Converting Petzl Duo to use 3.7v Li-ion Batteries  

With an increasing number of folk doing a 3.7v li-ion battery conversion on Petzl Duo, we thought we ought to pull up our big girl pants and have a go at it!

It was more straightforward than we recall from when we last attempted this, but still requires some basic skills and is probably not for the faint hearted or technically challenged! We have posted some photos and guidance below. If you need to ask us futher questions then this probably isn't for you. Just saying ;-)  

We would also suggest that anyone considering this conversion for the first time should use a broken / retired Duo, as it would be a shame to ruin a perfectly good lamp if things don't go quite to plan! Also, worth pointing out is that the Petzl Duo is a really practical caving lamp running 4x NiMh AA batteries (especially for remote expeditions etc), so the question begs whether or not this conversion is actually the right choice for you. Consider yourself warned :-) For the record, I still run my Duo on NiMh rechargeable AAs. 

CustomDUO v42 and Simple LED upgrade modules operate fine from 3.7v li-ion. In fact better than from AAs, as the power density of 18650 Li-ion cells is greater and will realise the highest outputs from the LED modules for longer !!

The basic procedure is to remove the cable from the battery box, and then gut the battery box by whatever means possible in order to leave a clear shell that can accommodate 2x 18650 batteries. Some people have reported that 3 cells are possible, but this is no doubt a very close fit.

We used 2x Panasonic NCR18650b 3400mAh 18650 cells wired in parallel, 1s2p, to provide 3.7v 6800mAh battery. The batteries were wired with a protection circuit PCB, which is important. Two cells fit into the battery box shell, but note that they are a very snug fit at the battery box lid (see photo). It might be necessary to hunt around for the right cells for the job, as there can be dimensional variations.

We used JST connectors on silicone leads for this conversion, as these can readily accommodate the current level required. Cheap leads and connectors are best avoided. It is best to make sure that this is not the weakest link in a robust solution. We used a cable tie at the base of the main lamp cable to ensure that it does not pull out of the box, and then simply flowed some epoxy resin into this corner of the box to seal. There is likely other equally or more effective approaches, but we were not minded to waste too much time on our conversion.

It did occur to us that before joining up the main Duo cable, that there was an ideal window of opportunity to thread the cable through holes in helmet, if so inclined. We did not, as threading under lip of helmet works just as well.

Extended run times using 3.7v 6800mAh li-ion battery arrangement with CustomDUO v42 module can be approximated as follows (remembering ofcourse that module is thermally regulated on higher settings to protect LEDs from over temperature operation);

• setting 1 (6800/150mA=)  45 hours

• setting 2 (6800/700mA=)  9 hours

• setting 3 (6800/350mA=)  19 hours

• setting 4 (6800/1400mA=)  4 hours

There is one issue with this conversion worth mention!!  While CustomDuo LED modules are ideally suited to 3.7v li-ion, the battery charge indicator is set up for 4xAA batteries (the normal Duo battery arrangement), so regardless of the level of charge of the battery pack it will basically always show the level of battery charge to be 1 of 5, i.e FLAT. Not a huge deal, but we will be certain to provide a user selectable option for 3.7v li-ion batteries when we next update the v42 software. (Incidentally, the v5 Omni module, that predated the v42, does have a battery charge level software calibration option for 3.7 li-ion batteries).

3xAA holder can be used easily as a backup for li-ion Duo conversion!! AAs don't care too much how they are treated, and therefore can very easily be dragged around cave without losing the plot...... Customduo LED modules happy enough running on 3xAA batteries (at least for this purpose).

IMPORTANT - 3.7v 18650 Li-ion batteries must be connected in parallel (or just one battery), so that the total voltage is 3.7v (approx 4.2v fully charged). 

Batteries must not be connected in series, because total voltage (7.4v for two batteries in series) will be too much, and module will be damaged. 

Charging 3.7v  li-ion batteries for Duo conversion

This 3.7v li-ion conversion can be easily and effectively charged with our new range of Monkey Tail USB chargers, offering connectivity with USB-A, micro USB and USB-C.

read more  

(Update January 2021)

18500 Li-ion batteries !!  An alternative DIY li-ion solution by Fjfell. Fjfell solution uses 3x18500 batteries, 50mm long (shorter than 18650s). A 6Ah pack is possible using 2Ah batteries arranged in parallel.

This solution has potential advantage that the 18500 cells are shorter in box than 18650 cells. 18650 cells are a very close fit, the stripped out Duo lid being the pinch point.