L2 batteries on my fenix tk75 (2009lm version) on turbo mode they last only 28 min.
I have test them on a Tk75 2600 lm version and they last a little longer ... 38 min on turbo mode (accumulated time)
Batteries only used 4-5 months, i really think other brands are doing better.
I have contacted Fénix Spain customer support and they are totally useless.

 

[BLUE LED]

Hi!
I must say i am kind of disappointed by Fenix ARB-L2 batteries on my fenix tk75 (2009lm version) on turbo mode they last only 28 min.
I have test them on a Tk75 2600 lm version and they last a little longer ... 38 min on turbo mode (accumulated time)
Batteries only used 4-5 months, i really think other brands are doing better.
I have contacted Fénix Spain customer support and they are totally useless.

It's not the battery at fault. It is the different versions on TK75 that you have. The timed step down on the original TK75 XM-L U2 is around 20 mins. Accumulated turbo time is around 40 minutes.

This was reduced to around 15 minutes on the 2nd version TK75 XM-L2 U2 2900 Lumens. Accumulated time on turbo around 30 minutes.

Instead of buying 2 versions I just moded my 1st gen TK75 2600 Lumens with 3 x XM-L2 U4. My accumulated time on turbo is 44 minutes.

 

[subwoofer]

Hi!
I must say i am kind of disappointed by Fenix ARB-L2 batteries on my fenix tk75 (2009lm version) on turbo mode they last only 28 min.
I have test them on a Tk75 2600 lm version and they last a little longer ... 38 min on turbo mode (accumulated time)
Batteries only used 4-5 months, i really think other brands are doing better.
I have contacted Fénix Spain customer support and they are totally useless.

As well as the answer BLUE LED has given, I would also point out that the way any cells are used affects their lifespan. Li-ion like to work in the 30-90% charge level range. If you keep them at 100% or regularly deep discharge them to below 30%, and especially if you leave them fully discharged, you increase the rate they degrade. Add to this a lot of cycles and you can 'wear out' any cells very fast.

If you hit the cell protection regularly, you should expect your cells to not last as long, and if you do, you should get them on charge asap to at least get them partially charged.

High drain also increases cell wear, so always using Turbo will also be harder on your cells.

There are many factors affecting cell life, and the typically quoted 'rechargeable for 500 cycles' does not take into account certain cell ageing factors.

Without knowing exactly how each cell has been treated, it is impossible to say if there is a cell related issue.

 

[theOS]

The batteries were always charged/used only with Fenix products (ARE-C2/TK75) , most of the times put to charge at 3.7 - 3.8 V remaining power.
I find this kind of disappointing due to the fact they only reach an autonomy of 37% from what Fenix promise.
Anyways i also had Panasonic batteries and they performed better than Fenix.

 

[BLUE LED]

I prefer Eagletac 3,500mAh and Nitecore D4 charger. It always charges to 4.20v

Good quality cells and charger.