Duo conversion using 18500

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I imagine this has been done before, but in our lockdown interlude I was inspired by various posts to have a go at a Duo conversion with an old one I had. We were already using several Duo inserts on Explorer helmets, but I am very reluctant to mess with them, so are using AA’s. Unless you commit to changing out cells on a longer trip, it’s a little difficult to use the full capability of the Custom Duo V42 - specifically running on full flood all the time.

I gutted it and put on another helmet. As you can see I moved the cable entry to one side. It is driving the warm-white version of the V42, as I also do in my Scurion. Being more used to carbide I suppose.

I started by making a twin 18650 (, which had a few trips. However, it’s rather cruel to the 20 year old plastic around the lid and seal as it is too tall. Probably it would be OK, but it offends my engineering purity.

I then made a triple 18500 ( as shown. This works pretty well and fits nicely. I also made a single 18650 as a spare because that can lie diagonally and not interfere with the lid.

I reckon you can make the whole thing including helmet and insert for about £140 with some judicious Ebaying, and you end up with a very decent light for the money.

Ian Ball:
What did you use to waterproof the holes?

and how did you make 3 cells fit vertically if 2 cells were a bad fit in the same orientation?


--- Quote from: Ian Ball on December 08, 2020, 10:53:10 am ---and how did you make 3 cells fit vertically if 2 cells were a bad fit in the same orientation?

--- End quote ---

18500 cells are shorter than 18650 (50mm instead of 65) :smartass:

It’s sealed with glue (poured into corner). You can make a pool with a glue gun (you could get this out again), or go for epoxy (not coming out). The cable is clamped both sides prior to that. Feeding through the helmet makes it a bit more robust (you could also seize the helmet entry). The cable now being near the corner of the box helps with glueing.

You need to be careful about the screw at the bottom depending on how enthusiastically you gutted the box, and whether you intend to use the bottom screw. Given the extra space with the 18500 maybe leave it alone.

The battery is double-wrapped, which makes it snug. It is a rattle fit without the wrap.

I did originally look at triple 18650, but it is very tricky to fit and probably even more unwise in terms of stressing the lid nearer the end.

Ian Ball:
Aaah pay attention Ian.

Please could you explain something to me energy density wise and the like, my maths and physics or maybe chemistry is not strong enough.

Would a lamp being powered by a LiIon cell of 2000mAh offering 3.7V achieve a longer run time than a NiMH 4.8V pack rated at 2000mAh? If the output was maintained at the 350mA mark (such as in Custom Duo setups).

If the current draw is 350mAs regardless of voltage then what effect does the lower voltage have in the system, reduced visible output, lower heat generation (wasted energy).

The reason I ask is I have a fair few older LiIon cells that are recycled out of laptop batteries and using an analyser I see they hold 2000mAh. 

However the four Ikea Ladda AA I use hold an average of 2300mAh.  And so would a single 18650 LiIon cell running more efficiently in terms of self discharge, loss of capacity in cold temperatures, efficiency at high drain make the switch to a single Li-Ion holding 2000mAh still be worth it over the current AA setup?  the benefit of reduced weight isn't something I see as an important benefit whereas the likelihood I bugger it up certainly is.

Obviously moving to a dual liion cell in parallel pack holding 4000mAps would be a better bet but the struggle to fit them in is off putting.


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