Modifying a Fenix HP-25

Hi, original thread in headlamps, I think due to the nature of the question it should be here instead, administrators feel free to delete original.

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Hi everyone, I have a Fenix HP25 which I personally think is the bee's knees, all except battery life.... This thing eats batteries like they are going out of style.

I recently began thinking that I might change the battery pack to rechargeable lithium ion. Anyone done this mod or a similar one before? Before going down the path of learning as much as I have, I looked out there to see what other headlamps similar to this one are using 18650s but nothing really fits the bill.

I contacted Fenix and they advised me that the input voltage range was 3.6 - 6.6V. The current for each LED is 785mA. Obviously this leads to the situation where if I use a 'dumb' 3 leg voltage regulator, I have to 'get rid of' anywhere between 1.8 - 0.4V to avoid damaging the inbuilt regulator if I use a couple of 18650s in series. With both LEDs on full power they would be together consuming about 5.8W (assuming that they are operating at 3.7V) which means I would be 'uselessly burning' between 2.8W and .6W depending on the state of the batteries. Not a great outcome.

I was on Fastech and saw they had a bunch of LED drivers, I thought this might be a great way to go, just replace the driver rather than have double regulation, but upon opening the headlamp it has a circuit board very different to the style that Fastech are selling. I suppose one (rather extreme) option would be to identify the regulator chip on the circuit board and change it to one that would handle 8.4V?

The other option is to just go with 1.2V NiMH Eneloops but I have my doubts that the headlamp would even function on its highest setting due to the lack of voltage.

This headlamp is perfect for my use as i really love the flood beam for close up work with the spot for outdoors.

If someone can suggest a viable option for conversion, I'll draw up the CAD files for a 18650 battery housing and stick them up on CPF for anyone who wants to get one printed.

Cheers!

Tristan.

If you're considering making a custom 3D-printed battery housing anyway, 4 NiMH AAs in series would put you neatly in the voltage range. 6V hot off the charger, 4.8V nominal, something like 4.0 - 4.4V at full depletion. Decent AA NiMH - think Eneloops - will happily put out 3 - 5A, which is more than you'd need. Less capacity than a decent 18650, though.

The snag is that going down to 3.6V could see one of your NiMH cells pushed to the brink of reverse charge, i.e. you'd have to be careful to change the cells as soon as the light started to lose power.

You could look into a buck regulator there are plenty on ebay that can take the ~8v of two 18650s and reduce that down to whatever voltage you desire for a few dollars. I would however suggest in that setup you use protected cells to prevent over draining them or worse.

maybe something like this
http://www.ebay.com/itm/Mini-MP1584EN-DC-DC-BUCK-Adjustable-Step-Down-Module-Effect-Higher-Than-LM2596-/191095423493?hash=item2c7e2cc205:g:gv0AAOSwrklVdVaF

you may be able to find room to fit a board this small somewhere.

Lynx_Arc said:

You could look into a buck regulator there are plenty on ebay that can take the ~8v of two 18650s and reduce that down to whatever voltage you desire for a few dollars. I would however suggest in that setup you use protected cells to prevent over draining them or worse.

maybe something like this
http://www.ebay.com/itm/Mini-MP1584...423493?hash=item2c7e2cc205:g:gv0AAOSwrklVdVaF

you may be able to find room to fit a board this small somewhere.

Click to expand...

Thanks for the tip. I am assuming that the original board contains a buck regulator, but only one that can handle a maximum of 6.6V. Do you think that there would be any problems feeding the input of this with an already regulated voltage (apart from the inefficiencies of a double regulator)?

At these lower amperage and differential voltage level I doubt the output of the 'new' buck regulator would be particularly noisy, but not having much experience with them in general I have no idea if the 'original' regulator would crap itself if there was anything other than pure DC being applied to it? Thoughts?

Also, after reading the various lithium battery posts on 'vent with flame' I am planing on exclusively using protected cells, especially as they will be directly adjacent to the back of my head!

TristanJ said:

Thanks for the tip. I am assuming that the original board contains a buck regulator, but only one that can handle a maximum of 6.6V. Do you think that there would be any problems feeding the input of this with an already regulated voltage (apart from the inefficiencies of a double regulator)?

At these lower amperage and differential voltage level I doubt the output of the 'new' buck regulator would be particularly noisy, but not having much experience with them in general I have no idea if the 'original' regulator would crap itself if there was anything other than pure DC being applied to it? Thoughts?

Also, after reading the various lithium battery posts on 'vent with flame' I am planing on exclusively using protected cells, especially as they will be directly adjacent to the back of my head!

Click to expand...

I'm not sure how the output would be from double buck regulation but I'm guessing it will be ok. As for the other board taking more than 6.6v I wouldn't count on it as I've found that electronics that buck regulate can handle less than nominal voltage but not too much more than nominal. Not knowing how the headlamp is wired it is possible that it has 2 circuits a buck regulator in the battery pack and a driver for the LED output. If this is the way it is wired then you could possibly totally replace the circuit in the battery pack in fact I would consider replacing the whole pack setup leaving the old pack intact and use a connector so you can swap pack setups if that would work. The opposite you could do is run 2 18650s in parallel and boost them to ~5-6v this would allow you the freedom to use 1 battery and eliminate the danger associated with series lithium ion battery circuit.

So it appears I have been vastly overthinking things...

I installed a fresh set of Alkalines (4) into the battery holder today, hooked up my multimeter, turned on both spot and flood to their highest settings and took some notes.

1. With brand new cells, voltage drooped straight to 3.4V.
2. After about 10 minutes, voltage has dropped to 3.25V, both LEDs still running fine at their highest levels.
3. After another 10 or so minutes the voltage dropped to 3.1 and the regulator kicks in, dropping both LEDs to their second highest setting.

Conclusion: I intend to draw up a custom housing to mount 2 Soshine 3400Ah protected 18650s in parallel, no extra electronics required. Output to the original headlamp will obviously be somewhere between 4.2V and 2.5V (depending on what droop I get sucking about 1.5A max, and when the under voltage protection kicks in) which will give me more than ample range and is well within the original specs the electronics can handle!

Some quick back of the envelope calculations lead me to think that the original 4 x AA battery pack would have about 15Wh (4 x 1.5V x 2.5Ah), the 18650s will give me 25Wh (2 x 3.7V x 3.4Ah). Both options will also weigh approximately the same as well (100g batteries only).

In addition I will never have to buy alkalines again and the Soshines are currently retaining for about $28 for a 4 pack at www.fasttech.com.

I could also potentially add an extra 18650 which would give me 37.5Wh, a very large capacity indeed for this light's current draw, and I don't think the extra 50g would even be noticeable on the head.

Thoughts?

I don't think using alkalines is a good indicator of things as the heavy currents have them unable to hold decent voltage while nimh would probably not drop in voltage nearly as much perhaps from about 5.6v with no load off the charger to about 5v with load if high enough amperage. It does sound like that you could use unprotected batteries but would have to use the first voltage drop as your indicator of time to change/recharge them. You could also if you didn't want to have to take the 18650 batteries out of the pack buy a usb charging module for about a dollar off ebay and install it in the box. Alkalines are not any good for most of todays high power LED lights as the current drains exceeding 0.5A have them slowly losing the competition between them and nimh batteries.

Lynx_Arc said:

I don't think using alkalines is a good indicator of things as the heavy currents have them unable to hold decent voltage while nimh would probably not drop in voltage nearly as much perhaps from about 5.6v with no load off the charger to about 5v with load if high enough amperage. It does sound like that you could use unprotected batteries but would have to use the first voltage drop as your indicator of time to change/recharge them. You could also if you didn't want to have to take the 18650 batteries out of the pack buy a usb charging module for about a dollar off ebay and install it in the box. Alkalines are not any good for most of todays high power LED lights as the current drains exceeding 0.5A have them slowly losing the competition between them and nimh batteries.

Click to expand...

Hi Lynx_Arc, thanks for your feedback. The alkalines were really just to see what voltage level triggered to step down from the highest to second highest brightness level. I was extremely surprised to see that happening at such a low voltage (3.1V). This test was the first step in the process of working out the output voltage of a external buck regulator but when I saw the actual level I realised that a couple of 18650s in parallel would be a lot simpler and more efficient. It will be interesting however to see what voltage a pair of freshly charged 18650s will sag to when asked to deliver about 1.5A. I have taken a look at a few battery discharge curves that people have posted on the forum and it seems that they don't immediately drop (like alkalines) but drop gently and progressively from around 3.7-4.2V to their protected lower level cutoff. My amperage ask is VERY conservative compared to most of the flashlights being talked about on this forum, and in actual real-world use I would be quite unlikely to be using both LEDs on full, rather I would be using either the flood or spot independently depending on the task I am doing. This means the actual real-world current draw would be somewhere about 800mA.

As for the charging module, as you say an inbuilt unit would be a lot simpler and cheaper but I intend to move towards 18650s for a bike light so I think I will go with a decent external charger so I can do both.

It still blows me away about just how far the brand new alkalines dropped though...

TristanJ said:

Hi Lynx_Arc, thanks for your feedback. The alkalines were really just to see what voltage level triggered to step down from the highest to second highest brightness level. I was extremely surprised to see that happening at such a low voltage (3.1V). This test was the first step in the process of working out the output voltage of a external buck regulator but when I saw the actual level I realised that a couple of 18650s in parallel would be a lot simpler and more efficient. It will be interesting however to see what voltage a pair of freshly charged 18650s will sag to when asked to deliver about 1.5A. I have taken a look at a few battery discharge curves that people have posted on the forum and it seems that they don't immediately drop (like alkalines) but drop gently and progressively from around 3.7-4.2V to their protected lower level cutoff. My amperage ask is VERY conservative compared to most of the flashlights being talked about on this forum, and in actual real-world use I would be quite unlikely to be using both LEDs on full, rather I would be using either the flood or spot independently depending on the task I am doing. This means the actual real-world current draw would be somewhere about 800mA.

As for the charging module, as you say an inbuilt unit would be a lot simpler and cheaper but I intend to move towards 18650s for a bike light so I think I will go with a decent external charger so I can do both.

It still blows me away about just how far the brand new alkalines dropped though...

Click to expand...

18650s vary in their internal resistance which is what determines the voltage drop under load. As they get larger in capacity their internal resistance increases and it reduces their ability to put out current. You can get 2500mah batteries that can put out over 20 amps while some 3400s won't even put out 10A without a struggle. If you use 2 in parallel then the internal resistance of the battery array (2 in parallel) is half that of one battery alone so it should reduce voltage drop.
Alkalines have a lot of internal resistance such that AAs struggle to put out more than 1 amp and capacity at about 20 ma is 2500mah while at 500ma maybe 1500mah and 1A maybe 800mah at 1.5A probably drops to 200mah at a lower voltage. Basically AA alkalines aren't good for more than 1A and at that current nimh AAs have a lot more runtime. I have a cheap ebay flashlight with a built in charger port (micro USB) I would say if you could fit a $1 board in there with no much effort I would do it so you don't have to carry along a charger with you just a cable. I keep a micro USB cable and 12v charger in my glove box as I can use the charger for other USB stuff and the cable for micro USB since it has become the standard input for now.