The SF M6-R: a regulated rechargeable SF M6 (HOLA)

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js

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THE SHORT VERSION ("just the facts, please")

This post documents a drop-in regulated, rechargeable NiMH battery pack and charging system specifically designed to work with the SureFire M6 running the MN21 HOLA. The battery pack fits in exactly the same space that the MB20 battery holder occupies, and does not require the springs to take up any extra length, nor does it require any modifications or additions to the M6 body, head, or switch.

The regulation voltage is set slightly higher than the highest voltage obtained from the MB20 battery holder with 6 fresh SF123A Lithium batteries, and the battery pack and integrated regulator stay in regulation over the entire 19 minute runtime. What this means, for those of you who are white-light, fresh-battery junkies, is that the lamp continues to deliver the same beautiful white and bright output for as long as the batteries have something to give.

The fast-charging system is based around the MAX712 fast charge IC and will charge the battery pack in just over 2 hours, at which point it automatically switches to a trickle-charge to maintain and equalize the cells. The charging system will not work from a 12 volt automobile adapter, unfortunately.

SOME INTRODUCTORY RAMBLING

Alright, that was the "abstract" portion for those who were impatient to have the salient facts ASAP. On to lots more details and pictures and discussion.

First of all, let me say that I have always been a bit annoyed by posts like "Well, at $22.50 per hour you can keep the SF M6; it's too rich for my blood." This has always seemed a bit like saying, "I think an M1 abrams is just stupid! Do you realize how much it would cost you to drive an hour to the coast and back?" The M1, like the M6, is not intended to be your EDC! I mean, as if anyone stands around for an hour, popping one full MB20 after another into an M6. It wasn't designed for recreational use.

Still, as this entire project tacitly admits, the $22.50/hr figure has a point to make, especially for those of us who primarily use our lights recreationally, even if we may like to pretend that we're spec ops insurgents on a reconnoiter. The desire for a rechargeable M6 has existed and manifested itself on the forums for a long time now. Before I started on this project I did an extensive search of all the CPF threads on the subject, and there were more than a few, some of them fairly old. Seems like every few months there is another "M6 on rechargeables?" or "Pilas in an M6?" question in the incandescent forum.

Why go to all the trouble to make an M6 rechargeable, though?, people might ask. After all, isn't there a SF 10X dominator to fill that role? This is what I thought at first too, but the short answer is that no, the 10X and the M6 are very different lights for different purposes. I won't go into too much detail regarding the merits of the M6, but suffice it to say that it is small and good to hand and has a shock isolated bezel and tactically correct tail switch. It is hard anodized with a pyrex lens, and it is very bright.

THE SF M6-R: A RECHARGEABLE M6

So, one day an idea of how to make a rechargeable SureFire M6 dropped on top of my head and I couldn't let it go. It's not a new idea--in fact it's fairly obvious, although the execution is far from simple!--but it's a good idea, and I found myself compelled to buy an M6 and all the stuff and at least try to make a rechargeable M6 a reality. It happened more or less like this: I noticed that the A sized NiMH batteries had the same diameter as the 123's that are so popular on CPF. Next I started thinking about what combinations of what lengths of A NiMH rechargeables would be a close match for multiples of 123's. Fortunately or unfortunately, the ONLY close match was with the M6, due to the complicated battery holder which introduced enough extra length so that three 2/3A's or two 4/5A's would more than fit in the length of the MB20, which holds 2 123's along it's length in a triple stack.

Now, of course, this is heretical, right? NiMH batteries? What's wrong with you, Jim, don't you know that Li-Ion are the best choice for a rechargeable M6?

Well, actually, no, I don't know that. udaman once accused us hotwire guys of failing to "think outside the box" because we weren't using Lithium chemistry batteries. I think that on CPF that "the box" IS Lithium chemistry. And this is understandable. Lithium batteries are pretty hard to beat in most ways, and have the highest energy densities of any battery chemistry currently in existence. Even so, the one downfall of currently available Li-Ion batteries is that they have high internal resistance and are not good at delivering high currents. First of all, they lose capacity and fail to deliver what they are nominally rated for, and second, they drop voltage due to their internal resistance, and third, they do not have good cycle life expectancy when run at these current rates. And, boy, does the M6 HOLA draw some current! Near 5 amps. And I'm not even getting into any safety issues of Lithium-ion and Lithium-poly rechargeable batteries.

That said, however, it looks to me as if two 18650 Li-ion batteries in series are a good match with the MN21and should provide 20 minutes of unregulated runtime. As for how many cycles they will last and how they will hold up, I do not know. Also, according to what I've read, these will fit inside a bored-out M4 body, which would give the same 500 lumen output of the M6 HOLA in an M4 configuration. Even so, I would still prefer my regulated, rechargeable NiMH pack in an M6 to these options.

A REGULATED M6

So back to the story at hand: once I had determined that the very same battery packs which I use for the TigerLight upgrades were a perfect fit in the M6 battery compartment, I had to wrestle with the problem of delivering the correct voltage to the M6 MN21 HOLA and also, if possible, the MN20 LOLA. I consulted Brock's flashlight page (which I now believe to be incorrect, BTW), made my own measurement, and finally, consulted with Willie Hunt, who is the SureFire engineer who sells the LVR flashlight light bulb regulators. The upshot was that in order to have a bright, white beam, I needed to provide a voltage under load of 6.8 volts to the MN21. This is too low for direct drive from either the 6 cell or the 9 cell pack, and thus I knew I needed a buck regulator. Also, it is too high for the 6 cell pack to stay in regulation over the entire run. Thus, my choice was clear: a 9 cell 2/3A NiMH battery pack with one of Willie Hunt's LVR regulators set at 6.8 volts and integrated into the pack to make a single, rather sophisticated regulated rechargeable battery pack.

The next challenge was to make the LVR fit comfortably in the space left between the circular battery compartment and the triple stack of 2/3A's--and it certainly was a challenge. I did this by having Willie leave out the ON/OFF/DIMMING button and by remote mounting the Power FET just a bit farther down the side of the battery pack.

There was more than just the space issue, of course. Integrating the regulator into a triple stack of three series cells involved two jumper braids to make the stacks into a 9 series configuration, and involved three connections to the LVR and two connections to rounded triangular metal end caps which had to be electrically isolated from the rest of the battery pack, and it required creating two additional connections directly to the + and - contact points of the battery pack for charging, because it is impossible to charge the pack through the LVR. This also required a set of male and female connectors which were both shielded and isolated so that neither the battery pack nor the charger could be inadvertently shorted.

I decided to use the new GP 1100 2/3A NiMH batteries, not so much because of the slightly great capacity than the KAN 1050's, nor because of the higher current capability (much more than is needed), but because they are slightly shorter than the KAN 1050's. Here is a picture of the GP batteries with a few of the KAN's. You can see that I removed the wrap from the batteries and also made new kapton insulators to replace the fiberboard ones. I needed to do this because the batteries would not seat together properly with the wrap in place and because the insulators were not glued in place and in my opinion were not suitably resistant enough to high temperatures to make a good end-to-end solder shield.

m6_r_1.jpg


After end-to-end soldering I wrapped each stack of three in kapton tape, although if I had nice thin shrink wrap I probably would have used that instead:

m6_r_2.jpg


Next I glued the stacks together into a triangle formation with RTV silicone adhesive.

THE LVR3I REGULATOR

This is the heart of the M6-R, Willie Hunt's LVR3I voltage regulator:

m6_r_4.jpg


It does it's job of voltage regulation very efficiently. In fact, Willie claims that it is 99 percent efficient. The LVR3I has a number of features that can be ordered as options, such as warning flashes, low-battery voltage cut-off, power-levels, and automatic low-level running at the end of battery cycle. As I had no room for the button, I could not have power-levels, and because the LVR is always connected to the battery pack, I could not have a low-battery cut-off, although the LVR will not function below 5 volts and will thus prevent the battery pack from being depleted to dangerously low levels. I did, however, order the M6-R LVR3I regulator set to give warning flashes when the battery reaches critically low levels (near .92 volts/cell). In addition, the LVR3I has a soft-start feature which limits the current flowing into the filament at start up. Due to the very low resistance of a cold filament and thus the very high startup current when direct driven, this is the time when most of lamps burn out. Limiting the startup current prolongs the life of the filament and allows one to run closer to the melting point of the filament for the brightest, whitest light possible from this technology.

Here is the pack with integrated LVR and charging connector. The wires to the left and right will be attached to the underside of the copper end caps.

m6_r_5.jpg


Here are the copper end caps before they have been tinned with 96.5Sn/3.5Ag solder. I use this silver bearing lead-free solder due to it's resistance to corrosion and oxidation and because it is the hardest of all the solders.

m6_r_3.jpg


And finally, here is the prototype pack getting a charge. (Keep in mind that it is a bit rough around the edges, being a proto-type and all):

m6_r_6.jpg


THE CHARGING SYSTEM

At first, I was going to make a simple LM317T based trickle charger for this setup, but upon investigation, I found that there were fast-charge IC controllers which greatly simplified building and designing a smart charger. With these chips it is actually so simple that it isn't much more work than an LM317T based circuit. I used Maxim-IC's MAX712 because it was readily available from Newark and their documentation was good and because others had already used the 712 and with good results: see this link for example. Note that I disagree with one calculation on this site (see below).

There are two especially nice things about the 712. One is that it is a zero delta V peak detect circuit, which means that if for some reason it fails to detect end of charge the first time, it has an infinite number of chances after that. The negative delta V circuit, on the other hand, will wait until it sees a voltage drop, so it only has one chance. The other nice thing about the 712 is that it switches over to a gentle C/16 slow charge after the completion of fast charging, and will thus equalize all the cells if they need it, and will maintain the battery pack at full charge until you go to use it. Even so, the current is not low enough to allow you to leave the pack connected to the charger 24/7, and it should be removed after no longer than several hours of trickle charging.

My circuit is exactly the same as the "Typical Operating Circuit" in the applications notes, with the addition of LED indicators for power and fast-charge, also described in the applications notes. This means, for those of you who might be thinking of building this circuit, that you will need to have enough current flowing into the V+ pin node to supply both the fast-charge indicator LED and the V+ pin. The V+ pin needs at least 5 mA, but can handle as much as 20mA, while the LED current must be exactly 5 mA. Thus the value of R1 should be such that 10 mA minimum flows into this node, and not the 5 mA described in the weblink above.

Here are a few pictures of the charging system:

m6_r_7.jpg


m6_r_8.jpg


This is just the proto-type, and will need to be tweaked a bit. In particular, I think that the heat sink I am using is overkill, and also that I could easily go to a smaller box. But these are minor issues; I am very happy with the performance of this circuit. Maxim-IC has done a great job with this chip.

And in case anyone is wondering, yes, I am all setup to use the same circuit (with different component values) to build a fast charger for the TigerLight. Details to follow in a separate thread. /ubbthreads/images/graemlins/smile.gif

SO, IS ANYONE INTERESTED?

I wanted to wait before posting this, but Ginseng dropped a teaser in the thread "What's the closest thing to a rechargeable M6?". Plus, I really want to get a feel for how much interest there is in this mod. brightnorm is already on board as a field tester, although I have not yet sent him his M6-R pack and charger. I will be doing so in the next week, or sooner if I can. And after he has had a chance to give me (and us) his impressions and suggestions, I would like to build three more battery packs and chargers for three additional field testers, if there is enough interest in this mod to warrant a future build run. I would like for one of the field testers to be from the UK or Europe so I can deal with the problem of the different line voltage and powersupply. All of the field testers should be flashlight veterans with a good knowledge of the subject, and they should all obviously own an M6.

So, is anyone interested? Price will really depend on how many packs I build. If there is enough interest, I may be able to get the LVR3I's for cheaper than the standard $50 rate. Assuming $50, that would price the battery pack at about $110, and the charging system should cost around $75 and probably less—I need to sit down and add up the parts and labor. In any case keep in mind that for $10 or so I would be happy to provide a nice connector set with banana plug terminations to allow you to plug into any of the excellent chargers commercially available. The Hitec CG-340 comes to mind, which can be had for $38, although it still needs a 12VDC power supply. Many people use a surplus Cosel switching supply for $24 from ebay. www.ramboy.com sells them, but there were very few left the last time I checked.

Anyway, there it is. A rechargeable M6 is not only not an impossibility, it's now a reality.
 
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kakster

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Im in for one /ubbthreads/images/graemlins/thumbsup.gif

Sorry if i missed it, but can you run the MN20 with this pack?
 

turbodog

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1) Good work.

2) I think you used too many cells. At 6.8 volts, each cell is down to .75 volts or so. Unless you used matched cells, at least 1 cell is certainly depleted by then and on it's way to reversing.
 

BC0311

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WOW, Jim! /ubbthreads/images/graemlins/happy14.gif /ubbthreads/images/graemlins/clap.gif

Britt
 

KevinL

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/ubbthreads/images/graemlins/faint.gif /ubbthreads/images/graemlins/bowdown.gif /ubbthreads/images/graemlins/bowdown.gif /ubbthreads/images/graemlins/bowdown.gif

Rechargeable AND regulated at one fell swoop... amazing!
 

js

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[ QUOTE ]
kakster said:
Im in for one /ubbthreads/images/graemlins/thumbsup.gif

Sorry if i missed it, but can you run the MN20 with this pack?

[/ QUOTE ]

No, you didn't miss it. Good question. I haven't actually tried yet. LOL! The short answer is definitely "yes", but the real question is "how much will the M6-R pack underdrive the MN20?"

As I mentioned, this post is a bit premature, but heck, it's a THREAD, right? I will add stuff as I get it or determine it. I was just going to let brightnorm determine this question, because I had to send my LVR back to Willie Hunt to have him modify it. I ordered it with a low battery voltage cutoff without realizing that when this is activated the regulator latches OFF until power is removed from it. Unfortuately, the LVR is always connected to the pack so removing power from it is a very non-convenient procedure involving removing the shrink wrap and covering tape over the LVR, and unsoldering and resoldering one lead. When I get BN's pack up and running I will try out the LOLA and post here.
 

js

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[ QUOTE ]
turbodog said:
1) Good work.

2) I think you used too many cells. At 6.8 volts, each cell is down to .75 volts or so. Unless you used matched cells, at least 1 cell is certainly depleted by then and on it's way to reversing.

[/ QUOTE ]

The warning flashes start at .92 volts/cell, and users are STRONGLY advised to turn off the M6 very shortly after getting the warning flashes. This is why the M6-R stays in regulation over the ENTIRE run! The regulation set-point is lower than the end-of-cycle battery voltage. This is the way I wanted it.
 

turbodog

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[ QUOTE ]
js said:
[ QUOTE ]
turbodog said:
1) Good work.

2) I think you used too many cells. At 6.8 volts, each cell is down to .75 volts or so. Unless you used matched cells, at least 1 cell is certainly depleted by then and on it's way to reversing.

[/ QUOTE ]

The warning flashes start at .92 volts/cell, and users are STRONGLY advised to turn off the M6 very shortly after getting the warning flashes. This is why the M6-R stays in regulation over the ENTIRE run! The regulation set-point is lower than the end-of-cycle battery voltage. This is the way I wanted it.

[/ QUOTE ]

Forgot about the warning. Yes, you're right. Multi cell packs are a pain though. If you build more and WANT to use matched cells I will match them for YOU for no charge. PM me.
 

brightnorm

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This is terrific! There has been much talk over the years about a rechargeable M6 but now it's a reality, and regulated to boot. Pretty d@mn impressive!

Brightnorm
 

pyro

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Re: The SF M6-R: a regulated rechargeable SF M6 (H

great!
i was also thinking about using 9 2/3A battery and LVR,
but i did not try it bacause i have no M6 and i had in mind that the LVR is much bigger and that there is no space to put it.
You solved that very nice!

I would add a spacer or some thin plate over the LVR to
protect it when the light gets dropped.
 

js

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Re: The SF M6-R: a regulated rechargeable SF M6 (H

[ QUOTE ]
pyro said:
great!
i was also thinking about using 9 2/3A battery and LVR,
but i did not try it bacause i have no M6 and i had in mind that the LVR is much bigger and that there is no space to put it.
You solved that very nice!

I would add a spacer or some thin plate over the LVR to
protect it when the light gets dropped.

[/ QUOTE ]

pyro,

If you'll notice in the picutres above, the LVR is attached face down to the battery pack, and the backside of the board has only wires on it. It is supported along the sides where it mates with the batteries by strips of mil spec silicone rubber, which keeps the components from touching the batteries. Thus, it is well protected as it is, and I have very little space for any sort of extra covering over the LVR beyond the kapton tape and shrink wrap. I suspect it would weather a drop just fine, although that's obviously never a good thing, and for combat or military purposes, the MB20 with SF123A's would be the better option.

Another thing, too, which I may tweak if I can, is that the soft-start feature of the LVR3I is very noticeable. Sometimes it can be a full second before the light reaches full brightness. While this is great for filament longevity, it's terrible for signaling in morse code or for surprise blinding someone. I will be asking Willie if he can reduce the time constant on the soft-start. In any case, this is one of the things to be perfected via the testing brightnorm will be doing for me. I guess I'd call it proto-type testing.

On another note, if anyone is interested in field testing specifically, please say so--again, assuming that there is enough interest to warrant a build run. Looks like that will be the case, from these initial posts, but we'll see.
 

Kiu

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Re: The SF M6-R: a regulated rechargeable SF M6 (H

Jim

Excellent work!! /ubbthreads/images/graemlins/wow.gif /ubbthreads/images/graemlins/happy14.gif
It seems you put many effort on it.
 

Sakugenken

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Re: The SF M6-R: a regulated rechargeable SF M6 (H

I am also very interested in this.
 

Glo_in_dark

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Re: The SF M6-R: a regulated rechargeable SF M6 (H

Count me in. I've wanted an M6 since it's conception - just could not justify the operational cost when used daily. WOW!!
Michael
 

js

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Re: The SF M6-R: a regulated rechargeable SF M6 (H

OK. Six people so far are interested. That's a good start.

As for lusting after an M6, I can totally relate. I've also wanted one since I learned about it, but it never would have flown with my wife. This way, I got to tell her that it was an investment and that I would at least break even on the project. Now I own an M6. /ubbthreads/images/graemlins/buttrock.gif It's a beauty of a light. The only thing that takes a bit of getting used to is the tail cap switch combined with the larger o.d. of the body. At first it hurt my hand to hold the switch down for any length of time, but with use I guess I developed the right muscles.
 

Xrunner

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Re: The SF M6-R: a regulated rechargeable SF M6 (H

/ubbthreads/images/graemlins/thumbsup.gif /ubbthreads/images/graemlins/popcorn.gif

-Mike
 
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