Advice requested: Old niterider HID to XML conversion

I've got an old niterider HID with a burned out bulb - I think it's a Slickrock ~ 800lm. The 13.2 V (I think) NiMH is still in great condition, and I'd like to figure out a way to salvage the head and battery. My plan was either to step down the voltage to ~ 6 V and run two XMLs in series or ~3 V and run them parallel with this driver (3.04 A output)
https://illuminationsupply.com/8xamc7135-304a-selectable-mode-driver-p-142.html

I already have the driver and XMLs around from an impulse order, so running them parallel is my ideal plan. I think running two XMLs at 1/2 max current seems ideal for cooling purposes, since I'm suspect of the heatsinking capabilities in the head, which seems to be mainly plastic. The driver has an input voltage of 2.8-6 V with low voltage power-down at 2.9V.

Would it be possible/not a terrible idea to do a voltage conversion in series with the driver in order to drop ~ 10 V in potential? I'm not really worried about complexity, but I'm looking to minimize new purchases. I also considered rewiring the battery pack, but I think that would not agree with my charger.

I'd appreciate any opinions, especially if they don't include buying more than $20 of new stuff. If this project is not doable at all, I'd like to hear that, too. I'd rather sell the thing than spend too much money trying to revive it.

EDIT: I should say, my main priority is salvaging the battery, so if you have other ideas for that (other than replacing the HID bulb with another HID bulb), I'm all ears.

I'd guess that your biggest challenge will be providing adequate heatsinking for the LEDs. I've converted a couple of old AA powered bike lights to use a 3 watt LED:

a Cateye Micro II:
http://www.flickr.com/photos/kurtsj00/sets/72157628920533643/

and an older Vista light:
http://www.flickr.com/photos/kurtsj00/sets/72157601241482040/

The Vista light is the cleaner looking conversion, and it took a bit of work to make a path to get the heat out of the housing and out to an external heatsink. Sometimes it can be challenge to figure out how to add the external heatsink without interfering with the mounting hardware or power switches.

Fitting the desire optic or reflector into the housing is the next challenge. It's worth doing some measuring and sketching to figure out if it'll all fit before committing to the design. You probably can't just use the reflector that's already in the light.

Plan B might be to just throw together a lighting head from scratch. I did this with a dynamo light where I just used some aluminum L channel and plexiglass. Not the most beautiful thing in the world, but no worse looking than my bike light conversions:
http://www.flickr.com/photos/kurtsj00/sets/72157621965148305/

While I understand the desire to use the driver that you already have, the fact that you'll have to buy another converter in order to convert the battery voltage down to 6v suggests that it might not be any more expensive to just buy a LED driver that can accept the battery voltage.

Other people have considered this sort of conversion in the past, and I don't recall anyone who has gone ahead with it after considering all of the changes required (but I might just have a poor memory). It is a lot of work to go through just to salvage a nicad battery that is probably close to the end of life itself.

Steve K said:

I'd guess that your biggest challenge will be providing adequate heatsinking for the LEDs. I've converted a couple of old AA powered bike lights to use a 3 watt LED:

a Cateye Micro II:
http://www.flickr.com/photos/kurtsj00/sets/72157628920533643/

and an older Vista light:
http://www.flickr.com/photos/kurtsj00/sets/72157601241482040/

Click to expand...

That's a neat idea for the heatsink on the Vista. Does that work in the rain?

Steve K said:

Other people have considered this sort of conversion in the past, and I don't recall anyone who has gone ahead with it after considering all of the changes required (but I might just have a poor memory). It is a lot of work to go through just to salvage a nicad battery that is probably close to the end of life itself.

Click to expand...

I've seen some of the speculation on this. I have a couple of voltage regulators sitting around, but I haven't really taken the time to see if they might work. What would be nice is to integrate the voltage regulator into a short NR extension cord to fit between the battery and the head so that, when the unfortunate days come that the NiMH passes on to that great charger in the sky, I could find an easy LiIon solution that didn't require another re-wire. I have (slightly) more experience with dynamo lighting where over-current/over-voltage is not (usually) an issue in basic circuits, so thinking about this job with a battery is a little beyond what I can do with my current knowledge. Is there anything inherently and obviously dumb (not just impractical) about a battery -> cable -> voltage protection ->cable -> head -> driver -> LED circuit that should scare me away from this?

This might be too much future-proofing to be practical. Though I don't tend to view "practical" as a virtue when I can learn something useful.

Steve K said:

While I understand the desire to use the driver that you already have, the fact that you'll have to buy another converter in order to convert the battery voltage down to 6v suggests that it might not be any more expensive to just buy a LED driver that can accept the battery voltage.

Click to expand...

I almost missed this part - I looked for a driver module that could do this, but didn't find one. I'd be open to this solution if there is a driver that can go from 13-14 V down to 3 or 6. Any suggestions from someone who has seen one of these? I'd be open to making my own if the parts list wasn't too extensive, but my experience here is NULL. It could be as simple as a single voltage, single current output - I don't need a low mode for this light.

BobRoss said:

That's a neat idea for the heatsink on the Vista. Does that work in the rain?

Click to expand...

I haven't used it in the rain. It should do tolerably well, though. I've forced RTV into the small gap between the housing and aluminum, both from the inside and outside.

BobRoss said:

I've seen some of the speculation on this. I have a couple of voltage regulators sitting around, but I haven't really taken the time to see if they might work. What would be nice is to integrate the voltage regulator into a short NR extension cord to fit between the battery and the head so that, when the unfortunate days come that the NiMH passes on to that great charger in the sky, I could find an easy LiIon solution that didn't require another re-wire. I have (slightly) more experience with dynamo lighting where over-current/over-voltage is not (usually) an issue in basic circuits, so thinking about this job with a battery is a little beyond what I can do with my current knowledge. Is there anything inherently and obviously dumb (not just impractical) about a battery -> cable -> voltage protection ->cable -> head -> driver -> LED circuit that should scare me away from this?

This might be too much future-proofing to be practical. Though I don't tend to view "practical" as a virtue when I can learn something useful.

Click to expand...

The ideal would be a driver that could handle the voltage from the nicad battery as well as any future lithium battery. A current regulated buck converter would seem to be ideal. As an electrical engineer who looks for excuses to make stuff, I usually browse the various semiconductor companies for a good controller IC instead of looking for pre-built drivers, so I can't make any recommendations.

Using a switching voltage regulator to knock down the battery voltage, and then hooking up a switching current regulator up might work, but... there could be issues. Switching regulators act like a negative resistance, and using one as a load for another could mess up the control loop stability (potentially). Control loop stability is not a simple matter, and using the regulator in an unconventional manner is inviting trouble.

Any chance that you could just update the battery pack now and use a battery configuration that the driver can accept? If the nicads are over 5 years old, they probably aren't in good shape and could be recycled without losing much.

Steve K said:

The ideal would be a driver that could handle the voltage from the nicad battery as well as any future lithium battery. A current regulated buck converter would seem to be ideal. As an electrical engineer who looks for excuses to make stuff, I usually browse the various semiconductor companies for a good controller IC instead of looking for pre-built drivers, so I can't make any recommendations.

Click to expand...

Well, I found a couple on amazon of all places. They are too big to fit in the head, but I got the chip # (lm2596).[EDIT - this chip is wrong] Very helpful!

This will be brand spanking new territory for me.

Steve K said:

Using a switching voltage regulator to knock down the battery voltage, and then hooking up a switching current regulator up might work, but... there could be issues. Switching regulators act like a negative resistance, and using one as a load for another could mess up the control loop stability (potentially). Control loop stability is not a simple matter, and using the regulator in an unconventional manner is inviting trouble.

Click to expand...

This is very helpful. I understand enough of it to not go down that road.

Steve K said:

Any chance that you could just update the battery pack now and use a battery configuration that the driver can accept? If the nicads are over 5 years old, they probably aren't in good shape and could be recycled without losing much.

Click to expand...

I can try out a niterider Li-Ion from a minewt dual I have. The only catch is that the wiring on the cords is different, even though the connectors are the same. I assume this is to prevent plugging in the wrong battery and blowing a light. They have three cords in each wire, but only two of them do something, and the orientation of the two that do something depends on the battery. The negative terminal wire is identical, so I will just combine the other two wires in the head.

Thanks for all your help! I'll post pictures when I get around to this.

EDIT: chip was incorrect

BobRoss said:

Well, I found a couple on amazon of all places. They are too big to fit in the head, but I got the chip # (lm2596). Very helpful!

Click to expand...

The LM2596 is a voltage regulator, so it's not a good choice for driving a LED. A current regulator is a better choice.
I went to the T.I. web site to look up a better converter, but their web site is profoundly unhelpful. I've sent an e-mail to the local T.I. applications engineer in hopes that he can suggest something suitable and tell the folks at the home office that they really need to fix their web site. (T.I. bought National Semiconductor, who made the LM2596)

By comparison, Linear Technology has a much better website:
http://www.linear.com/products/step-down_(buck)_led_drivers
The LT3477 looks promising, but is a challenge to solder by hand. Maybe someone sells a driver based on this chip?

Steve K said:

The LM2596 is a voltage regulator, so it's not a good choice for driving a LED. A current regulator is a better choice.
I went to the T.I. web site to look up a better converter, but their web site is profoundly unhelpful. I've sent an e-mail to the local T.I. applications engineer in hopes that he can suggest something suitable and tell the folks at the home office that they really need to fix their web site. (T.I. bought National Semiconductor, who made the LM2596)

By comparison, Linear Technology has a much better website:
http://www.linear.com/products/step-down_%28buck%29_led_drivers
The LT3477 looks promising, but is a challenge to solder by hand. Maybe someone sells a driver based on this chip?

Click to expand...

Oops! Thanks. I'll edit my last post.